Cueing effects on semantic and perceptual categorization: ERPs reveal differential effects of validity as a function of processing stage.

Valid cueing has been shown to accelerate target identification and improve decision accuracy. However, the precise nature and extent to which biasing influences the successive stages of target processing remain unclear. The present event-related potential (ERP) study used a "hybrid" task that combined features of standard cued-attention and task-switching paradigms in order to explore the effects of expectation on both identification and categorization of centrally presented stimuli. Subjects made semantic judgments (living/nonliving) on word targets ("bunny"), and perceptual judgments (right/left) on arrow targets ("<"). Target expectancy was manipulated using cues that were valid (60 percent of trials), invalid (10 percent), or neutral (30 percent). Invalidly cued targets required task-set switching before categorization could commence, and resulted in RT costs relative to validly or neutrally cued targets. Additional benefits from valid-cueing were only observed for word targets. Invalid cueing of both arrow and word targets modulated early posterior visual potentials (P1/N1) and elicited a subsequent anterior P3a (270 ms). The temporal relationship of these effects suggests that the P3a indexed domain-general task-set switching processes recruited in response to the detection of unexpected perceptual information. Subsequent to the P3a and immediately preceding the behavioral response, validly cued targets elicited enhanced stimulus-specific waveforms (arrows: parietal positivity [P290], words: inferior temporal negativity [late ITN: 400-600 ms]). The degree of neural enhancement relative to the invalid and neutral conditions mirrored the magnitude of corresponding RT benefits, suggesting that these waveforms indexed categorization, decision processes or both. Together, these results suggest that valid cueing increases the neural efficiency of initial stimulus identification, facilitating transmission of information to subsequent categorization stages, where increased neural activity leads to behavioral benefits.