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A

A-beta fiber
A wide-diameter, myelinated sensory nerve fiber that transmits signals from mechanical stimulation.
A-delta fiber
An intermediate-sized, myelinated sensory nerve fiber that transmits pain and temperature signals.
abducens (VI) nerves
The sixth pair of cranial nerves, which innervate the lateral rectus muscle of the eyeballs.
absolute metrical depth cue
A depth cue that provides quantifiable information about distance in the third dimension (e.g., his nose sticks out 4 centimeters in front of his face).
absolute pitch
A rare ability whereby some people are able to very accurately name or produce notes without comparison to other notes.
absolute threshold
The minimum amount of stimulation necessary for a person to detect a stimulus 50% of the time.
absorb
To take up something—such as light, noise, or energy—and not transmit it at all.
acceleration
A change in velocity. Mathematically, acceleration is the derivative of velocity. In words, linear acceleration indicates a change in linear velocity; angular acceleration indicates a change in angular velocity.
accessory olfactory bulb (AOB)
A neural structure found in nonhuman animals that is smaller than the main olfactory bulb and located behind it and that receives input from the vomeronasal organ.
accidental viewpoint
A viewing position that produces some regularity in the visual image that is not present in the world (e.g., the sides of two independent objects lining up perfectly).
accommodation
The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects).
achromatopsia
An inability to perceive colors that is caused by damage to the central nervous system.
acoustic reflex
A reflex that protects the ear from intense sounds, via contraction of the stapedius and tensor tympani muscles.
acoustic startle reflex
The very rapid motor response to a sudden sound. Very few neurons are involved in the basic startle reflex, which can also be affected by emotional state.
active sensing
Sensing that includes self-generated probing of the environment. Besides our vestibular sense, other active human senses include vision and touch. Animal active sensing includes the use of echoes by whales and bats, the use of electrical signals by some fishes, and the use of whiskers/antennae by fishes, insects, and nocturnal rodents.
acuity
The smallest spatial detail that can be resolved at 100% contrast.
adaptation
A reduction in response caused by prior or continuing stimulation.
adapting stimulus
A stimulus whose removal produces a change in visual perception or sensitivity.
additive color mixture
A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together.
aerial perspective or haze
A depth cue based on the implicit understanding that light is scattered by the atmosphere. More light is scattered when we look through more atmosphere. Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct.
afferent fiber
A neuron that carries sensory information to the central nervous system. Compare efferent fiber.
afferent signals
Information flowing inward to the central nervous system from sensors in the periphery. Passive sensing would rely exclusively on such sensory inflow, providing a traditional view of sensation. See also afferent fiber.
age-related macular degeneration (AMD)
A disease associated with aging that affects the macula. AMD gradually destroys sharp central vision, making it difficult to read, drive, and recognize faces. There are two forms of AMD: wet and dry.
agnosia
A failure to recognize objects in spite of the ability to see them. Agnosia is typically due to brain damage.
akinetopsia
A rare neuropsychological disorder in which the affected individual has no perception of motion.
amacrine cell
A retinal cell found in the inner synaptic layer that makes synaptic contacts with bipolar cells, ganglion cells, and other amacrine cells.
ambiguous figure
A visual stimulus that gives rise to two or more interpretations of its identity or structure.
amblyopia
A developmental disorder characterized by reduced spatial vision in an otherwise healthy eye, even with proper correction for refractive error. Also known as lazy eye.
amplitude
In reference to vestibular sensation, the size (increase or decrease) of a head movement (e.g., angular velocity, linear acceleration, tilt).
amplitude or intensity
In reference to hearing, the magnitude of displacement (increase or decrease) of a sound pressure wave. Amplitude is perceived as loudness.
ampulla
An expansion of each semicircular-canal duct that includes that canal’s cupula, crista, and hair cells, where transduction occurs.
amygdala-hippocampal complex
The conjoined regions of the amygdala and hippocampus, which are key structures in the limbic system. This complex is critically involved in the unique emotional and associative properties of olfactory cognition.
analgesia
Decreasing pain sensation during conscious experience.
anamorphosis or anamorphic projection
Use of the rules of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion.
angular acceleration
The rate of change of angular velocity. Mathematically, the integral of angular acceleration is angular velocity, and the integral of angular velocity is angular displacement. Angular acceleration, angular velocity, and angular displacement all mathematically represent angular motion.
angular motion
Rotational motion like the rotation of a spinning top or swinging saloon doors that rotate back and forth.
anisometropia
A condition in which the two eyes have different refractive errors (e.g., one eye is farsighted and the other not).
anomia
An inability to name objects in spite of the ability to see and recognize them (as shown by usage). Anomia is typically due to brain damage.
anosmia
The total inability to smell, most often resulting from sinus illness or head trauma.
anterior cingulate cortex (ACC)
A region of the brain associated with the perceived unpleasantness of a pain sensation.
aperture
An opening that allows only a partial view of an object.
aperture problem
The fact that when a moving object is viewed through an aperture (or a receptive field), the direction of motion of a local feature or part of the object may be ambiguous.
apparent motion
The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession.
aqueous humor
The watery fluid in the anterior chamber of the eye.
aromatherapy
The manipulation of odors to influence, mood, performance, and well-being as well as the physiological correlates of emotion such as heart rate, blood pressure, and sleep.
articulation
The act or manner of producing a speech sound using the vocal tract.
astigmatism
A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea.
attack
The part of a sound during which amplitude increases (onset).
attention
Any of the very large set of selective processes in the brain. To deal with the impossibility of handling all inputs at once, the nervous system has evolved mechanisms that are able to bias processing to a subset of things, places, ideas, or moments in time.
attention deficit hyperactivity disorder (ADHD)
a common childhood disorder that can continue into adulthood. Symptoms include difficulty focusing attention as well as problems controlling behavior.
attentional blink
The tendency not to perceive or respond to the second of two different target stimuli amid a rapid stream of distracting stimuli if the observer has responded to the first target stimulus within 200–500 milliseconds before the second stimulus is presented.
audibility threshold
The lowest sound pressure level that can be reliably detected at a given frequency.
auditory nerve
A collection of neurons that convey information from hair cells in the cochlea to (afferent) and from (efferent) the brain stem.
auditory stream segregation
The perceptual organization of a complex acoustic signal into separate auditory events for which each stream is heard as a separate event.
autonomic nervous system
The part of the nervous system that is responsible for regulating many involuntary actions and that innervates glands, heart, digestive system, etc.
azimuth
The angle of a sound source on the horizontal plane relative to a point in the center of the head between the ears. Azimuth is measured in degrees, with 0 degrees being straight ahead. The angle increases clockwise toward the right, with 180 degrees being directly behind.

B

balance
The neural processes of postural control by which weight is evenly distributed, enabling us to remain upright and stable.
balance system
The sensory systems, neural processes, and muscles that contribute to postural control. Specific components include the vestibular organs, kinesthesis, vestibulo-spinal pathways, skeletal bones, and postural control muscles. Because of its crucial contributions to balance, some even informally refer to the vestibular system as the “balance system” and the vestibular organs as the “balance organs.” But the balance system is much more than just the vestibular system, and the vestibular system contributes to more than just balance.
basal cell
One of the three types of cells in the olfactory epithelium. Basal cells are the precursor cells to olfactory sensory neurons.
basic color terms
Color words that are single words (like “blue,” not “sky blue”), are used with high frequency, and have meanings that are agreed upon by speakers of a language.
basic taste
Any of the four taste qualities that are generally agreed to describe human taste experience: sweet, salty, sour, bitter.
basilar membrane
A plate of fibers that forms the base of the cochlear partition and separates the middle and tympanic canals in the cochlea.
Bayesian approach
A way of formalizing the idea that our perception is a combination of the current stimulus and our knowledge about the conditions of the world—what is and is not likely to occur. The Bayesian approach is stated mathematically as Bayes’ theorem—P(A|O) = P(A) x P(O|A)/P(O)—which enables us to calculate the probability (P) that the world is in a particular state (A) given a particular observation (O).
belt area
A region of cortex, directly adjacent to the primary auditory cortex (A1), with inputs from A1, where neurons respond to more complex characteristics of sounds.
binaral rivalry
Competition between the two nostrils for odor perception. When a different scent is presented to each nostril simultaneously, we perceive each scent to be alternating back and forth with the other, and not a blend of the two scents.
binding problem
The challenge of tying different attributes of visual stimuli (e.g., color, orientation, motion), which are handled by different brain circuits, to the appropriate object so that we perceive a unified object (e.g., red, vertical, moving right).
binocular
Referring to two eyes.
binocular depth cue
A depth cue that relies on information from both eyes. Stereopsis is the primary example in humans, but convergence and the ability of two eyes to see more of an object than one eye sees are also binocular depth cues.
binocular disparity
The differences between the two retinal images of the same scene. Disparity is the basis for stereopsis, a vivid perception of the three-dimensionality of the world that is not available with monocular vision.
binocular rivalry
The competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes.
binocular summation
The combination (or “summation”) of signals from each eye in ways that make performance on many tasks better with both eyes than with either eye alone.
biological motion
The pattern of movement of living beings (humans and animals).
bipolar cell
A retinal cell that synapses with either rods or cones (not both) and with horizontal cells, and then passes the signals on to ganglion cells.
bitter
One of the four basic tastes; the taste quality, generally considered unpleasant, produced by substances like quinine or caffeine.
blood oxygen level–dependent (BOLD) signal
The ratio of oxygenated to deoxygenated hemoglobin that permits the localization of brain neurons that are most involved in a task.
body image
The impression of our bodies in space.

