Model for Contrast Enhancement in the Insect Visual System

The author proposed a hypothetical model of neural interaction in the lamina gan-glionaris of grasshoppers (Northrop, 1975) that may account for the anomalous resolution observed in the grasshopper's multimodal and DCMD units. The model makes use of an engineering technique called multiplicative signal processing

(MSP). The model assumes that an ommatidium has six identical retinula cells that send their axons to synapse on one monopolar cell in the lamina cartridge serving that ommatidium (Horridge, 1966). Assume that the light intensity on all the rhab-doms is equal, and large enough so that each retinula cell depolarization is in its logarithmic region. Thus, for the kth depolarization,

The constants D and B are chosen to give the best fit to the Vr vs. I curve.

Next, assume that all six of the retinula cell axons converge on one monopolar cell, inducing a hyperpolarizing postsynaptic potential, given by

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