Color vision are mixed together or diluted with white light, the number of distinguishable chromaticities can be quite high. In very low light levels, vision is scotopic: light is detected by rod cells of the retina. Rods are maximally sensitive to wavelengths near 500 nm, and play little, if any, role in colour vision. In brighter light, such as daylight, vision is photopic: light is detected by cone cells which are responsible for colour vision. Cones are sensitive to a range of wavelengths, but are most sensitive to wavelengths near 555 nm. Between these regions, mesopic vision comes into play and both
Retina horizontal cell proposed. Horizontal cells span across photoreceptors and summate inputs before synapsing onto photoreceptor cells. Horizontal cells may also synapse onto bipolar cells, but this remains uncertain. There is a greater density of horizontal cells towards the central region of the retina. In the cat, it is observed that A-type horizontal cells have a density of 225 cells/mm near the center of the retina and a density of 120 cells/mm in more peripheral retina. Horizontal cells and other retinal interneuron cells are less likely to be near neighbours of the same subtype than would occur by chance, resulting in ‘exclusion zones’