NEURONAL CORRELATES OF VISIBILITY AND INVISIBILITY IN THE PRIMATE VISUAL-SYSTEM

A brief visual target stimulus may be rendered invisible if it is imme diately preceded or followed by another stimulus. This class of illusi ons, known as visual masking, may allow insights into the neural mecha nisms that underlie visual perception. We have therefore explored the temporal characteristics of masking illusions in humans, and compared them with corresponding neuronal responses in the primary visual corte x of awake and anesthetized monkeys. Stimulus parameters that in human s produce forward masking (in which the mask precedes the target) supp ress the transient on-response to the target in monkey visual cortex. Those that produce backward masking (in which the mask comes after the target) inhibit the transient after-discharge, the excitatory respons e that occurs just after the disappearance of the target. These result s suggest that, for targets that can be masked (those of short duratio n), the transient neuronal responses associated with onset and turning off of the target may be important in its visibility.