The internal structure of stopping as revealed by a sensory detection task.

An important aspect of everyday behaviour is the ability to cancel a prepared movement. In Experiment 1, subjects prepared a response, and then either executed it in response to a subsequent Go signal, or cancelled the movement if a NoGo signal occurred. Subjects had to detect weak shocks, which were delivered after the signals on some trials. Results were compared to a prior instruction condition in which subjects knew at the start of the trial if they should move or not. We found that detection rates on move trials were lower than on non-move trials, consistent with sensory suppression. There was no difference between conditions in detection for move trials. However, detection rates for non-move trials were significantly lower in the NoGo than in the prior instruction condition, suggesting an element of sensory suppression associated with actions, which are prepared, but then inhibited before execution. In Experiment 2, the delay between the NoGo signal and shock was varied. Detection rates improved monotonically as the interval increased from 0 up to 200 ms. The recovery from sensory suppression offers a new way of measuring the processes triggered by a NoGo signal. Our results suggest that when a prepared movement is inhibited the dismantling of the sensory consequences of the motor command takes at least 200 ms.