The role of S-cones in human vision

The human S-cone ERG and single neuron responses from cells mediating the signals of short wavelength sensitive cones (S-cones) were examined and compared with the responses of long (L) and middle (M) wavelength sensitive cones. The S-cone system contributes a relatively small signal to the total cone ERG; it can be selectively light adapted; its b-wave is slower than that of L- and M-cone b-wave; and it lacks a d-wave. Transient tritanopia, a striking feature of S-cone on-retinal ganglion cells, is relatively weak at the level of the ERG. The responses of geniculate neurons were studied using a slowly moving border of energy and wavelength contrast. The ability of cells to respond to wavelength contrast across a border in which energy contrast was reversed was tested in all major varieties of retino-geniculate neurons in the macaque monkey. Cells mediating the signals of S-cones are unique in responding to wavelength rather than energy contrast. The most effective stimulus for such cells is white/yellow wavelength contrast at minimum energy contrast. It is suggested that the S-cone system's major role is to detect wavelength (chromatic) contrast and in particular white and grey from yellow and brown at minimal energy (brightness) contrast.