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Abstract

The perception of brightness in animals or humans is currently not well understood, as demonstrated by our inability to explain illusions that rely on mid and/or high level mechanisms. Although animal studies have shown the activation of neurons in the early visual pathways whose functional properties correlate with the perception of brightness, the components of a “brightness pathway” (in analogy to the motion pathway) have not yet been described.
Perception of brightness has been explored in psychophysical experiments in monkeys. Rhesus monkeys, trained to discriminate the brightness of two test fields, also succeeded in performing the tasks of brightness induction and White's effect (Huang et al. 2002). In all three tasks, the performance of monkey observers was similar to that of human observers. This indicates that monkeys perceive surface brightness in a way comparable to humans.
So far, only retinal contributions to the perception of brightness have been successfully investigated. Results related to the contribution of S cones to brightness perception have been reported. In physiological experiments in anaesthetized monkeys, responses of V1 neurons to S cone isolating stimuli showed that S cones account for about 10\% of the total amplitude of modulation (Chatterjee \& Callaway 2003). For human observers, flicker fusion experiments in 73 color normal human observers indicated that on average S-cone input accounts for about 5\% of the total brightness signal.