Cost of cone coupling to trichromacy in primate fovea

Cone synaptic terminals couple electrically to their neighbors. This reduce s the amplitude of temporally uncorrelated voltage differences between neig hbors. For an achromatic stimulus coarser than the cone mosaic, the uncorre lated voltage difference between neighbors represents mostly noise; so nois e is reduced more than the signal. Here coupling improves signal-to-noise r atio and enhances contrast sensitivity. But for a chromatic stimulus the un correlated voltage difference between neighbors of different spectral type represents mostly signal; so signal would be reduced more than the noise. T his cost of cone coupling to encoding chromatic signals was evaluated using a compartmental model of the foveal cone array. When cones sensitive to mi ddle (M) and long (L) wavelengths alternated regularly, and the conductance between a cone and all of its immediate neighbors was 1000 pS (similar to 2 connexons/cone pair), coupling reduced the difference between the L and M action spectra by nearly fivefold, from about 38% to 8%. However, L and M cones distribute randomly in the mosaic, forming small patches of like type , and within a patch the responses to a chromatic stimulus are correlated. In such a mosaic, coupling still reduced the difference between the I, and M action spectra, but only by 2.4-fold, to about 18%. This result is indepe ndent of the L/M ratio. Thus "patchiness" of the L/M mosaic allows cone cou pling to improve achromatic contrast sensitivity while minimizing the cost to chromatic sensitivity. (C) 2000 Optical Society of America [S0740-3232(0 0)02103-7].