Fig. 3. Photoreceptor inputs are differentially filtered by bipolar cell glutamate receptors. A Schematic showing b2 and b3 bipolar cells contacting a single cone. B Depolarization of the cone elicited responses in b2 and b3 bipolar cells that were simultaneously recorded (upper panel). Scaled versions of the simultaneously recorded responses (lower panel) illustrate the difference in response time course, which is attributable to differential filtering by the b2 and b3 bipolar cell synapses. (Panel B is reprinted from [8], with permission from Elsevier.)

mate receptor subtypes differentially filter photoreceptor input in distinct morphological bipolar cell types to transmit different features of the visual input, as illustrated in Fig. 3 [8–10]. This filtering causes some bipolar cells to respond transiently and other bipolar cells to respond in a sustained fashion, enabling separate bipolar cells to extract distinct temporal and spatial features of the visual input. Further separation of color and luminosity information also occurs by distinct bipolar cells [13]. These morphologically and functionally distinct bipolar cell types transmit different representations of the visual scene to the IPL for additional processing [14].

FUNCTIONAL STRATIFICATION OF THE IPL

Bipolar cells convey different representations of photoreceptor input to the IPL by slow, graded potentials. When the ON and OFF classes of bipolar cells are depolarized by increments or decrements of illumination, respectively, they release glutamate, which excites postsynaptic ganglion cells and amacrine cells. The excitatory output of each bipolar cell class is shaped by amacrine cell synaptic inputs to bipolar cell terminals, as described in detail, the section on Presynaptic Inhibition. Additional signal processing, attributable to amacrine and ganglion cell interactions, also occurs in the IPL, before visual information is sent to the higher visual centers.

ON and OFF Response Stratification

The axon terminals of ON and OFF bipolar cells stratify in two distinct sublaminae of the IPL, where they contact the dendrites of ON and OFF ganglion and amacrine cells. These two strata are where synaptic connections are made by the pathways that encode