Thus far we have considered only the responses of individual photoreceptors. However, spatial interactions in the retina also play an important role in regulation of the light response.
Photoreceptors and horizontal cells form layers of cells through which their potential can spread laterally. The output from the photoreceptors is directed toward the horizontal cells, but the response of the horizontal cells also influences the photorecep-tors, forming a feedback loop with spatial interactions. Here we examine two models of this structure. The simpler, due to Peskin (1976), was originally constructed as a model for the horseshoe crab eye, but has wider applicability.
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