Stop task after-effects: the extent of slowing during the preparation and execution of movement

In the stop task, response time to the go signal is increased when the immediately preceding trial involves the presentation of a stop signal. A recent explanation suggests that these "after-effects" are due to mechanisms that occur prior to the completion of response selection processes, but it is possible that they instead may reflect a slowed motor response (i.e., deliberate slowing after response selection). The participants completed a novel stop task that allows a differentiation between the time taken to prepare a movement (which incorporates response selection processes) and the time taken to execute a movement (i.e., speed of motor response). If mechanisms underlying stop task after-effects occur prior to the completion of response selection processes, then slowing should only occur during movement preparation. Movement preparation and execution time during go trials were analysed according to the characteristics of the preceding trial. Slowing after a stop trial was found during movement preparation time (regardless of inhibition success on that stop trial), and it further increased during this period when the primary task stimulus was repeated. There was also evidence for general after-effects during movement execution time, but no effect of repetition. These findings support the current theoretical accounts that suggest that repetition-based stop task after-effects are attributable to a mechanism that occurs prior to the completion of response selection processes, and also indicate a possible switch to a more conservative response set (as in signal detection theory terms) that results in deliberate slowing of movement.