Perceptual and motor conflict in the Stroop effect: event-related brain potential and electromyography studies

Szűcs, Denes, Soltész, Fruzsina and S., White (2008) Perceptual and motor conflict in the Stroop effect: event-related brain potential and electromyography studies. [in special issue: 10th International Conference on Cognitive Neuroscience, Bodrum, Turkey, 1 Sep - 5 Sep, 2008] Frontiers in Human Neuroscience, 2. (doi:10.3389/conf.neuro.09.2009.01.211).

Abstract

In a series of event-related brain potential (ERP) and electromyography (EMG) studies [Sz?cs et al., 2007, Behav Brain Funct; Sz?cs & Soltész, 2007 Neuropsychologia; Sz?cs & Soltész, 2008, Brain Res; Sz?cs, Soltész & White, submitted] we investigated the role of perceptual and motor conflict in the Stroop effect. We used the numerical Stroop paradigm where participants had to judge which of the two simultaneously presented numbers was larger physically or numerically. The physical-size comparison task is analogous to the classical color-word Stroop paradigm in that the irrelevant numerical meaning of numbers affects physical magnitude judgments. The lateralized readiness potential (LRP) was used to guide the separation of perceptual and motor conflict. In the first three studies, there were two major findings. First, between 150-250 ms we found early ERP markers of perceptual conflict. Second, at around 400 ms, after the onset of the LRP, we found late conflict-related ERP amplitude effects. We hypothesized that these late effects may be related to resolving motor response competition, or to slow, context-related stimulus-conflict detection. In a fourth study we used EMG electrodes placed on response hands to directly investigate the presence of motor conflict at the level of effectors in the Stroop paradigm. Incorrect response activation detected by EMG was temporally contingent with late ERP effects identified by our previous studies. Hence, we conclude that late ERP effects in the Stroop paradigm are at least partially related to motor response competition. Altogether our studies establish reliable ERP markers of perceptual and motor conflict in the Stroop effect.

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