P300-based Stroop study with low probability and target Stroop oddballs: the evidence still favors the response selection hypothesis.

This paper addresses the issue of the locus of action in cognitive processing during Stroop effects. It uses the P300 latency to assess stimulus processing effects, but, for the first time, under conditions in which Stroop stimuli are rare and target stimuli. The study was also concerned with demonstrating that apparent P300s during verbal responding are in fact uninterpretable due to contamination of EEG by speech-related artifact. Three studies were presented. In Study 1, there were 3 blocks, each containing 1 of 3 types of rare Stroop stimuli (p = .15), congruent, neutral, and incongruent. There were also 3 response modes: button press (BUTTON), speaking aloud (VERBAL), and speaking to self (SILENT). Three sessions were used, each for a different response style. The only task was to name the color on each trial. In the 2 non-verbal blocks, Reaction Time (RT) varied by stimulus type; congruent < neutral < incongruent. P300 latency was the same across blocks in these non-verbal conditions in which one saw the classic Pz > Cz > Fz distribution. The much larger, speech artifact-contaminated "P300s" in the VERBAL blocks did suggest a Stroop effect, especially at Fz and Cz, where "P300s" were larger than at Pz. In Study 2, there were 2 response modes, VERBAL and SILENT, and only two rare Stroop stimuli; neutral and incongruent, 1 per block. In each of these blocks, one word-color combination was a designated target requiring a unique response. The subject was to name the color followed by a yes or no to categorize the target or non-target. Again the RT for incongruents was greater than RT for neutrals, without a parallel effect in P300 latency. Again, the rostral ERPs appeared artifactual in the VERBAL condition. Study 3 was a replication of the second study, except that motivated subjects, versus Psychology pool recruits, were used. The latency-RT correlation still failed to obtain. Thus, using classic P300-eliciting antecedents-rare and target (Stroop) stimuli-this study supports the view that the locus of Stroop interference is in response processing.