Linear color opponency in carp retinal ganglion cells

Double opponent (i.e. red/green cone opponent in center and surround of the receptive field) ganglion cells in photopic carp retina were studied. It was found that a subclass of these cells (about 30%) was linearly opponent. For these cells the response to stimulation of the receptive field center could be constructed by summation of the responses due to stimulation of the red and green cone components separately. These linear opponent cells had smaller receptive field sizes than the nonlinear double opponent ganglion cells. Both strong (i.e. showing a marked sign inversion when wavelength is varied) and weak color opponency (i.e. showing no such inversion) can be found in the same linear double opponent ganglion cells. It is shown that such different behaviour can be understood from the change in amplitude, when intensity is varied, of the red and green cone components of the center responses of the cells. For small spots and low intensities, the red cone component dominates, which results in weak opponency; for large spots or high intensities strong opponency is observed.