Animation 45.1: Information Processing in the Retina

INTRODUCTION

Visual processing begins in the retina. The photoreceptors—rods and cones— are located in the back of the retina, and are responsible for phototransduction. The photoreceptors modulate the activity of the bipolar cells, which in turn connect with ganglion cells. The axons of the ganglion cells form the optic nerve, which carries information from the retina to the brain. The bipolar cells and ganglion cells are organized in such a way that each cell responds to light falling on a small circular patch of the retina, which defines the cell's receptive field. Both bipolar cells and ganglion cells have two basic types of retinal receptive fields: on-center/off-surround and off-center/on-surround. The center and its surround are always antagonistic and tend to cancel each other's activity. In this tutorial, we will examine the responses of these two classes of retinal ganglion cells to stimulation of different portions of their receptive fields.

Video titled: Animation 45.1: Information Processing in the Retina

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CONCLUSION

The ability of retinal ganglion cells to detect differences in the level of illumination between the center and surround of their receptive field explains why uniform illumination of the visual field is less effective in activating a ganglion cell than is a well placed spot or a line or edge passing through the center of the cell's receptive field. This information is relayed to the primary visual cortex, where cortical neurons combine the input from many ganglion cells into the building blocks of visual images. For example, cells in the cortex that receive input from the retina (relayed from the thalamus) do not respond simply to light or dark in the visual field, but more typically to bars of light with a particular orientation. The input from these cells, in turn, converges upon other neurons that respond to bars of light moving in a particular direction in a particular part of the visual field. These are the first steps in the identification of edges, and ultimately form perception—a critical process for making sense of our visual world.

Textbook Reference: Key Concept 45.2 Nervous System Functions Rely on Neural Circuits

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