is inhibited by yellow light, via red and green signaling bipolar cells, that makes contacts at the outer IPL to process blue-yellow color opponency.

Directional-Selective Ganglion Cells

A subset of ganglion cells responds to the movement of images, but only in preferred directions. These ganglion cells are called directionally selective (D-S) cells and are a favorite of retinal electrophysiologists because they serve as model output cells that reveal the complexity of synaptic computation in the IPL. Although D-S ganglion cells were first described by Barlow and colleagues [66] over 40 years ago, we still do not know the synaptic interactions responsible for their behavior. What is known is that image motion in the preferred direction excites D-S cells, which are then inhibited after a time lag. By contrast, image motion in the null direction elicits a long-lasting inhibition that shunts the excitatory input, resulting in no response. One study suggested that this D-S computation occurs at the level of the ganglion cell dendrites, which integrate excitatory and inhibitory inputs [67]. However, other studies challenged this notion and suggested that the input neurons to the D-S cells are themselves directionally selective [68–70]. One point of agreement between most studies is that starburst amacrine cells are critical for D-S responses in ganglion cells. The genetic ablation of starburst cells, which provide both cholinergic and GABAergic input to D-S cells, results in the elimination D-S responses [71]. Thus, starburst amacrine cells are most likely critical players in the formation of D-S ganglion cell response, but precise roles of presynaptic and postsynaptic computations still need to be worked out.

Intrinsically Photosensitive Ganglion Cells

Recent work revealed that 1–3% of ganglion cells contain the photopigment melanopsin and can directly sense light [72]. These ganglion cells respond to light even when rod and cone inputs are disrupted, consistent with the notion that they are intrinsically photosensitive. Intrinsic light responses are long lasting and activated by moderate- to-bright light intensities. This class of ganglion cell also receives input from highersensitivity rod and cone synaptic pathways, which elicit responses by these neurons at lower light intensities. These ganglion cells have extensive dendritic fields that completely tile the retina, allowing this system to respond to light impinging on any part of the retina. Intrinsically photosensitive ganglion cells project to areas of the brain that control pupillary reflexes and the light entrainment of circadian rhythms.

CONCLUSIONS

The IPL is a critical stage in visual processing. Parallel streams of information arrive in the IPL via distinct bipolar cell types. This information is then further shaped by synaptic interaction between inhibitory amacrine cells with the bipolar cell inputs and ganglion cell outputs of the IPL. The ability to detect the direction of movement and certain colors occurs because of processing in the IPL. Many questions about IPL processing still remain. The challenge of future research is to apply new techniques such as molecular genetic approaches of disrupting circuitry and powerful multielectrode recording techniques to gain additional insights into IPL function.

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