Cone-Opponent Ganglion Cells in the Primate Fovea Tuned to Non-Cardinal Color Directions
A long-standing question in vision science is how the three cone photoreceptor types – long (L), medium (M) and short (S) wavelength sensitive – combine to generate our perception of color. Hue perception can be described along two opponent axes: red-green and blue-yellow. Psychophysical measurements of color appearance indicate that the cone inputs to the red-green and blue-yellow opponent axes are M vs. L+S and L vs. M+S, respectively. However, the “cardinal directions of color space” revealed by psychophysical measurements of color detection thresholds following adaptation are L vs. M and S vs. L+M. These cardinal directions match the most common cone-opponent retinal ganglion cells (RGCs) in the primate retina. Accordingly, the cone opponency necessary for color appearance is thought to be established in cortex. However, small populations with the appropriate M vs. L+S and L vs. M+S cone-opponency have been reported in large surveys of cone inputs to primate RGCs and their projecti