C

C fiber
A narrow-diameter, unmyelinated sensory nerve fiber that transmits pain and temperature signals.
C tactile (CT) afferent
A narrow-diameter, unmyelinated sensory nerve fiber that transmits signals from pleasant touch.
cataract
An opacity of the crystalline lens.
categorical perception
For speech as well as other complex sounds and images, the phenomenon by which the discrimination of items is no better than the ability to label items.
change blindness
The failure to notice a change between two scenes. If the gist, or meaning, of the scene is not altered, quite large changes can pass unnoticed.
characteristic frequency (CF)
The frequency to which a particular auditory nerve fiber is most sensitive.
chemosignal
Any of various chemicals emitted by humans that are detected by the olfactory system and that may have some effect on the mood, behavior, hormonal status, and/or sexual arousal of other humans.
chord
A combination of three or more musical notes with different pitches played simultaneously.
chorda tympani
The branch of cranial nerve VII (the facial nerve) that carries taste information from the anterior, mobile tongue (the part that can be stuck out). The chorda tympani exits the tongue with the lingual branch of the trigeminal nerve (cranial nerve V) and then passes through the middle ear on its way to the brain.
chromophore
The light-catching part of the visual pigments of the retina.
cilium (pl. cilia)
Any of the hairlike protrusions on the dendrites of olfactory sensory neurons. The receptor sites for odorant molecules are on the cilia, which are the first structures involved in olfactory signal transduction.
circadian
Referring to the biological cycle that recurs approximately every 24 hours, even in the absence of cues to time of day (via light, clocks, etc.).
circumvallate papillae
Circular structures that form an inverted V on the rear of the tongue (three to five on each side, with the largest in the center). Circumvallate papillae are moundlike structures, each surrounded by a trench (like a moat). These papillae are much larger than fungiform papillae.
closure
In reference to perception, closure is the name of a Gestalt principle that holds that a closed contour is preferred to an open contour.
coarticulation
The phenomenon in speech whereby attributes of successive speech units overlap in articulatory or acoustic patterns.
cochlea
A spiral structure of the inner ear containing the organ of Corti.
cochlear nucleus
The first brain stem nucleus at which afferent auditory nerve fibers synapse.
cochlear partition
The combined basilar membrane, tectorial membrane, and organ of Corti, which are together responsible for the transduction of sound waves into neural signals.
cognitive habituation
The psychological process by which, after long-term exposure to an odor, one no longer has the ability to detect that odor or has very diminished detection ability.
cold fiber
A sensory nerve fiber that fires when skin temperature decreases.
color-anomalous
A better term for the commonly used term color-blind. Most “color-blind” individuals can still make discriminations based on wavelength. Those discriminations are different from the norm—that is, anomalous.
color assimilation
A color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other.
color constancy
The tendency of a surface to appear the same color under a fairly wide range of illuminants.
color contrast
A color perception effect in which the color of one region induces the opponent color in a neighboring region.
color space
The three-dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors.
column
A vertical arrangement of neurons. Neurons within a single column tend to have similar receptive fields and similar orientation preferences.
common fate
Gestalt grouping rule stating that the tendency of sounds to group together will increase if they begin and/or end at the same time.
comparator
An area of the visual system that receives one copy of the command issued by the motor system when the eyes move (the other copy goes to the eye muscles). The comparator compares the image motion signal with the eye motion signal and can compensate for the image changes caused by the eye movement.
complex cell
A cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions.
computed tomography (CT)
An imaging technology that uses X-rays to create images of slices through volumes of material (e.g., the human body).
conductive hearing loss
Hearing loss caused by problems with the bones of the middle ear.
cone
A photoreceptor specialized for daylight vision, fine visual acuity, and color.
cone monochromat
An individual with only one cone type. Cone monochromats are truly color-blind.
cone of confusion
A region of positions in space where all sounds produce the same time and level (intensity) differences (ITDs and ILDs).
cone-opponent cell
A cell type—found in the retina, lateral geniculate nucleus, and visual cortex—that, in effect, subtracts one type of cone input from another.
congenital prosopagnosia
A form of “face blindness” apparently present from birth, as opposed to “acquired prosopagnosia,” which would typically be the result of an injury to the nervous system.
conjunction search
Search for a target defined by the presence of two or more attributes (e.g., a red, vertical target among red horizontal and blue vertical distractors).
continuity constraint
In stereopsis, the observation that, except at the edges of objects, neighboring points in the world lie at similar distances from the viewer. This is one of several constraints that have been proposed as helpful in solving the correspondence problem.
contralateral
Referring to the opposite side of the body (or brain).
contralesional field
The visual field on the side opposite a brain lesion. For example, points to the left of fixation are contralesional to damage in the right hemisphere of the brain.
contrast
The difference in luminance between an object and the background, or between lighter and darker parts of the same object.
contrast sensitivity function (CSF)
A function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus.
contrast threshold
The smallest amount of contrast required to detect a pattern.
convergence
The ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Convergence reduces the disparity of that feature to zero (or nearly zero).
cornea
The transparent “window” into the eyeball.
correspondence problem
1. In binocular vision, the problem of figuring out which bit of the image in the left eye should be matched with which bit in the right eye. The problem is particularly vexing when the images consist of thousands of similar features, like dots in random dot stereograms. 2. In motion detection, the problem faced by the motion detection system of knowing which feature in Frame 2 corresponds to a particular feature in Frame 1.
corresponding retinal points
Two monocular images of an object in the world are said to fall on corresponding points if those points are the same distance from the fovea in both eyes. The two foveas are also corresponding points.
cortical magnification
The amount of cortical area (usually specified in millimeters) devoted to a specific region (e.g., 1 degree) in the visual field.
cranial nerves
Twelve pairs of nerves (one for each side of the body) that originate in the brain stem and reach sense organs and muscles through openings in the skull.
cribriform plate
A bony structure riddled with tiny holes that separates the nose from the brain at the level of the eyebrows. The axons from the olfactory sensory neurons pass through the tiny holes of the cribriform plate to enter the brain.
crista
Any of the specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla.
criterion
In reference to signal detection theory, an internal threshold that is set by the observer. If the internal response is above criterion, the observer gives one response (e.g., “yes, I hear that”). Below criterion, the observer gives another response (e.g., “no, I hear nothing”).
critical bandwidth
The range of frequencies conveyed within a channel in the auditory system.
critical period
A phase in the life span during which abnormal early experience can alter normal neuronal development. Critical periods are proposed for the development of binocular vision and development of a first human language.
cross-adaptation
The reduction in detection of one odorant following exposure to a prior odorant. Cross-adaptation is presumed to occur because the components of the odors (or the odorants) in question share one or more olfactory receptors for their transduction, but the order in which odorants are presented also plays a role.
cross-modality matching
The ability to match the intensities of sensations that come from different sensory modalities. This ability allows insight into sensory differences. For example, a listener might adjust the brightness of a light until it matches the loudness of a tone.
crossed disparity
The sign of disparity created by objects in front of the plane of fixation (the horopter). The term crossed is used because images of objects located in front of the horopter appear to be displaced to the left in the right eye, and to the right in the left eye.
cue
A stimulus that might indicate where (or what) a subsequent stimulus will be. Cues can be valid (giving correct information), invalid (incorrect), or neutral (uninformative).
cultural relativism
In sensation and perception, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment.
cycle
For a grating, a pair consisting of one dark bar and one bright bar.
cycles per degree
The number of pairs of light and dark bars (cycles of a grating) per degree of visual angle.
Cyclopean
Referring to stimuli that are defined by binocular disparity alone. Named after the one-eyed Cyclops of Homer’s Odyssey.
cytochrome oxidase (CO)
An enzyme used to reveal the regular array of “CO blobs,” which are spaced about 0.5 millimeter apart in the primary visual cortex.

D

decay
The part of a sound during which amplitude decreases (offset).
decibel (dB)
A unit of measure for the physical intensity of sound. Decibels define the difference between two sounds as the ratio between two sound pressures. Each 10:1 sound pressure ratio equals 20 dB, and a 100:1 ratio equals 40 dB.
decoding
The process of determining the nature of a stimulus from the pattern of responses measured in the brain or, potentially, in an artificial system like a computer network. The stimulus could be a sensory stimulus or it could be an internal state (e.g., the contents of a dream).
deep neural network (DNN)
A type of “machine learning” in artificial intelligence in which a computer is programmed to learn something (here object recognition). First the network is “trained” using input for which the answer is known (“that is a cow”). Subsequently, the network can provide answers from input that it has never seen before.
dermis
The inner of two major layers of skin, consisting of nutritive and connective tissues, within which lie the mechanoreceptors.
deuteranope
An individual who suffers from color blindness that is due to the absence of M-cones.
dichoptic
Referring to the presentation of two different stimuli, one to each eye. Different from binocular presentation, which could involve both eyes looking at a single stimulus.
diffuse bipolar cell
A bipolar retinal cell whose processes are spread out to receive input from multiple cones.
diopter (D)
A unit of measurement of the optic power of a lens. It is equal to the reciprocal of the focal length, in meters. A 2-diopter lens will bring parallel rays of light into focus at ½ meter (50 cm).
diplopia
Double vision. If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic.
direction
The line one moves along or faces, with reference to the point or region one is moving toward or facing.
directional transfer function (DTF)
A measure that describes how the pinna, ear canal, head, and torso change the intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth and elevation).
distractor
In visual search, any stimulus other than the target.
divergence
The ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Divergence reduces the disparity of that feature to zero (or nearly zero).
dizziness
A commonly used lay term that nonspecifically indicates any form of perceived spatial disorientation, with or without instability.
doctrine of specific nerve energies
A doctrine, formulated by Johannes Müller, stating that the nature of a sensation depends on which sensory fibers are stimulated, rather than how they are stimulated.
dorsal column–medial lemniscal (DCML) pathway
The route from the spinal cord to the brain that carries signals from skin, muscles, tendons, and joints.
dorsal horn
A region at the rear of the spinal cord that receives inputs from receptors in the skin.
double dissociation
The phenomenon in which one of two functions, such as first- and second- order motion, can be damaged without harm to the other, and vice versa.
double-opponent cell
A cell type, found in the visual cortex, in which one region is excited by one cone type, combination of cones, or color and inhibited by the opponent cones or color (e.g., R+/G–). Another adjacent region would be inhibited by the first input and excited by the second (thus, in this example, R–/G+).
dualism
The idea that the mind has an existence separate from the material world of the body.
duplex
In reference to the retina, consisting of two parts: the rods and cones, which operate under different conditions.

