Perceptual criterion and motor threshold: a signal detection analysis of the relationship between perception and action.

Although a large number of studies have demonstrated that a motor response to a visual stimulus is, at least to some extent, independent of the perceptual response, little effort has been spent on the investigation of the explicit characteristics of this independency. In the present experiment, observers were presented with an S1-S2 stimulus-pair, with S1 within the threshold range and with S2 highly suprathreshold. S2 was displayed either at the same location as S1 (masked condition), or some degree to the left or right of S1 (non-masked). Both the observers' sensitivity to S1 and simple RTs elicited by the stimulus pair were jointly assessed on a trial-by-trial basis. Response times decreased with increasing S1 contrast for perceptual Hits both when S1 was masked by S2 and when it was not, but for Misses only when S1 was masked, though to a lesser extent than for Hits. When RTs are collapsed across perceptual Hits and Misses for any given S1-contrast, they were independent of whether S1 was masked or not. The data indicate that the motor system has a fixed, high-energy threshold, whereas the perceptual system has a d'-dependent criterion that can either be higher or lower than the motor threshold-depending on the particular conditions.