Neural mechanisms for forming a perceptual decision.

Cognitive and behavioral responses to environmental stimuli depend on an evaluation of sensory signals within the cerebral cortex. The mechanism by which this occurs in a specific visual task was investigated with a combination of physiological and psychophysical techniques. Rhesus monkeys discriminated among eight possible directions of motion while directional signals were manipulated in visual area MT. One directional signal was generated by a visual stimulus and a second signal was introduced by electrically stimulating neurons that encoded a specific direction of motion. The decisions made by the monkeys in response to the two signals allowed a distinction to be made between two possible mechanisms for interpreting directional signals in MT. The monkeys tended to cast decisions in favor of one or the other signal, indicating that the signals exerted independent effects on performance and that an interactive mechanism such as vector averaging of the two signals was not operative. Thus, the data suggest a mechanism in which monkeys chose the direction encoded by the largest signal in the representation of motion direction, a "winner-take-all" decision process.