Stroop interference and the timing of selective response activation.

OBJECTIVE: To examine the exact timing of selective response activation in a manual color-word Stroop task. METHODS: Healthy individuals performed two versions of a manual color-word Stroop task, varying in the probability of incongruent color-words, while EEG was recorded. RESULTS: Stroop interference effect was manifested as longer reaction times for incongruent relative to congruent color-words, and was larger in a task version where incongruent color-words were presented less frequently. Whereas the congruence between color display and word meaning did not affect average stimulus- and response-locked lateralized readiness potential (LRP) onset latencies nor response-locked LRP amplitudes, P3 peak latencies were longer and stimulus-locked LRPs were smaller for incongruent than congruent trials. CONCLUSIONS: These data are consistent with the idea that behavioral Stroop interference reflects delays in processing stages preceding color-based selective response activation in a subset of trials. They also do not exclude additional delays after color-based selective response activation, at least up until some 200ms before the overt response. SIGNIFICANCE: This chronometric analysis allows for a parcellation of the Stroop interference process that may be applied in psychopathology