ModiWcation of planned actions

To elucidate the time course and processes underlying pre-movement modiWcation of planned actions, participants prepared to make an action at a time of their own choosing within a speciWed temporal window. In some conditions, participants prepared to make a single right index Wnger key press, whereas in others, they prepared to make a sequence of two key presses consisting of a right index Wnger key press followed by a right middle Wnger key press. On a proportion of trials, their internal preparation was interrupted by an auditory tone, in response to which they made either: the same action as they were intending, a diVerent action requiring an additional eVector (i.e. switch from preparing a single right index Wnger key press to executing a right index, middle Wnger sequence), or a diVerent action requiring one less eVector (i.e. switch from preparing a right index, middle Wnger sequence to executing a right index Wnger key press). For unmodiWed actions, switching from an internally generated to an externally triggered mode of response production produced a signiWcant reaction time cost (RT cost) for both single and sequential actions, with the cost for single actions being signiWcantly greater than that for sequential actions. Given that the RT cost did not increase as the complexity of the actions increased it is unlikely that the source of the cost is related to motor execution processes, and it is suggested that it may arise at a higher level cognitive stage of processing. In addition, reaction times to produce modiWed actions were signiWcantly greater than those to produce unmodiWed actions. Finally, it took signiWcantly longer to produce modiWed actions requiring one less eVector than to produce modiWed actions requiring one more eVector. We suggest that two time-consuming processes are involved in switching between internally generated and externally triggered actions that are modiWed or unmodiWed: a trigger switch cost when the same action has to be produced in response to an external trigger as opposed to an internal trigger, and a switch cost reXecting changes in the pattern of executed motor commands when modiWcation is necessary. It is suggested that such processes may be mediated by regions of the frontal lobes.