perceived hue must be curved, so that the Abney effect is accounted for. The chromaticity diagrams that have been corrected for the Abney effect are therefore excellent illustrations of the non-linear nature of the visual system. Also, the Abney effect does not disallow any and all straight lines on chromaticity diagrams. One may mix two monochromatic lights and not see a shift in hue, thereby suggesting a straight-line plot for the different levels of mixture would be appropriate on a chromaticity diagram. Physiology The opponent process model of the visual system is composed of two chromatic neural channels and one achromatic neural channel. The chromatic channels consist of a red-green channel and a yellow-blue channel and transmit color information. The achromatic channel is responsible for luminance, or white-black discrimination. Hue and saturation are perceived due to varying amounts of activity in these neural channels consisting of axon pathways from retinal ganglion cells. These three channels are tied closely to reaction time in response to colors. The achromatic neural channel has a faster response time than the chromatic neural channels under most conditions. The functions of these channels are task-dependent. Some activities are dependent on one channel or the other, as well as both channels. When a colored stimulus is summed with a white stimulus, both the chromatic and achromatic channels are activated. The achromatic channel will have a slightly slowed response time, since it must adjust to the different luminance; however, despite this delayed response, the speed of the achromatic channel will still be faster than that of the chromatic channel. In these conditions of summed stimuli, the magnitude of the signal emitted by the achromatic channel will be stronger than of the chromatic channel. The coupling of a faster response with a higher amplitude from the achromatic channel means that reaction time will most likely depend on the luminance and saturation levels of the stimuli. The customary explanations for color vision explain the difference in hue perception as elemental sensations that are inherent to the physiology of the observer. However, no specific physiological constraints or theories have been able to explain the response to each unique hue. To this end, both the observer’s spectral sensitivity and relative number of cone types have proven to not play any significant role in perceiving different hues. Perhaps the environment plays a larger role in the perception of unique hues than the different