E

ear canal
The canal that conducts sound vibrations from the pinna to the tympanic membrane and prevents damage to the tympanic membrane.
eccentricity
The distance between the retinal image and the fovea.
efference copy or corollary discharge signal
The phenomenon in which outgoing (efferent) signals from the motor cortex are copied as they exit the brain and are rerouted to other areas in the sensory cortices.
efferent commands
Information flowing outward from the central nervous system to the periphery. A common example is motor commands that regulate muscle contraction. The copy of such motor commands is often called an efferent copy. See also efferent fiber.
efferent fiber
A neuron that carries information from the central nervous system to the periphery. Compare afferent fiber.
egocenter
The center of a reference frame used to represent locations relative to the body.
electroencephalography (EEG)
A technique that, using many electrodes on the scalp, measures electrical activity from populations of many neurons in the brain.
emmetropia
The condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball.
end stopping
The process by which a cell in the cortex first increases its firing rate as the bar length increases to fill up its receptive field, and then decreases its firing rate as the bar is lengthened further.
endogenous
In reference to spatial attention, a form of top-down (knowledge-driven) control in which attention is voluntarily directed toward the site where the observer anticipates a stimulus will occur.
endogenous cue
In directing attention, a cue that is located in (endo) or near the current location of attention.
endogenous opiate
A chemical released by the body that blocks the release or uptake of neurotransmitters necessary to transmit pain sensations to the brain.
ensemble statistics
The average and distribution of properties like orientation or color over a set of objects or over a region in a scene.
entorhinal cortex
A phylogenetically old cortical region that provides the major sensory association input into the hippocampus. The entorhinal cortex also receives direct projections from olfactory regions.
entry-level category
For an object, the label that comes to mind most quickly when we identify it (e.g., “bird”). At the subordinate level, the object might be more specifically named (e.g., “eagle”); at the superordinate level, it might be more generally named (e.g., “animal”).
epidermis
The outer of two major layers of skin.
equal-loudness curve
A graph plotting sound pressure level (dB SPL) against the frequency for which a listener perceives constant loudness.
equilibrium
In reference to the vestibular system, our vestibular sense comprised of spatial orientation perception—encompassing our perception of linear motion, angular motion, and tilt—combined with reflexive vestibular responses like posture, vestibulo-autonomic reflexes, and vestibulo-ocular reflexes.
equiluminant
Referring to stimuli that vary in color but not in luminance.
esotropia
Strabismus in which one eye deviates inward.
Euclidean
Referring to the geometry of the world, so named in honor of Euclid, the ancient Greek geometer of the third century BCE. In Euclidean geometry, parallel lines remain parallel as they are extended in space, objects maintain the same size and shape as they move around in space, the internal angles of a triangle always add to 180 degrees, and so forth.
event-related potential (ERP)
A measure of electrical activity from a subpopulation of neurons in response to particular stimuli that requires averaging many EEG recordings.
exogenous
In reference to spatial attention, a form of bottom-up (stimulus-driven) attention reflexively (involuntarily) directed toward the site at which a stimulus has abruptly appeared.
exogenous cue
In directing attention, a cue that is located out (exo) at the desired final location of attention.
exotropia
Strabismus in which one eye deviates outward.
exploratory procedure
A stereotyped hand movement pattern used to touch objects in order to perceive their properties; each procedure is best for determining one (or more) object properties.
extinction
In visual attention, the inability to perceive a stimulus to one side of the point of fixation (e.g., to the right) in the presence of another stimulus, typically in a comparable position in the other visual field (e.g., on the left side).
extrastriate body area (EBA)
A region of extrastriate visual cortex in humans that is specifically and reliably activated by images of the body other than the face.
extrastriate cortex
The region of cortex bordering the primary visual cortex and containing multiple areas involved in visual processing.

F

familiar size
A depth cue based on knowledge of the typical sizes of objects such as humans or pennies.
feature integration theory
Anne Treisman’s theory of visual attention, which holds that a limited set of basic features can be processed in parallel preattentively, but that other properties, including the correct binding of features to objects, require attention.
feature search
Search for a target defined by a single attribute, such as a salient color or orientation.
Fechner’s law
A principle describing the relationship between stimulus and resulting sensation that says the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity.
feed-forward process
A process that carries out a computation (e.g., object recognition) one neural step after another, without need for feedback from a later stage to an earlier stage.
figure-ground assignment
The process of determining that some regions of an image belong to a foreground object (figure) and other regions are part of the background (ground).
filiform papillae
Small structures on the tongue that provide most of the bumpy appearance. Filiform papillae have no taste function.
filter
An acoustic, electrical, electronic, or optical device, instrument, computer program, or neuron that allows the passage of some range of parameters (e.g., orientations, frequencies) and blocks the passage of others.
first-order motion
The motion of an object that is defined by changes in luminance.
flavor
The combination of true taste (sweet, salty, sour, bitter) and retronasal olfaction.
focal distance
The distance between the lens (or mirror) and the viewed object, in meters.
focus of expansion
The point in the center of the horizon from which, when we’re in motion (e.g., driving on the highway), all points in the perspective image seem to emanate. The focus of expansion is one aspect of optic flow.
foliate papillae
Folds of tissue containing taste buds. Foliate papillae are located on the rear of the tongue lateral to the circumvallate papillae, where the tongue attaches to the mouth.
formant
A resonance of the vocal tract. Formants are specified by their center frequency and are denoted by integers that increase with relative frequency.
Fourier analysis
A mathematical procedure by which any signal can be separated into component sine waves at different frequencies. Combining these sine waves will reproduce the original signal.
fovea
A small pit located near the center of the macula and containing the highest concentration of cones, and no rods. It is the portion of the retina that produces the highest visual acuity and serves as the point of fixation.
frame of reference
The coordinate system used to define locations in space.
free fusion
The technique of converging (crossing) or diverging the eyes in order to view a stereogram without a stereoscope.
frequency
For sound, the number of times per second that a pattern of pressure change repeats. Frequency is perceived as pitch.
functional magnetic resonance imaging (fMRI)
A variant of magnetic resonance imaging that makes it possible to measure localized patterns of activity in the brain. Activated neurons provoke increased blood flow, which can be quantified by measuring changes in the response of oxygenated and deoxygenated blood to strong magnetic fields.
fundamental frequency
The lowest-frequency component of a complex periodic sound.
fundus
The back layer of the retina: what the eye doctor sees through an ophthalmoscope.
fungiform papillae
Mushroom-shaped structures (maximum diameter 1 millimeter) that are distributed most densely on the edges of the tongue, especially the tip. Taste buds (an average of six per papilla) are buried in the surface.
fusiform face area (FFA)
A region of extrastriate visual cortex in humans that is specifically and reliably activated by human faces.

G

G protein–coupled receptor (GPCR)
Any of the class of receptors that are present on the surface of olfactory sensory neurons. All GPCRs are characterized by a common structural feature of seven membrane-spanning helices.
ganglion cell
A retinal cell that receives visual information from photoreceptors via two intermediate neuron types (bipolar cells and amacrine cells) and transmits information to the brain and midbrain.
gate control theory
A description of the pain-transmitting system that incorporates modulating signals from the brain.
geon
In Biederman’s recognition-by-components model, any of the “geometric ions” out of which perceptual objects are built.
Gestalt
In German, literally “form.” In reference to perception, a school of thought stressing that the perceptual whole could be greater than the apparent sum of the parts.
Gestalt grouping rules
A set of rules describing which elements in an image will appear to group together. The original list was assembled by members of the Gestalt school of thought.
glabrous
In reference to skin, lacking hair.
global superiority effect
The finding in various experiments that the properties of the whole object take precedence over the properties of parts of the object.
glomerulus (sing. glomeruli)
Any of the spherical conglomerates containing the incoming axons of the olfactory sensory neurons. Each OSN converges onto two glomeruli (one medial, one lateral).
good continuation
1. In reference to vision, a Gestalt grouping rule stating that two elements will tend to group together if they seem to lie on the same contour. 2. In reference to hearing, a Gestalt grouping rule stating that sounds will tend to group together as continuous if they seem to share a common path, similar to a shared contour for vision.
graded potential
An electrical potential that can vary continuously in amplitude.
granular cells
Like mitral cells, granular cells are at the deepest level of the olfactory bulb. They comprise an extensive network of inhibitory neurons, integrate input from all the earlier projections, and are thought to be the basis of specific odorant identification.
graviception
The physiological structures and processes that sense the relative orientation of gravity with respect to the organism.
gravity
A force that attracts a body toward the center of the Earth.
guided search
Search in which attention can be restricted to a subset of possible items on the basis of information about the target item’s basic features (e.g., its color).
gustation
The sense of taste.

