Clocking perceptual processing speed: From chance to 75% correct in less than 30 milliseconds

THE NEURAL BASIS OF CHOICE BEHAVIOR HAS BEEN INTENSELY STUDIED WITH LABORATORY TASKS IN WHICH A SUBJECT SEES A STIMULUS AND MAKES A CORRESPONDING MOTOR RESPONSE, BUT THE ISSUE OF TIMING HAS BEEN HARD TO TACKLE: How much time is necessary to make the perceptual judgment versus executing the motor report? When and how does a subject commit to a particular choice, and what neural mechanisms determine that? A major limitation has been that reaction times (RTs) are affected by sensory and motor factors (e.g., task difficulty, urgency, expectation) that can be covertly traded. Recently, we designed a task that overcomes these problems and allows us to construct a new curve that unambiguously reveals how a subject's perceptual judgment unfolds in time. Specifically, the slope of this "tachometric" curve depends on the perceptual difficulty of the task and the perceptual capacity of the subject, but not on motor execution. This technique shows that monkeys can make accurate color discriminations in less than 30 ms. More importantly, it provides a novel metric for correlating the time courses of pyschophysical and neuronal responses, opening up a new avenue for investigating choice behaviors in a wide variety of experimental conditions.