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Perceptual decision making in less than 30 milliseconds
Terrence R Stanford, Swetha Shankar, Dino P Massoglia, M Gabriela Costello & Emilio Salinas
In perceptual discrimination tasks, a subjects response time is determined by both sensory and motor processes. Measuringthe time consumed by the perceptual evaluation step alone is therefore complicated by factors such as motor preparation, task difficulty and speed-accuracy tradeoffs. Here we present a task design that minimizes these confounding factors and allows usto track a subjects perceptual performance with unprecedented temporal resolution. We find that monkeys can make accurate color discriminations in less than 30 ms. Furthermore, our simple task design provides a tool for elucidating how neuronal activity relates to sensory as opposed to motor processing, as demonstrated with neural data from cortical oculomotor neurons. In these cells, perceptual information acts by accelerating and decelerating the ongoing motor plans associated with correct and incorrect choices, as predicted by a race-to-threshold model, and the time course of these neural events parallels the time course of the subjects choice accuracy.
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Perceptual decision-making capacity has been intensely studied in psychophysical and neurophysiological experiments as a function of signal quality, strength and subjective value110. However, there is a fundamental question that has been more difficult to address: how long does it take to make a perceptual judgment? This issue is relevant to many real-life situations in which a choice must be made very quickly. For example, a driver sees a traffic light and must rapidly decide whether to step on the brake or the accelerator. The ensuing action on the pedal will take at least a few hundred milliseconds to be initiated11,12. How much of this reaction time is dedicated to the perceptual analysis of the visual scenethat is, to determining whether the light is red or green?
The basic measurement seems deceptively simple. In well-trained macaque monkeys, which are the subjects of our study, a fixational eye movement (saccade) to a highly salient, unambiguous target takes ~150200 ms to execute13. This must be the time that the motor apparatus needs to produce an unambiguous response, so the difference between this number and the reaction time in a two-alternative forced-choice task...