H

hair cell
Any cell that has stereocilia for transducing mechanical movement in the inner ear into neural activity sent to the brain; some hair cells also receive inputs from the brain.
haptic perception
Knowledge of the world that is derived from sensory receptors in skin, muscles, tendons, and joints, usually involving active exploration.
harmonic spectrum
The spectrum of a complex sound in which energy is at integer multiples of the fundamental frequency.
helicotrema
The opening that connects the tympanic and vestibular canals at the apex of the cochlea.
hertz (Hz)
A unit of measure for frequency. One hertz equals one cycle per second.
heterodimer
A chain of two molecules that are different from each other.
heuristic
A mental shortcut.
high-spontaneous fiber
An auditory nerve fiber that has a high rate (more than 30 spikes per second) of spontaneous firing; high-spontaneous fibers increase their firing rate in response to relatively low levels of sound.
holistic processing
Processing based on analysis of the entire object or scene and not on adding together a set of smaller parts or features.
homologous regions
Brain regions that appear to have the same function in different species.
homunculus
A maplike representation of regions of the body in the brain.
horizontal cell
A specialized retinal cell that contacts both photoreceptor and bipolar cells.
horopter
The location of objects whose images lie on corresponding points. The surface of zero disparity.
hue
The perceptual attribute of colors that enables them to be classed as similar to red, green, or blue, or something in between.
hyperalgesia
An increased or heightened response to a normally painful stimulus.
hypercolumn
A 1-millimeter block of striate cortex containing two sets of columns, each covering every possible orientation (0–180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye.
hyperopia
Farsightedness, a common condition in which light entering the eye is focused behind the retina and accommodation is required in order to see near objects clearly.
hyperpolarization
An change in membrane potential such that the inner membrane surface becomes more negative than the outer membrane surface.

I

illuminant
The light that illuminates a surface.
illusory conjunction
An erroneous combination of two features in a visual scene—for example, seeing a red X when the display contains red letters and Xs but no red Xs.
illusory contour
A contour that is perceived even though nothing changes from one side of it to the other in an image.
image
A picture or likeness.
imbalance
Lack of balance; unsteadiness; nearly falling over.
inattentional blindness
A failure to notice—or at least to report—a stimulus that would be easily reportable if it were attended.
incus
The middle of the three ossicles, connecting the malleus and the stapes.
inferior colliculus
A midbrain nucleus in the auditory pathway.
inferotemporal (IT) cortex
Part of the cerebral cortex in the lower portion of the temporal lobe, important in object recognition.
inhibition of return
The relative difficulty in getting attention (or the eyes) to move back to a recently attended (or fixated) location.
inner ear
A hollow cavity in the temporal bone of the skull, and the structures within this cavity: the cochlea and the semicircular canals of the vestibular system canals.
inner segment
The part of a photoreceptor that lies between the outer segment and the cell nucleus.
insular cortex
The primary cortical processing area for taste—the part of the cortex that first receives taste information. Also called the insula or the gustatory cortex.
interaural level difference (ILD)
The difference in level (intensity) between a sound arriving at one ear versus the other.
interaural time difference (ITD)
The difference in time between a sound arriving at one ear versus the other.
interocular transfer
The transfer of an effect (such as adaptation) from one eye to the other.
inverse-square law
A principle stating that as distance from a source increases, intensity decreases faster such that decrease in intensity is equal to the distance squared. This general law also applies to optics and other forms of energy.
ipsilateral
Referring to the same side of the body (or brain).
ipsilesional field
The visual field on the same side as a brain lesion.
iris
The colored part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding and contracting the pupil.
isointensity curve
A map plotting the firing rate of an auditory nerve fiber against varying frequencies at varying intensities.

J

just noticeable difference (JND) or difference threshold
The smallest detectable difference between two stimuli, or the minimum change in a stimulus that enables it to be correctly judged as different from a reference stimulus.
juxtaglomerular neurons
The first layer of cells surrounding the glomeruli. They are a mixture of excitatory and inhibitory cells and respond to a wide range of odorants. The selectivity of neurons to specific odorants increases in a gradient from the surface of the olfactory bulb to the deeper layers.

K

kinesthesia
Perception of the position and movement of our limbs in space.
kinesthetic
Referring to perception involving sensory mechanoreceptors in muscles, tendons, and joints.
koniocellular
Referring to cells in the koniocellular layer of the lateral geniculate nucleus of the thalamus. Konio from the Greek for “dust” referring to the appearance of the cells.
koniocellular cell
A neuron located between the magnocellular and parvocellular layers of the lateral geniculate nucleus. This layer is known as the koniocellular layer.

L

L-cone
A cone that is preferentially sensitive to long wavelengths; colloquially (but not entirely accurately) known as a “red cone.”
labeled lines
A theory of sensory coding in which each nerve fiber carries a particular stimulus quality. For example, a taste nerve fiber that responds best to sucrose but also responds with small responses to other stimuli carries only sweet.
lateral geniculate nucleus (LGN)
A structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex.
lateral inhibition
Antagonistic neural interaction between adjacent regions of the retina.
lateral superior olive (LSO)
A relay station in the brain stem where inputs from both ears contribute to detection of the interaural level difference.
learned taste aversion
The avoidance of a novel flavor after it has been paired with gastric illness. The smell, not the taste, of the substance is key for the learned aversion response in humans.
lens
The lens inside the eye that enables the changing of focus.
lesion
In reference to neurophysiology, 1. (n) A region of damaged brain. 2. (v) To destroy a section of the brain.
limbic system
The group of neural structures that includes the olfactory cortex, the amygdala, the hippocampus, the piriform cortex, and the entorhinal cortex. The limbic system is involved in many aspects of emotion and memory. Olfaction is unique among the senses for its direct connection to the limbic system.
linear acceleration
The rate of change of linear velocity. Mathematically, the integral of linear acceleration is linear velocity, and the integral of linear velocity is linear displacement, which is also referred to as “translation.” Linear acceleration, linear velocity, and linear displacement all mathematically represent linear motion.
linear motion
Translational motion like the predominant movement of a train car or bobblehead doll.
linear perspective
A depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image.
lordosis
The position that females of some species (e.g., pigs and rats) need to assume in order to be impregnated. It involves the downward curving of the spinal column and exposure of the genitals.
loudness
The psychological aspect of sound related to perceived intensity (amplitude).
low-spontaneous fiber
An auditory nerve fiber that has a low rate (less than 10 spikes per second) of spontaneous firing; low-spontaneous fibers require relatively intense sound before they will fire at higher rates.
luminance-defined object
An object that is delineated by differences in reflected light.

M

M ganglion cell
A ganglion cell resembling a little umbrella that receives excitatory input from diffuse bipolar cells and feeds the magnocellular layer of the lateral geniculate nucleus.
M-cone
A cone that is preferentially sensitive to middle wavelengths; colloquially (but not entirely accurately) known as a “green cone.”
macula
1. In reference to vision, The pigmented region with a diameter of about 5.5 mm near the center of the retina. It is sometimes referred to as the macula lutea (from the Latin) because of its yellow appearance. 2. In reference to the vestibular system, any of the specialized detectors of linear acceleration and gravity found in each otolith organ.
magnetic resonance imaging (MRI)
An imaging technology that uses the responses of atoms to strong magnetic fields to form images of structures like the brain. The method can be adapted to measure activity in the brain, as well (see functional magnetic resonance imaging).
magnetoencephalography (MEG)
A technique, similar to electroencephalography, that measures changes in magnetic activity across populations of many neurons in the brain.
magnitude estimation
A psychophysical method in which the participant assigns values according to perceived magnitudes of the stimuli.
magnocellular layer
Either of the bottom two neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically larger than those in the top four layers.
main olfactory bulb (MOB)
The rounded extension of the brain just above the nose that is the first region of the brain where smells are processed. In humans we refer simply to olfactory bulb(s); in nonhuman animals with accessory olfactory bulbs, we distinguish between main and accessory.
malleus
One of the three ossicles. The malleus receives vibration from the tympanic membrane and is attached to the incus.
masking
Using a second sound, frequently noise, to make the detection of another sound more difficult.
materialism
The idea that the only thing that exists is matter, and that all things, including the mind and consciousness, are the results of interaction between bits of matter.
mathematical integration
Computing an integral—one of the two main operations in calculus (the other, the inverse operation, is differentiation). Velocity is the integral of acceleration. Change of position is the integral of velocity.
mechanoreceptor
A sensory receptor that responds to mechanical stimulation (pressure, vibration, or movement).
medial geniculate nucleus
The part of the thalamus that relays auditory signals to the temporal cortex and receives input from the auditory cortex.
medial superior olive (MSO)
A relay station in the brain stem where inputs from both ears contribute to detection of the interaural time difference.
Meissner corpuscle
A specialized nerve ending associated with fast-adapting (FA I) fibers that have small receptive fields.
melanopsin
A photopigment, found in a class of photoreceptive retinal ganglion cells, that is sensitive to ambient light.
melody
A sequence of notes or chords perceived as a single coherent structure.
Merkel cell neurite complex
A specialized nerve ending associated with slowly adapting (SA I) fibers that have small receptive fields.
mesoptic
Referring to the middle range of light intensities.
metamers
Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences.
method of adjustment
A method of limits in which the participant controls the change in the stimulus.
method of constant stimuli
A psychophysical method in which many stimuli, ranging from rarely to almost always perceivable (or rarely to almost always perceivably different from a reference stimulus), are presented one at a time. Participants respond to each presentation: “yes/no,” “same/different,” and so on.
method of limits
A psychophysical method in which the particular dimension of a stimulus, or the difference between two stimuli, is varied incrementally until the participant responds differently.
metrical depth cue
A depth cue that provides quantitative information about distance in the third dimension.
microsaccade
An involuntary, small, jerklike eye movement.
microvilli
Slender projections of the cell membrane on the tips of some taste bud cells that extend into the taste pore.
mid-spontaneous fiber
An auditory nerve fiber that has a medium rate (10–30 spikes per second) of spontaneous firing. The characteristics of mid-spontaneous fibers are intermediate between low- and high-spontaneous fibers.
mid-level (or middle) vision
A loosely defined stage of visual processing that comes after basic features have been extracted from the image (low-level, or early, vision) and before object recognition and scene understanding (high-level vision).
middle canal
One of three fluid-filled passages in the cochlea. The middle canal is sandwiched between the tympanic and vestibular canals and contains the cochlear partition. Also called scala media.
middle ear
An air-filled chamber containing the middle bones, or ossicles. The middle ear conveys and amplifies vibration from the tympanic membrane to the oval window.
middle temporal area (MT)
An area of the brain thought to be important in the perception of motion. Also called V5 in humans.
midget bipolar cell
A small bipolar cell in the central retina that receives input from a single cone.
mitral cell
The deepest layer of neurons in the olfactory bulb. Each mitral cell responds to only to a few specific odorants.
monocular
Referring to one eye.
monocular depth cue
A depth cue that is available even when the world is viewed with one eye alone.
monosodium glutamate (MSG)
The sodium salt of glutamic acid (an amino acid).
motion aftereffect (MAE)
The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object.
motion parallax
An important depth cue that is based on head movement. The geometric information obtained from an eye in two different positions at two different times is similar to the information from two eyes in different positions in the head at the same time.
myopia
Nearsightedness, a common condition in which light entering the eye is focused in front of the retina and distant objects cannot be seen sharply.

