Event-Related Potentials with the Stroop colour-word task: Timing of semantic conflict

Event-Related Potentials (ERPs) elicited by congruent and incongruent colour-word stimuli of a Stroop paradigm, in a task in which participants were required to judge the congruence/incongruence of the two dimensions of the stimuli, were recorded in order to study the timing of the semantic conflict. The reaction time to colour-word incongruent stimuli was significantly longer than the reaction time to congruent stimuli (the Stroop effect). A temporal Principal Components Analysis was applied to the data to identify the ERP components. Three positive components were identified in the 300–600 ms interval in response to the congruent and incongruent stimuli: First P3, P3b and PSW. The factor scores corresponding to the First P3 and P3b components were significantly smaller for the incongruent stimuli than for the congruent stimuli. No differences between stimuli were observed in the factor scores corresponding to the PSW or in the ERP latencies. We conclude that the temporal locus of the semantic conflict, which intervenes in generating the Stroop effect, may occur within the time interval in which the First P3 and P3b components are identified, i.e. at approximately 300–450 ms post-stimulus. We suggest that the semantic conflict delays the start of the response selection process, which explains the longer reaction time to incongruent stimuli.