Ganglion Cell Receptive Fields

Studies of the electrical activity of ganglion cells have yielded some interesting results. In the dark, each ganglion cell discharges spontaneously at a slow rate. When the room lights are turned on, the firing rate of many (but not all) ganglion cells increases slightly. With some ganglion cells, however, if a small spot of light is directed at the center of their receptive fields, a large increase in firing rate results. Surprisingly, then, a small spot of light can be a more effective stimulus than larger areas of light!

When the spot of light is moved only a short distance away from the center of the receptive field, the ganglion cell responds in the opposite manner. The ganglion cell that was stimulated with light at the center of its receptive field is inhibited by light in the periphery of its field. The responses produced by light in the center and by light in the "surround" of the visual field are antagonistic. Those ganglion cells that are stimulated by light at the center of their visual fields are said to have on-center fields; those that are inhibited by light in the center and stimulated by light in the surround have offcenter fields (fig. 10.45).

The reason wide illumination of the retina has a weaker effect than pinpoint illumination is now clear; diffuse illumination gives the ganglion cell conflicting orders—on and off. Because of the antagonism between the center and surround of ganglion cell receptive fields, the activity of each ganglion cell is a result of the difference in light intensity between the center and surround of its visual field. This is a form of lateral inhibition that helps to accentuate the contours of images and improve visual acuity.

276 Chapter Ten

Ganglion Cell Receptive Fields

On-center Field

On-center Field

Spot of light

Light on center stimulates, light on surround inhibits, ganglion cell

Spot of light

Off-center Field

Light on center stimulates, light on surround inhibits, ganglion cell

Light on center inhibits, light on surround stimulates, ganglion cell

Bar of light

On Off

Light across both center and surround is less effective at stimulating ganglion cell

Bar of light on retina

On center Off surround

Bar of light on retina

On center Off surround

Simple cortical neurons in cerebral cortex

Ganglion cells in retina

Simple cortical neurons in cerebral cortex

Ganglion cells in retina

■ Figure 10.46 Stimulus requirements for simple cortical neurons. Cortical neurons called simple cells have rectangular receptive fields that are best stimulated by slits of light of particular orientations. This may be due to the fact that these simple cells receive input from ganglion cells that have circular receptive fields along a particular line.

■ Figure 10.45 Ganglion cell receptive fields. Each ganglion cell receives input from photoreceptors in the retina that are part of the ganglion cell's "receptive field." Because of the antagonism between the field's center and its surround, an image that falls across the entire field has less effect than one that only excites just the center or surround. Because of this, edges of an image are enhanced, improving the clarity of vision.

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