N

nasal dominance
The asymmetry characterizing the intake of air by the two nostrils, which leads to differing sensitivity to odorants between the two nostrils. Nasal dominance alternates nostrils throughout the day, but there is no predictability about when the nostrils alternate.
Necker cube
An outline that is perceptually bi-stable. Unlike the situation with most stimuli, two interpretations continually battle for perceptual dominance.
negative afterimage
An afterimage whose polarity is the opposite of the original stimulus. Light stimuli produce dark negative afterimages. Colors are complementary; for example, red produces green, and yellow produces blue.
neglect
As a neurological symptom, in visual attention: (1) The inability to attend or respond to stimuli in the contralesional visual field (typically, the left field after right parietal damage). (2) Ignoring half of the body or half of an object.
neural plasticity
The ability of neural circuits to undergo changes in function or organization as a result of previous activity.
neuroimaging
A set of methods that generate images of the structure and/or function of the brain. In many cases, these methods allow us to examine the brain in living, behaving humans.
neurotransmitter
A chemical substance used in neuronal communication at synapses.
neutral point
The point at which an opponent color mechanism is generating no signal. If red-green and blue-yellow mechanisms are at their neutral points, a stimulus will appear achromatic. (The black-white process has no neutral point.)
nociceptor
A sensory receptor that responds to painful input, such as extreme heat or pressure.
nonaccidental feature
A feature of an object that is not dependent on the exact (or accidental) viewing position of the observer.
nonmetrical depth cue
A depth cue that provides information about the depth order (relative depth) but not depth magnitude (e.g., his nose is in front of his face).
nontaster (of PTC/PROP)
An individual born with two recessive alleles for the TAS2R38 gene and unable to taste the compounds phenylthiocarbamide (PTC) and propylthiouracil (PROP).

O

occlusion
A cue to relative depth order in which, for example, one object obstructs the view of part of another object.
octave
The interval between two sound frequencies having a ratio of 2:1.
ocular dominance
The property of the receptive fields of striate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other.
oculomotor (III) nerves
The third pair of cranial nerves, which innervate all the extrinsic muscles of the eye except the lateral rectus and the superior oblique muscles, and which innervate the elevator muscle of the upper eyelid, the ciliary muscle, and the sphincter muscle of the pupil.
odor
The translation of a chemical stimulus into the sensation of an odor percept. For example, “The cake has a chocolate odor.”
odor hedonics
The liking dimension of odor perception, typically measured by ratings of an odor’s perceived pleasantness, familiarity, and intensity.
odorant
A molecule that is defined by its physicochemical characteristics, and that can be translated by the nervous system into the perception of a smell. For example, “You smelled the odorant methyl salicylate, which has the odor of wintergreen mint.”
odorant receptor (OR)
The region on the cilia of olfactory sensory neurons where odorant molecules bind.
OFF bipolar cell
A bipolar cell that hyperpolarizes in response to an increase in light captured captured by the cones.
OFF-center cell
A cell that increases firing in response to a decrease in light intensity in its receptive-field center.
olfaction
The sense of smell.
olfactory bulb
A blueberry-sized extension of the brain just above the nose, where olfactory information is first processed. There are two olfactory bulbs, one in each brain hemisphere, corresponding to the right and left nostrils.
olfactory cleft
A narrow space at the back of the nose into which air flows and where the olfactory epithelium is located.
olfactory epithelium
A secretory mucous membrane in the human nose whose primary function is to detect odorants in inhaled air. Located on both sides of the upper portion of the nasal cavity and the olfactory clefts, the olfactory epithelium contains three types of cells: olfactory sensory neurons, basal cells, and supporting cells.
olfactory (I) nerves
The first pair of cranial nerves. The axons of the olfactory sensory neurons bundle together after passing through the cribriform plate to form the olfactory nerve, which conducts impulses from the olfactory epithelia in the nose to the olfactory bulb.
olfactory sensory neuron (OSN)
One of three cell types—the main one—in the olfactory epithelium. OSNs are small neurons located beneath a mucous layer in the epithelium. The cilia on the OSN dendrites contain the receptor sites for odorant molecules.
olfactory tract
The bundle of axons of the mitral and tufted cells within the olfactory bulb that sends odor information to the primary olfactory cortex.
olfactory white
The olfactory equivalent of white noise or the color white. When at least 30 odorants of equal intensity that span olfactory physiochemical and psychological (perceptual) space are mixed, they produce a resultant odor perception that is the same as that of every other mixture of 30 odorants meeting the same span and equivalent intensity criteria, even though the various mixtures do not share any common odorants.
ON bipolar cell
A bipolar cell that depolarizes in response to an increase in light captured by the cones.
ON-center cell
A cell that increases firing in response to an increase in light intensity in its receptive-field center.
opponent color theory
The theory that perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black-white.
optic array
The collection of light rays that interact with objects in the world that are in front of a viewer. Term coined by J. J. Gibson.
optic flow
The pattern of apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene.
optic (II) nerves
The second pair of cranial nerves, which arise from the retina and carry visual information to the thalamus and other parts of the brain.
optokinetic nystagmus (OKN)
A reflexive eye movement in which the eyes will involuntarily track a continually moving object.
orbitofrontal cortex (OFC)
The part of the frontal lobe of the cortex that lies behind the bone (orbit) containing the eyes. The OFC is responsible for the conscious experience of olfaction, as well as the integration of pleasure and displeasure from food; and it has been referred to as the secondary olfactory cortex and the secondary taste cortex. The OFC is also involved in many other functions, and it is critical for assigning affective value to stimuli—in other words, determining hedonic meaning.
organ of Corti
A structure on the basilar membrane of the cochlea that is composed of hair cells and dendrites of auditory nerve fibers.
orientation tuning
The tendency of neurons in striate cortex to respond optimally to certain orientations and less to others.
orthonasal olfaction
Sniffing in and perceiving odors through our nostrils, which occurs when we are smelling something that is in the air.
oscillatory
Referring to back-and-forth movement that has a constant rhythm.
ossicle
Any of three tiny bones of the middle ear: malleus, incus, and stapes.
otitis media
Inflammation of the middle ear, commonly in children as a result of infection.
otoconia
Tiny calcium carbonate stones in the ear that provide inertial mass for the otolith organs, enabling them to sense gravity and linear acceleration.
otolith organ
Either of two mechanical structures (utricle and saccule) in the vestibular system that sense both linear acceleration and gravity.
otosclerosis
Abnormal growth of the middle-ear bones that causes hearing loss.
outer ear
The external sound-gathering portion of the ear, consisting of the pinna and the ear canal.
outer segment
The part of a photoreceptor that contains photopigment molecules.
oval window
The flexible opening to the cochlea through which the stapes transmits vibration to the fluid inside.

