Modulation of brain activity by selective task sets observed using event-related potentials.

We investigated the ability of subjects to shift dynamically between selective task sets, using informative trial-by-trial cues. Two tasks were used which involved non-overlapping neural systems and different hemispheric specialization. In a verbal task, subjects decided whether a letter string was a real word or a non-word. In a spatial task, subjects decided whether an angle was acute or obtuse. A behavioural experiment showed that performance improved when cues predicted the upcoming task (80% validity), compared to when neutral cues did not afford selective task sets. Event-related potentials (ERPs) revealed brain activity related to forming selective task expectations, to switching tasks, and to the modulation of target processing as a function of such expectations and switches. Activity predicting the probable task started over parietal electrodes 160 ms after cue presentation, while activity related to task switching started at frontal electrodes around 280 ms. Both types of activities developed before target onset. Target processing was significantly influenced by the validity of the cue prediction, including strong modulation of language-related potentials. These results show that it is possible to switch dynamically between task sets involving distinct neural systems, even before the appearance of an imperative target stimulus, and that the nature of the task sets can influence neural activity related to task-set reconfiguration. Selective task sets can in turn modulate the processing of target stimuli. The effects also apply to the case of foveally presented words, whose processing has often been hypothesized to be automatic and outside the influence of selective attention.