We suppose that E is the excitation of a receptor by light and that I is the inhibition of the receptor from the layer of horizontal cells. The photoreceptor response is R = E — I. A light stimulus L causes an excitation E in the receptor and E decays with time constant t. The response of the receptor R provides an input into a layer of inhibitory cells, which are laterally connected, and so the inhibition spreads laterally by diffusion and decays with time constant 1. The model equations are
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