Figure 7.

Spatial summation of 1F achromatic and 2F chromatic components. Spatial nonlinearities in cat Y cells result in a 2F response to counterphase modulation of gratings symmetrically positioned about the receptive field center, which has been attributed to a subunit structure. With counterphase modulation of chromatic gratings, a null could be found which indicates that the non-linear response is not due to a subunit structure. (A, B) Amplitude and phase of responses of two MC cells to counterphase modulation (0.2 cpd, 4.88 Hz) of achromatic (30% contrast) and red-green chromatic gratings (27% RMS contrast for M and L cones). For the achromatic gratings, there is a 1^st harmonic response minimum and phase reversal at 2 phase loci, with little 2^nd harmonic response. This is consistent with linear spatial summation. For the 2F chromatic response, there are minima close to the minima for achromatic modulation. 2F phase is not affected. These data are consistent with linear spatial summation within chromatic mechanisms prior to nonlinear summation.