Task switching: a high-density electrical mapping study.

Flexibly switching between tasks is one of the paradigmatic functions of so-called "executive control" processes. Neuroimaging studies have implicated both prefrontal and parietal cortical regions in the processing necessary to effectively switch task. Beyond their general involvement in this critical function, however, little is known about the dynamics of processing across frontal and parietal regions. For instance, it remains to be determined to what extent these areas play a role in preparing to switch task before arrival of the stimulus to be acted upon and to what extent they play a role in any switching processes that occur after the stimulus is presented. Here, we used the excellent temporal resolution afforded by high-density mapping of brain potentials to explore the time course of the processes underlying (1) the performance of and (2) the preparation for a switch of task. We detail the contributions of both frontal and parietal processes to these two aspects of the task-switching process. Our data revealed a complex pattern of effects. Most striking was a period of sustained activity over bilateral parietal regions preceding the switch trial. Over frontal regions, activity actually decreased during this same period. Strongest sustained frontal activity was in fact seen for trials on which no switch was required. Further, we find that the first differential activity associated with switching task was over posterior parietal areas (220 ms), whereas over frontal scalp, the first differential activity is found more than 200 ms later. These and other effects are interpreted in terms of a "competition" model in which preparing to switch task is understood as the beginning of a competition between the potentially relevant tasks that is resolved during the switch trial. Our findings are difficult to account for with models that posit a strong role for frontal cortical regions in "reconfiguring" the system during switches of task.