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.
Light activates the photoreceptors, which modulate the activity of bipolar cells. These cells, in turn, connect with ganglion cells located at the front of the retina. The axons of the ganglion cells form the optic nerve, which carries information to the brain. Two other types of neurons—horizontal cells and amacrine cells—are primarily responsible for lateral interactions within the retina.
The bipolar cells and ganglion cells are organized in such a way that each cell responds to a small circular patch of the retina, which defines the cell's receptive field. The receptive fields of retinal ganglion cells are concentric, consisting of a roughly circular central area and a surrounding ring.
Retinal ganglion cells have two basic types of 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.
First let's look at the response of an on-center ganglion cell to a spot of light. When no light is falling on the receptive field, a spontaneous level of activity is recorded from the ganglion cell. Notice that when the light enters the surround region of this on-center ganglion cell, the level of activity recorded in the cell decreases. Conversely, a spot of light in the center of the receptive field increases the firing rate. A maximal response in an on-center ganglion cell is achieved when the entire center of the receptive field is illuminated. Likewise, if we illuminate only the surround using a ring of light, the ganglion cell is maximally inhibited. Note that if we illuminate both the center and surround region, the response is just above baseline (center effects are slightly stronger than surround).
Now let's look at the response of an off-center ganglion cell. Notice that when the light enters the surround region of this off-center ganglion cell, the level of activity recorded in the cell increases. Conversely, a spot of light in the center of the receptive field decreases the firing rate. If we illuminate the entire center of an off-center ganglion cell, the cell is maximally inhibited. A maximal response is achieved when the entire surround of the receptive field is illuminated. As with the on-center ganglion cell, if we illuminate both the center and surround region, the response changes very little from baseline.
As you can see, these data explain why uniform illumination of the visual field is less effective in activating a ganglion cell than is a well-placed small spot or a line or edge passing through the center of the cell's receptive field. This organization makes the ganglion cells sensitive to luminance contrast.
To understand the importance of luminance contrast, note how the response rate changes depending on the position of the receptive field. By combining information from adjacent receptive fields, the brain can thus construct information about edges and ultimately shapes.
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.