The Madrid card sorting test (MCST): a task switching paradigm to study executive attention with event-related potentials.

Event-related potentials (ERPs) provide valuable information about the fast brain dynamics subserving cognitive functions such as attention and working memory. Most ERP studies employ cognitive paradigms with a fixed task-set (i.e., press a button to coloured targets), but few have measured ERPs time-locked to shifts in set using a task-switching paradigm. The Madrid card sorting test (MCST) is a dual task protocol in which feedback cues signal unpredictable shifts in set (i.e., from 'sort cards by colour' to 'sort cards by shape'). This protocol allows for an integrated analysis of ERPs to both feedback cues and target card events, providing separate ERP features for the shifting, updating, and rehearsal of attention sets in working memory. Two of these ERP indices are the frontal and posterior aspects of the P300 response. Feedback cues that direct a shift in set also elicit a frontally distributed P3a potential (300-400 ms). Instead, target card events evoke increasingly larger posterior P3b (350-600 ms) activity as the new task set becomes gradually rehearsed. The observed modulations in the frontal and posterior aspects of the P300 response system are interpreted from current models of prefrontal cortex function in the executive control of attention.