P

P ganglion cell
A small ganglion cell that receives excitatory input from single midget bipolar cells in the central retina and feeds the parvocellular layer of the lateral geniculate nucleus.
Pacinian corpuscle
A specialized nerve ending associated with fast-adapting (FA II) fibers that have large receptive fields.
panpsychism
The idea that the mind exists as a property of all matter—that is, that all matter has consciousness.
Panum’s fusional area
The region of space, in front of and behind the horopter, within which binocular single vision is possible.
papilla (pl. papillae)
Any of multiple structures that give the tongue its bumpy appearance. From smallest to largest, the papilla types that contain taste buds are fungiform, foliate, and circumvallate; filiform papillae, which do not contain taste buds, are the smallest and most numerous.
parabelt area
A region of cortex, lateral and adjacent to the belt area, where neurons respond to more complex characteristics of sounds, as well as to input from other senses.
parahippocampal place area (PPA)
A region of extrastriate visual cortex in humans that is specifically and reliably activated more by images of places than by other stimuli.
parallel search
A search in which multiple stimuli are processed at the same time.
parallelism
A rule for figure-ground assignment stating that parallel contours are likely to belong to the same figure.
parietal lobe
In each cerebral hemisphere, a lobe that lies toward the top of the brain between the frontal and occipital lobes.
parvocellular
Referring to cells in the parvocellular layers of the lateral geniculate nucleus of the thalamus. Parvo from the Greek for “small” referring to the size of the cells.
parvocellular layer
Any of the top four neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically smaller than those in the bottom two layers.
perception
The act of giving meaning to a detected sensation.
period
In reference to hearing, the time required for a full wavelength of an acoustic sine wave to pass by a point in space.
phantom limb
Sensation perceived from a physically amputated limb of the body.
phase
1. A fraction of the cycle of the sine wave described in degrees (0° to 360°) or radians (0π to 2π). In reference to hearing, phase can be used to describe fractions of a period that relate to time. 2. The relative position of a grating.
phase locking
Firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a given frequency. (The neuron need not fire on every cycle, but each firing will occur at the same point in the cycle.)
pheromone
A chemical emitted by one member of a species that triggers a physiological or behavioral response in another member of the same species. Pheromones are signals for chemical communication and may or may not have any smell.
phonation
The process through which vocal folds are made to vibrate when air pushes out of the lungs.
photoactivation
Activation by light.
photon
A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties.
photopic
Referring to light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors (that is, drive them to their maximum responses).
photoreceptor
A light-sensitive receptor in the retina.
pictorial depth cue
A cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures.
pinna (pl. pinnae)
The outer, funnel-like part of the ear.
pitch
The psychological aspect of sound related mainly on the frequency of vibration.
place code
Tuning of different parts of the cochlea to different frequencies, in which information about the particular frequency of an incoming sound wave is coded by the place along the cochlear partition that has the greatest mechanical displacement.
placebo effect
Decreasing pain sensation when people think they’re taking an analgesic drug but actually are not.
polysensory
Referring to blending multiple sensory systems.
positivism
A philosophical position arguing that all we really have to go on is the evidence of the senses, so the world might be nothing more than an elaborate hallucination.
positron emission tomography (PET)
An imaging technology that enables us to define locations in the brain where neurons are especially active by measuring the metabolism of brain cells using safe radioactive isotopes.
preattentive stage
The processing of a stimulus that occurs before selective attention is deployed to that stimulus.
presbyopia
Literally “old sight.” The age-related loss of accommodation, which makes it difficult to focus on near objects.
primary auditory cortex (A1)
The first area within the temporal lobes of the brain responsible for processing acoustic information.
primary olfactory cortex or piriform cortex
The neural area where olfactory information is first processed. It comprises the amygdala, parahippocampal gyrus, and interconnected areas, and it interacts closely with the entorhinal cortex.
primary visual cortex (V1), area 17, or striate cortex
The area of the cerebral cortex of the brain that receives direct inputs from the lateral geniculate nucleus, as well as feedback from other brain areas.
primer pheromone
A pheromone that triggers a physiological (often hormonal) change among conspecifics. This effect usually involves prolonged pheromone exposure.
principle of univariance
The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discriminations based on wavelength.
probability summation
The increased detection probability based on the statistical advantage of having two (or more) detectors rather than one.
projective geometry
For purposes of studying perception of the three-dimensional world, the geometry that describes the transformations that occur when the three-dimensional world is projected onto a two-dimensional surface. For example, parallel lines do not converge in the real world, but they do in the two-dimensional projection of that world.
proprioception
Perception mediated by kinesthetic and internal receptors.
prosopagnosia
An inability to recognize faces.
protanope
An individual who suffers from color blindness that is due to the absence of L-cones.
proximity
A Gestalt grouping rule stating that the tendency of two features to group together will increase as the distance between them decreases.
psychoacoustics
The study of the psychological correlates of the physical dimensions of acoustics; a branch of psychophysics.
psychophysics
The science of defining quantitative relationships between physical and psychological (subjective, perceptual) events.
pupil
The dark, circular opening at the center of the iris in the eye, where light enters the eye.

Q

qualia (sing. quale)
In reference to philosophy, private conscious experiences of sensation or perception.

R

random dot stereogram (RDS)
A stereogram made of a large number (often in the thousands) of randomly placed dots. Random dot stereograms contain no monocular cues to depth. Stimuli visible stereoscopically in random dot stereograms are Cyclopean stimuli.
rapid serial visual presentation (RSVP)
An experimental procedure in which stimuli appear in a stream at one location (typically the point of fixation) at a rapid rate (typically about eight per second).
rate saturation
The point at which a nerve fiber is firing as rapidly as possible and further stimulation is incapable of increasing the firing rate.
rate-intensity function
A graph plotting the firing rate of an auditory nerve fiber in response to a sound of constant frequency at increasing intensities.
reaction time (RT)
A measure of the time from the onset of a stimulus to a response.
realism
A philosophical position arguing that there is a real world to sense.
receiver operating characteristic (ROC) curve
In reference to studies of signal detection, the graphical plot of the hit rate as a function of the false-alarm rate. If these are the same, points fall on the diagonal, indicating that the observer cannot tell the difference between the presence and absence of the signal. As the observer’s sensitivity increases, the curve bows upward toward the upper left corner. That point represents a perfect ability to distinguish signal from noise (100% hits, 0% false alarms).
receptive field
The region on the retina in which visual stimuli influence a neuron’s firing rate.
receptor adaptation
The biochemical phenomenon, that occurs after continual exposure to an odorant, whereby receptors are no longer available to respond to the odorant and detection ceases.
receptor potential
A change in voltage across the membrane of a sensory receptor cell (in the vestibular system, a hair cell) in response to stimulation.
recognition-by-components model
Biederman’s model of object recognition, which holds that objects are recognized by the identities and relationships of their component parts.
reflect
To redirect something that strikes a surface—especially light, sound, or heat—usually back toward its point of origin.
reflectance
The percentage of light hitting a surface that is reflected and not absorbed into the surface. Typically reflectance is given as a function of wavelength.
reflexive eye movement
A movement of the eye that is automatic and involuntary.
refract
1. To alter the course of a wave of energy that passes into something from another medium, as water does to light entering it from the air. 2. To measure the degree of refraction in a lens or eye.
refractive error
A very common disorder in which the image of the world is not clearly focused on the retina. The most common refractive errors are: myopia, hyperopia, astigmatism and presbyopia.
Reissner’s membrane
A thin sheath of tissue separating the vestibular and middle canals in the cochlea.
relatability
The degree to which two line segments appear to be part of the same contour.
related color
A color, such as brown or gray, that is seen only in relation to other colors. For example, a “gray” patch in complete darkness appears white.
relative height
As a depth cue, the observation that objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image. Objects farther away will be seen as higher in the image.
relative metrical depth cue
A depth cue that could specify, for example, that object A is twice as far away as object B without providing information about the absolute distance to either A or B.
relative size
A comparison of size between items without knowing the absolute size of either one.
releaser pheromone
A pheromone that triggers an immediate behavioral response among conspecifics.
response enhancement
An effect of attention on the response of a neuron in which the neuron responding to an attended stimulus gives a bigger response.
retina
A light-sensitive membrane in the back of the eye that contains photoreceptors and other cell types that transduce light into electrochemical signals and transmits them to the brain through the optic nerve.
retinitis pigmentosa (RP)
A progressive degeneration of the retina that affects night vision and peripheral vision. RP commonly runs in families and can be caused by defects in a number of different genes that have recently been identified.
retronasal olfaction
Perceiving odors through the mouth while breathing and chewing. This is what gives us the experience of flavor.
retronasal olfactory sensation
The sensation of an odor that is perceived when chewing and swallowing force an odorant in the mouth up behind the palate into the nose. Such odor sensations are perceived as originating from the mouth, even though the actual contact of odorant and receptor occurs at the olfactory mucosa.
reverse-hierarchy theory
A theory that fast, feed-forward processes can give your crude information about objects and scenes based on activity in high-level parts of the visual cortex. You become aware of details when activity flows back down the hierarchy of visual areas to lower-level areas where the detailed information is preserved.
rhodopsin
The visual pigment found in rods.
rod
A photoreceptor specialized for night vision.
rod monochromat
An individual with no cones of any type. In addition to being truly color-blind, rod monochromats are badly visually impaired in bright light.
round window
A soft area of tissue at the base of the tympanic canal that releases excess pressure remaining from extremely intense sounds.
Ruffini ending
A specialized nerve ending associated with slowly adapting (SA II) fibers that have large receptive fields.

