sign-inverting response was only recently explained with the discovery of a special class of hyperpolarizing (metabotropic) glutamate receptors localized to bipolar cells with ON responses. This arrangement leads to the establishment of ON and OFF channels at the level of the photoreceptor-to-bipolar cell synapse in the outer plexi-form layer. Separate ON and OFF channel responses are maintained throughout much of the visual pathway, including ganglion cells, as well as cells in the lateral geniculate and in the visual cortex. Both channels respond to increases and decreases in light levels but in opposite directions. From an engineering perspective, this doubling of the visual signal by creating a mirror image provides an internal comparison mechanism that could help in decreasing noise and maintaining fidelity of the visual signal as it is sent to the visual cortex.

Center Versus Surround Receptive Fields

These ON and OFF responses result from the direct synaptic activation of first-order to second-order to third-order neurons that form the straight-through pathway within a given module. In addition to these vertical responses, horizontal interactions between adjacent modules are mediated by retinal interneurons: horizontal cells at the level of the outer plexiform layer and amacrine cells in the inner plexiform layer. Horizontal cells are depolarized along with OFF bipolar cells in response to glutamate released from photoreceptors (Fig. 7). Horizontal cells are inhibitory interneurons so they in effect produce an inhibitory influence that spreads from any given active module to adjacent modules. Thus, the dark activation of an OFF bipolar cell via the straight-through pathway from a photoreceptor may be counterbalanced by inhibitory input from horizontal cells if surrounding modules are also being activated by darkness at the same time. In other words, if a central module and its surrounding modules are exposed to uniform darkness, then OFF bipolar cells will not be active; the same holds true for ON channels. Likewise, both ON and OFF channels are essentially silent when a central module and its surrounding modules are exposed to uniform levels of light.

The functional classification of ON and OFF responses denotes the response of a bipolar cell to stimulation via the straight-through pathway—namely, the central portion of the module. Because of inhibitory horizontal cells, a bipolar cell is responsive not only to input from the straight-through (central) pathway, but also to adjacent or surrounding pathways as well, and the response to central output is opposite that from the surround under similar lighting conditions. The arrangement is called center versus surround inhibition. Each bipolar cell receives central input from a small circular region of retina containing one or a small group of cones and surround input from a larger area encircling the central region. These regions together comprise the area of the retina from which a given bipolar cell will receive input, and they define the receptive field for that particular cell. Each bipolar cell has a distinctive receptive field. If a receptive field map of all bipolar cells were displayed on the retina, the entire surface area would be covered by overlapping circles, with those near the fovea being very small and those in the peripheral retina being much larger.

Because of center versus surround inhibition, an OFF bipolar cell will be maximally stimulated when the portion of the retina representing the center of the receptive Held of the cell is relatively dark and the surrounding area is light. Under these conditions, the bipolar cell will be receiving direct dark stimulation from the central pathway but no inhibitory input from surrounding OFF pathways. Similarly, OFF bipolar cells will be maximally inhibited when a small spot of light surrounded by a dark annulus is centered over its receptive field. The opposite is true for ON bipolar cells. They are maximally stimulated when their receptive fields are illuminated by a small spot of light surrounded by a dark annulus and maximally inhibited by a small dark spot surrounded by a bright annulus. Conditions for maximal stimulation or inhibition can be artificially produced under laboratory conditions, but they are rarely encountered in the real world. During normal visual experiences, individual receptive fields may be uniformly dark or light (in which case there is no bipolar response), or they may be nonuniformly illuminated such as in the case when an edge of light falls across the receptive field. As long as the receptive field is nonuniformly illuminated, the bipolar cell will respond by being either partially depolarized or hyperpolarized. Thus, bipolar cells act as local edge detectors.

The receptive field properties of bipolar cells represent a series of templates that selectively encode information about boundaries of light and dark in the visual field. As this coded information is transmitted through neurons in the primary visual pathway, other processing steps are performed; however, the basic center versus surround property is retained through many synaptic relays. Bipolar cells, ganglion cells, lateral geniculate cells, and several types of cells in the visual cortex all have circular ON or OFF center versus surround receptive fields.

Magnocellular and Parvocellular Pathways

There is a large variation in the receptive field sizes of ganglion cells. One type of ganglion cell has a relatively small receptive field size, roughly equivalent to the receptive field size of one bipolar cell or a small group of bipolar cells. These cells can encode finely detailed information about the shape and texture of objects in the visual field and constitute the parvocellular (P) pathway. Another type of ganglion cell has a large receptive field equivalent to the summed receptive field sizes of hundreds of bipolar cells. This highly convergent pathway, called the magnocellular (M) pathway, is not capable of distinguishing fine detail; it is better suited for the task of mapping the general location of objects within the visual field. A third type of ganglion cell, not as well understood, has more complex receptive field properties and responds specifically to the movement of objects.

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