Evidence for a noise gain control mechanism in human vision

For small, brief targets incremental threshold is known to obey the de Vries-Rose law: threshold rises in direct proportion to the square-root of background intensity. We present data demonstrating a square-root law for brightness matching as well. The square-root law for brightness is obtained over the full range of scotopic vision, and the low intensity end of photopic vision. The classic theory of de Vries and Rose explains the square-root law on the basis of increased variability of the photon count as the background increases. Our brightness matching data instead indicates that the mean signal level is reduced by a factor which is inversely proportional to the standard deviation of the photon count. This result is consistent with the idea that in the retina there exists a gain control mechanism that is sensitive to the variance in the photon input, rather than to the mean illuminance. The importance of this idea to the modelling of retinal gain controls is discussed.