Spatiotemporal brain dynamics during preparatory set shifting: MEG evidence.

Humans can flexibly alter a plan of action to adjust their behavior adaptively in changing environments. Functional neuroimaging has shown distinct patterns of activation across a frontoparietal network responsible for switching and updating such plans of action or 'task sets.' However, little is known about the temporal order of activations within prefrontal or across with posterior regions subserving set-shifting operations. Here, whole-head magnetoencephalography (MEG) was used to explore the spatiotemporal brain dynamics in a modified version of the Wisconsin card-sorting test (WCST). Our task was designed to examine preparation of set-shifting rather than set-acquisition operations time locked to context-informative cues. Three cortical regions showed a larger number of MEG activity sources in response to shift and relative to nonshift cues: (a) inferior frontal gyrus (IFG; BA 45, 47/12), (b) anterior cingulate cortex (ACC; BA 24, 32), and (c) supramarginal gyrus (SMG; BA 40). Importantly, the timing of MEG activation differed across these regions. The earliest shift-related MEG activations were detected at the IFG (100-300 ms postcue onset), followed by two further peaks at the ACC (200-300 and 400-500 ms) and the SMG (300-400 and 500-600 ms). Several other prefrontal and posterior cortical areas were similarly activated by both shift and nonshift preparatory cues. The resulting temporal pattern of interactions within prefrontal and across with posterior association cortices is coherent with current models of task switching and provides novel information about the temporal course of brain activations responsible for the executive control of attention.