Different time courses of Stroop and Garner effects in perception--an event-related potentials study

Visual integration between target and irrelevant features leads to effects of irrelevant feature congruency (Stroop) or variation (Garner) on target classification performance. Presenting closed geometrical shapes as stimuli, we obtained Stroop and Garner effects of one part of their contour on another, in response times and error rates. The correlates of these effects in brain activity were observed in event-related potentials (ERP). Stroop effects occurred in ERP amplitude of the N1 and N2 components, starting about 170 ms after stimulus onset; Garner effects occurred in amplitude of the rising part of the P3 component, starting about 330 ms after stimulus onset. A subsequent point-wise analysis of Stroop and Garner effects in ERP showed that they belong to different, cascaded processing stages. The difference in time course between Stroop and Garner effects in ERP is in accordance with the view that both are produced by different mechanisms, the former sensitive to interference within presentations and the latter sensitive to interference between presentations. The brief interval of 330-370 ms after stimulus onset when these two mechanisms overlap may correspond to the central processing bottleneck, responsible for the combinations of Stroop and Garner effects generally found in response times.