Fig. 1.
(A) On the computerized stop-signal task, subjects respond rapidly to left- or right-facing arrows on screen with corresponding motor responses, and they attempt to inhibit responses when an auditory stop-signal sounds. Over the course of the task, the time between stimulus onset and occurrence of the stop-signal is varied by means of a tracking algorithm. This permits calculation of the SSRT, which reflects an estimate of the time taken to internally suppress prepotent motor responses [for further details of calculation, see (18, 23)]. The average response time for Go trials is also recorded. (B) On the probabilistic learning task, volunteers make a two-alternative forced choice between two stimuli (one red, one green) on each trial. The “correct” stimulus (always the first stimulus touched) receives an 8:2 ratio of positive:negative feedback, and the opposite ratio is given for the “incorrect” stimulus. Feedback is provided in the form of “CORRECT” or “INCORRECT” appearing on screen after each choice. Ability to acquire the stimulus-reward association on the basis of this degraded feedback is assessed by the number of errors made before reaching criterion, defined as eight consecutive correct responses to the maximally rewarded stimulus. After 40 trials (stage 1), the contingencies reverse for the subsequent 40 trials (stage 2) (i.e., if “red” was previously correct, then “green” becomes correct). Ability to reverse the previously acquired stimulus-reward association is assessed by the number of perseverative errors to the previously maximally rewarded stimulus. Ability to acquire the new stimulus-reward association is again assessed by the number of errors made before reaching criterion. The detrimental effect of misleading negative feedback on learning is assessed by means of an overall “feedback sensitivity” score. This is defined as the overall likelihood that the volunteer inappropriately switched to choose the incorrect stimulus after misleadingly being informed that his or her correct response on the previous trial was not correct.