Electrophysiological measurement of rapid shifts of attention during visual search

Woodman, Geoffrey F.; Luck, Steven J. · 1999 · OpenAlex-citations

DOI: 10.1038/23698

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Summary

This study addresses a longstanding controversy in visual perception regarding whether visual search operates via serial attention shifts or parallel processing. While behavioral data often show that search time increases with the number of distractors, this pattern could reflect either the serial application of attention to individual objects or capacity limitations within a parallel system. Because behavioral measures lack moment-by-moment resolution of attentional distribution, the authors utilized the N2pc component of the event-related potential (ERP) waveform. The N2pc is a negative voltage deflection occurring 200–300 ms post-stimulus that lateralizes to the hemisphere contralateral to an attended object, providing a continuous electrophysiological marker of covert attention. To distinguish between serial and parallel models, the researchers conducted three experiments using modified visual search paradigms that biased the order of attentional shifts. In the first experiment, subjects searched for a target among four colored squares and distractors. The target appeared in a "high-probability" color (C75) on 75% of trials and a "low-probability" color (C25) on 25%, biasing subjects to attend to C75 first. When C75 and C25 were in opposite hemifields, the N2pc initially lateralized to the hemisphere contralateral to C75 (200–300 ms) and then shifted to the hemisphere contralateral to C25 (300–450 ms). This interhemispheric switching paralleled an 80 ms difference in behavioral response times, supporting a serial shift of attention. A second experiment confirmed this pattern on target-present trials, showing that attention remained on C75 if it was the target, but shifted to C25 if C75 was a distractor. To rule out the possibility that the probability manipulation artificially induced a serial strategy, a third experiment exploited natural biases toward items near fixation. Subjects searched for red items located either near or far from fixation. When near and far items were in opposite hemifields, the N2pc again shifted from the hemisphere contralateral to the near item to that of the far item. Crucially, control trials with single items showed no intrinsic timing differences between near and far processing, confirming that the shift reflected attentional movement rather than processing speed variations. Statistical analyses confirmed significant interactions between stimulus configuration and N2pc measurement intervals across all experiments. The findings provide direct electrophysiological evidence that visual search involves rapid, serial shifts of attention among objects. While a completely flexible parallel model could theoretically emulate these results, the data rule out the majority of parallel models that do not involve rapid redistribution of attentional resources. The study demonstrates that the N2pc component can track the moment-by-moment direction of attention, resolving the ambiguity inherent in behavioral measures and supporting the view that attention is applied serially to items in a visual search array.

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• visual search