S

S-cone
A cone that is preferentially sensitive to short wavelengths; colloquially (but not entirely accurately) known as a “blue cone.”
saccade
A type of eye movement, made both voluntarily and involuntarily, in which the eyes rapidly change fixation from one object or location to another.
saccadic suppression
The reduction of visual sensitivity that occurs when we make saccadic eye movements. Saccadic suppression eliminates the smear from retinal image motion during an eye movement.
saccule
One of the two otolith organs. A saclike structure that contains the saccular macula. Also called sacculus.
salience
The vividness of a stimulus relative to its neighbors.
salty
One of the four basic tastes; the taste quality produced by the cations of salts (e.g., the sodium in sodium chloride produces the salty taste). Some cations also produce other taste qualities (e.g., potassium tastes bitter as well as salty). The purest salty taste is produced by sodium chloride (NaCl), common table salt.
scatter
To disperse something—such as light—in an irregular fashion.
scene-based guidance
Information in our understanding of scenes that helps us find specific objects in scenes (e.g., objects do not float in air, faucets are near sinks).
scotopic
Referring to light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors.
second-order motion
The motion of an object that is defined by changes in contrast or texture, but not by luminance.
selective attention
The form of attention involved when processing is restricted to a subset of the possible stimuli.
semicircular canal
Any of three toroidal tubes in the vestibular system that sense angular motion.
sensation
The ability to detect a stimulus and, perhaps, to turn that detection into a private experience.
sense of angular motion
The perceptual modality that senses rotation.
sense of linear motion
The perceptual modality that senses translation.
sense of tilt
The perceptual modality that senses head inclination with respect to gravity.
sensitivity
1. The ability to perceive via the sense organs. 2. Extreme responsiveness to radiation, especially to light of a specific wavelength. 3. The ability to respond to transmitted signals. 4. In reference to signal detection theory, a value that defines the ease with which an observer can tell the difference between the presence and absence of a stimulus or the difference between Stimulus 1 and Stimulus 2.
sensorineural hearing loss
Hearing loss due to defects in the cochlea or auditory nerve.
sensory conflict
Sensory discrepancies that arise when sensory systems provide conflicting information. For example, vision may indicate that you are stationary while the vestibular system tells you that you are moving (or vice versa).
sensory exafference
Change in afference caused by external stimuli. For the vestibular system, vestibular afference evoked by passive head motion would yield sensory exafference. Compare sensory reafference.
sensory integration
The process of combining different sensory signals. Typically, combining several signals yields more accurate and/or more precise information than can be obtained from individual sensory signals. This is not the mathematical process of integration learned in calculus (e.g., the integral of acceleration is velocity).
sensory reafference
Change in afference caused by self-generated activity. For the vestibular system, vestibular afference evoked by an active self-generated head motion would yield sensory reafference. Compare sensory exafference.
serial self-terminating search
A search from item to item, ending when a target is found.
set size
The number of items in a visual display.
shape-pattern theory
The current dominant biochemical theory for how chemicals come to be perceived as specific odors. Shape-pattern theory contends that different scents—as a function of the fit between odorant shape and OR shape—activate different arrays of olfactory receptors in the olfactory epithelia. These various arrays produce specific firing patterns of neurons in the olfactory bulbs, which then determine the particular scent we perceive.
sharper tuning
An effect of attention on the response of a neuron in which the neuron responding to an attended stimulus responds more precisely. For example, a neuron that responds to lines with orientations from –20 degrees to +20 degrees might come to respond to ±10-degree lines.
signal detection theory
A psychophysical theory that quantifies the response of an observer to the presentation of a signal in the presence of noise. Measures obtained from a series of presentations are sensitivity (d′) and criterion of the observer.
similarity
A Gestalt grouping rule stating that the tendency of two features to group together will increase as the similarity between them increases. For example, in hearing, the tendency of two sounds to group together will increase as the acoustic similarity between them increases.
simple cell
A cortical neuron whose receptive field has clearly defined excitatory and inhibitory regions.
simultagnosia
An inability to perceive more than one object at a time. Simultagnosia is a consequence of bilateral damage to the parietal lobes (Balint syndrome).
sine wave
A simple, smoothly changing oscillation that repeats across space. Higher-frequency sine waves have more oscillations, and lower frequencies have fewer oscillations, over a given distance. 1. In reference to hearing, a waveform for which variation as a function of time is a sine function. Also called pure tone. 2. In reference to vision, a pattern for which variation in a property like brightness or color as a function of space is a sine function.
sine wave grating
A grating with a sinusoidal luminance profile as shown in Figure 3.4A.
single-opponent cell
Another way to refer to cone-opponent cells, in order to differentiate them from double-opponent cells.
sinusoidal
Referring to any oscillation, such as a sound wave or rotational motion, whose waveform is that of a sine curve. The period of a sinusoidal oscillation is the time that it takes for one full back-and-forth cycle of the motion to occur. The frequency of a sinusoidal oscillation is defined as the numeral 1 divided by the period.
smooth pursuit
A type of voluntary eye movement in which the eyes move smoothly to follow a moving object.
somatosensation
Collectively, sensory signals from the skin, muscles, tendons, joints, and internal receptors.
somatosensory area 1 (S1)
The primary receiving area for touch in the cortex.
somatosensory area 2 (S2)
The secondary receiving area for touch in the cortex.
somatotopic
Referring to spatial mapping in the somatosensory cortex in correspondence to spatial events on the skin.
somatotypical
Referring to normal somatosensation.
sour
One of the four basic tastes; the taste quality produced by the hydrogen ion in acids.
source segregation or auditory scene analysis
Processing an auditory scene consisting of multiple sound sources into separate sound images.
spatial disorientation
Any impairment of spatial orientation. More specifically, any impairment of our sense of linear motion, angular motion, or tilt.
spatial frequency
The number of cycles of a grating (e.g., changes in light and dark) per unit of visual angle (usually specified in cycles per degree).
spatial layout
The description of the structure of a scene (e.g., enclosed, open, rough, smooth) without reference to the identity of specific objects in the scene.
spatial orientation
A sense consisting of three interacting modalities: perception of linear motion, angular motion, and tilt.
spatial-frequency channel
A pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies.
specific anosmia
The inability to smell one specific compound amid otherwise normal smell perception.
specific hungers theory
The idea that deficiency of a given nutrient produces craving (a specific hunger) for that nutrient. Curt Richter first proposed this theory and demonstrated that cravings for salty or for sweet are associated with deficiencies in those substances. However, the idea proved wrong for other nutrients (e.g., vitamins).
spectral power distribution
The physical energy in a light as a function of wavelength.
spectral reflectance function
The percentage of a particular wavelength that is reflected from a surface.
spectral sensitivity
The sensitivity of a cell or a device to different wavelengths on the electromagnetic spectrum.
spectrogram
In sound analysis, a three-dimensional display that plots time on the horizontal axis, frequency on the vertical axis, and amplitude (intensity) on a color or gray scale.
spectrum
A representation of the relative energy (intensity) present at each frequency.
spinothalamic pathway
The route from the spinal cord to the brain that carries most of the information about skin temperature and pain.
staircase method
A psychophysical method for determining the concentration of a stimulus required for detection at the threshold level. The staircase method is an example of a method of limits. A stimulus (e.g., odorant) is presented in an ascending concentration sequence until detection is indicated, and then the concentration is shifted to a descending sequence until the response changes to “no detection.” This ascending and descending sequence is typically repeated several times, and the concentrations at which reversals occur are averaged to determine the threshold detection level of that odorant for a given individual. Also called reverse staircase method.
stapedius
The muscle attached to the stapes; tensing the stapedius decreases vibration.
stapes
One of the three ossicles. Connected to the incus on one end, the stapes presses against the oval window of the cochlea on the other end.
stereoacuity
A measure of the smallest binocular disparity that can generate a sensation of depth.
stereoblindness
An inability to make use of binocular disparity as a depth cue. This term is typically used to describe individuals with vision in both eyes. Someone who has lost one (or both) eyes is not typically described as “stereoblind.”
stereocilium
Any of the hairlike extensions on the tips of hair cells in the cochlea that, when flexed, initiate the release of neurotransmitters.
stereoisomers
Isomers (molecules that can exist in different structural forms) in which the spatial arrangements of the atoms are mirror-image rotations of one another, like a right and left hand.
stereopsis
The ability to use binocular disparity as a cue to depth.
stereoscope
A device for simultaneously presenting one image to one eye and another image to the other eye. Stereoscopes can be used to present dichoptic stimuli for stereopsis and binocular rivalry.
Stevens’s power law
A principle describing the relationship between stimulus and resulting sensation that says the magnitude of subjective sensation is proportional to the stimulus magnitude raised to an exponent.
stimulus onset asynchrony (SOA)
The time between the onset of one stimulus and the onset of another.
strabismus
A misalignment of the two eyes such that a single object in space is imaged on the fovea of one eye and on a nonfoveal area of the other (turned) eye.
structural description
A description of an object in terms of the nature of its constituent parts and the relationships between those parts.
structuralism
A school of thought believing that complex objects or perceptions could be understood by analysis of the components.
substantia gelatinosa
A region of interconnecting neurons in the dorsal horn of the spinal cord.
subtraction method
In function magnetic imaging, brain activity is measured in two conditions: one with and one without the involvement of the mental process of interest. Subtracting the two conditions shows regions of brain specifically activated by that process.
subtractive color mixture
A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remainder will contribute to the perception of color.
superior colliculus
A structure in the midbrain that is important in initiating and guiding eye movements.
superior olive
An early brain stem region in the auditory pathway where inputs from both ears converge.
supertaster
An individual who experiences the most intense taste sensations. Some stimuli are dramatically more intense for supertasters than for medium tasters or nontasters. Supertasters also tend to experience more intense oral burn and oral touch sensations. A variety of factors may contribute to this heightened perception, among the most important is the density of fungiform papillae.
supporting cell
One of the three types of cells in the olfactory epithelium. Supporting cells provide metabolic and physical support for the olfactory sensory neurons.
suppression
In vision, the inhibition of an unwanted image. Suppression occurs frequently in people with strabismus.
surroundedness
A rule for figure-ground assignment stating that if one region is entirely surrounded by another, it is likely that the surrounded region is the figure.
sweet
One of the four basic tastes; the taste quality produced by some sugars, such as glucose, fructose, and sucrose. These three sugars are particularly biologically useful to us, and our sweet receptors are tuned to them. Some other compounds (e.g., saccharin, cyclamate, aspartame) are also sweet.
symmetry
A rule for figure-ground assignment stating that symmetrical regions are more likely to be seen as figure.
synapse
The junction between neurons that permits information transfer.
synaptic terminal
The location where axons terminate at the synapse for transmission of information by the release of a chemical transmitter.
syncopation
Any deviation from a regular rhythm.

