The same restrictions on negative frequency and self-inhibition (Kjj = 0) apply as in the dyadic case. Ratliff et al. (l963) observed that the Kkj decreases as the physical distance between the kth and jth ommatidium increases. This decrease is reasonable both from a functional and anatomical point of view. The terminal arborizations of nerve fibers branch as they become more distant from their source; thus their density and their effectiveness in causing inhibition of E-cell firing decreases with the radial distance from their source ommatidium.

Figure 5.3-5 illustrates schematically the instantaneous frequencies of three optic nerve fibers corresponding to the E-cells in three separate ommatidia stimulated individually with ON/OFF illumination (N = 3). From 0 3 t 3 tl, I3 is ON and I, and !2 are OFF; r, = r2 = 0 and r3 = r3max = e3. At t,, I, goes ON. Note the decrement in r3max caused by r, > 0, also the overshoot in r, and the corresponding undershoot in r3. When I2 comes ON at t2, there are corresponding decrements in both r, and r3, again with undershoots caused by the overshoot in r2. Note that for t > t4, Il = I3 =

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