| Summary: | This study aimed to clarify the neural substrates of behavioral switch and restart costs in intermittently instructed task-switching paradigms. Event-related potentials (ERPs) were recorded while participants were intermittently cued to switch or repeat their categorization rule (<em>Switch task</em>), or else they performed two perceptually identical control conditions (<em>NoGo</em> and <em>Oddball</em>). The three tasks involved different task-sets with distinct stimulus-response associations in each, but identical visual stimulation, consisting of frequent colored shapes (<em>p</em> = 0.9) and randomly interspersed infrequent black shapes (<em>p</em> = 0.1; ‘+’ and ‘x’ symbols). Behavioral restart costs were observed in the fi rst target responses following all black shapes in the Switch and NoGo tasks – but not in the Oddball task – and corresponded with enhanced fronto-centrally distributed early cue-locked P3 activity (peak latency 325–375 ms post-cue onset at the vertex). In turn, behavioral <em>switch costs</em> were associated with larger late cue-locked P3 amplitudes in the Switch task only (peak latency 400–450 ms post-cue onset at mid-parietal sites). Together with our information theoretical estimations, ERP results suggested that restart and switch costs indexed two neural mechanisms related to the preparatory resolution of uncertainty: (1) the intermittent re-activation of task-set information, and (2) the updating of stimulus-response mappings within an active task set, as indexed by early and late cue-locked P3 activations, respectively. In contrast, target-locked P3 activations refl ected a functionally distinct mechanism related to the implementation of task-set information. We conclude that task-switching costs consist of both switch-specifi c and switch-unspecifi c processes during the preparation and execution stages of task performance.
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