T

tactile
Referring to the result of mechanical interactions with the skin.
tactile agnosia
The inability to identify objects by touch.
target
The goal of a visual search.
tastant
Any stimulus that can be tasted.
taste
Sensations evoked by solutions in the mouth that contact receptors on the tongue and the roof of the mouth that then connect to axons in cranial nerves VII, IX, and X.
taste bud
A globular cluster of cells that has the function of creating neural signals conveyed to the brain by the taste nerves. Some of the cells in a taste bud have specialized sites on their apical projections that interact with taste stimuli. Some of the cells form synapses with taste nerve fibers.
taste receptor cell
A cell within the taste bud that contains sites on its apical projection that can interact with taste stimuli. These sites fall into two major categories: those interacting with charged particles (e.g., sodium and hydrogen ions), and those interacting with specific chemical structures.
taster (of PTC/PROP)
An individual born with one or both dominant alleles for the TAS2R38 gene and able to taste the compounds phenylthiocarbamide (PTC) and propylthiouracil (PROP). PTC/PROP tasters who also have a high density of fungiform papillae are PROP supertasters.
tau (τ)
Information in the optic flow that could signal time to collision (TTC) without the necessity of estimating either absolute distances or rates. The ratio of the retinal image size at any moment to the rate at which the image is expanding is tau, and TTC is proportional to tau.
tectorial membrane
A gelatinous structure, attached on one end, that extends into the middle canal of the ear, floating above inner hair cells and touching outer hair cells.
template
The internal representation of a stimulus that is used to recognize the stimulus in the world. Unlike its use in, for example, making a key, a mental template is not expected to actually look like the stimulus that it matches.
tempo
The perceived speed of the presentation of sounds.
temporal code
Tuning of different parts of the cochlea to different frequencies, in which information about the particular frequency of an incoming sound wave is coded by the timing of neural firing as it relates to the period of the sound.
temporal integration
The process by which a sound at a constant level is perceived as being louder when it is of greater duration. The term also applies to perceived brightness, which depends on the duration of light.
tensor tympani
The muscle attached to the malleus; tensing the tensor tympani decreases vibration.
tetrachromatic
Referring to the rare situation (in humans, at least) where the color of any light is defined by the relationships of four numbers—the outputs of those four receptor types.
texture gradient
A depth cue based on the geometric fact that items of the same size form smaller images when they are farther away. An array of items that change in size smoothly across the image will appear to form a surface tilted in depth.
texture segmentation
Carving an image into regions of common texture properties.
texture-defined object or contrast-defined object
An object that is defined by differences in contrast, or texture, but not by luminance.
thermoreceptor
A sensory receptor that signals information about changes in skin temperature.
threshold tuning curve
A graph plotting the thresholds of a neuron or fiber in response to sine waves with varying frequencies at the lowest intensity that will give rise to a response.
tilt
To attain a sloped position like that of the Leaning Tower of Pisa.
tilt aftereffect
The perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation.
timbre
The psychological sensation by which a listener can judge that two sounds with the same loudness and pitch are dissimilar. Timbre quality is conveyed by harmonics and other high frequencies.
time to collision (TTC)
The time required for a moving object (such as a cricket ball) to hit a stationary object (such as a batsman’s head). TTC = distance/rate.
tip link
A tiny filament that stretches from the tip of a stereocilium to the side of its neighbor.
tip-of-the-nose phenomenon
The inability to name an odor, even though it is very familiar. Contrary to the tip-of-the-tongue phenomenon, one has no lexical access to the name of the odor, such as first letter, rhyme, number of syllables, and so on, when in the tip-of-the-nose state. This is an example of how language and olfactory perception are deeply disconnected.
tone chroma
A sound quality shared by tones that have the same octave interval.
tone height
A sound quality corresponding to the level of pitch. Tone height is monotonically related to frequency.
tonotopic organization
An arrangement in which neurons that respond to different frequencies are organized anatomically in order of frequency.
topographical mapping
The orderly mapping of the world in the lateral geniculate nucleus and the visual cortex.
touch
The sensations caused by stimulation of the skin, muscles, tendons, and joints.
transduce
To convert from one form of energy to another (e.g., from light to neural electrical energy, or from mechanical movement to neural electrical energy). Neurons use electrical signals in their communications.
transmit
To convey something (e.g., light) from one place or thing to another.
transparent
Referring to the characteristic of a material that allows light to pass through it with no interruption such that objects on the other side can be clearly seen.
triangle test
A test in which a participant is given three odorants to smell, of which two are the same and one is different. The participant is required to state which is the odd odor out. Typically, the order in which the three odorants are given (e.g., same, same, different; different, same, same; same, different, same) is manipulated and the test is repeated several times for greater accuracy.
trichromacy or trichromatic theory of color vision
The theory that the color of any light is defined in our visual system by the relationships of three numbers—the outputs of three receptor types now known to be the three cones. Also called the Young-Helmholtz theory.
trigeminal (V) nerve
The fifth cranial nerve, which transmits information about the “feel” of an odorant (e.g., mint feels cool, cinnamon feels warm), as well as pain and irritation sensations (e.g., ammonia feels burning).
tritanope
An individual who suffers from color blindness that is due to the absence of S-cones.
trochlear (IV) nerves
The fourth pair of cranial nerves, which innervate the superior oblique muscles of the eyeballs.
tufted cells
The next layer of cells after the juxtaglomerular neurons. They respond to fewer odorants than the juxtaglomerular cells, but more than neurons at the deepest layer of cells.
two-point touch threshold
The minimum distance at which two stimuli (e.g., two simultaneous touches) are just perceptible as separate.
two-tone suppression
A decrease in the firing rate of one auditory nerve fiber due to one tone, when a second tone is presented at the same time.
tympanic canal
One of three fluid-filled passages in the cochlea. The tympanic canal extends from the round window at the base of the cochlea to the helicotrema at the apex. Also called scala tympani.
tympanic membrane
The eardrum; a thin sheet of skin at the end of the outer ear canal. The tympanic membrane vibrates in response to sound.

U

umami
The taste sensation produced by monosodium glutamate.
uncrossed disparity
The sign of disparity created by objects behind the plane of fixation (the horopter). The term uncrossed is used because images of objects located behind the horopter will appear to be displaced to the right in the right eye, and to the left in the left eye.
unique hue
Any of four colors that can be described with only a single color term: red, yellow, green, blue. Other colors (e.g., purple or orange) can also be described as compounds (reddish blue, reddish yellow).
uniqueness constraint
In stereopsis, the observation that a feature in the world is represented exactly once in each retinal image. This constraint simplifies the correspondence problem.
unrelated color
A color that can be experienced in isolation.
utricle
One of the two otolith organs. A saclike structure that contains the utricular macula. Also called utriculus.

V

vanishing point
The apparent point at which parallel lines receding in depth converge.
vection
An illusory sense of self-motion caused by moving visual cues when one is not, in fact, actually moving.
velocity
The speed and direction in which something moves. Mathematically, velocity is the integral of acceleration. In words, linear velocity is distance divided by time to traverse that distance; angular velocity is rotation angle divided by time to traverse that angle.
velocity storage
Prolongation of a rotational response by the brain beyond the duration of the rotational signal provided to the brain by the semicircular canals; typically yielding responses that are nearer the actual rotational motion than the signal provided by the canals.
vergence
A type of eye movement in which the two eyes move in opposite directions; for example, both eyes turn toward the nose (convergence) or away from the nose (divergence).
vertigo
A sensation of rotation or spinning. The term is often used more generally to mean any form of dizziness.
vestibular canal
One of three fluid-filled passages in the cochlea. The vestibular canal extends from the oval window at the base of the cochlea to the helicotrema at the apex. Also called scala vestibuli.
vestibular organs
The set of five sense organs—three semicircular canals and two otolith organs—located in each inner ear that sense head motion and head orientation with respect to gravity. See also vestibular system.
vestibular system
The vestibular organs as well as the vestibular neurons in cranial nerve VIII and the central neurons that contribute to the functional roles that the vestibular system participates in.
vestibulo-ocular reflex (VOR)
A short-latency reflex that helps stabilize vision by counterrotating the eyes when the vestibular system senses head movement.
vestibulocochlear (VIII) nerves
The eighth pair of cranial nerves, which connect the inner ear with the brain, transmitting impulses concerned with hearing and spatial orientation. The vestibulocochlear nerve is composed of the cochlear nerve branch and the vestibular nerve branch.
vibration theory
An alternative to shape-pattern theory for describing how olfaction works. Vibration theory proposes that every odorant has a different vibrational frequency and that molecules that produce the same vibrational frequencies will smell the same.
Vieth-Müller circle
The location of objects whose images fall on geometrically corresponding points in the two retinas. If life were simple, this circle would be the horopter, but life is not simple.
visual acuity
A measure of the finest detail that can be resolved by the eyes.
visual angle
The angle that an object subtends at the eye.
visual crowding
The deleterious effect of clutter on peripheral object recognition.
visual search
Search for a target in a display containing distracting elements.
visual-field defect
A portion of the visual field with no vision or with abnormal vision, typically resulting from damage to the visual nervous system.
vitalism
The idea that there is a force in life that is distinct from physical entities.
vitreous humor
The transparent fluid that fills the vitreous chamber in the posterior part of the eye.
vocal tract
The airway above the larynx used for the production of speech. The vocal tract includes the oral tract and nasal tract.
volley principle
The idea that multiple neurons can provide a temporal code for frequency if each neuron fires at a distinct point in the period of a sound wave but does not fire on every period.
vomeronasal organ (VNO)
Found in nonhuman animals, it is a chemical-sensing organ at the base of the nasal cavity with a curved tubular shape. The VNO evolved to detect chemicals that cannot be processed by ORs, such as large and/or aqueous molecules, the types of molecules that constitute pheromones. Also called Jacobson’s organ.

W

warmth fiber
A sensory nerve fiber that fires when skin temperature increases.
wave
An oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium.
wavelength
The distance required for one full cycle of oscillation for a sine wave.
Weber fraction
The constant of proportionality in Weber’s law.
Weber’s law
The principle describing the relationship between stimulus and resulting sensation that says the just noticeable difference (JND) is a constant fraction of the comparison stimulus.
white noise
Noise consisting of all audible frequencies in equal amounts. White noise in hearing is analogous to white light in vision, for which all wavelengths are present.