The neural correlates of parallel and serial search

Abstract

For almost half a century, target-distractor similarity has been known to induce different visual search modes. When a target is highly salient, it can pop out, suggesting parallel processing of all items irrespective of set size. By contrast, high similarity among items requires item-by-item comparison with an attentional template, a characteristic of serial search. Despite this long-standing distinction, little is known about the neural correlates of search modes, as typical differences in visual displays confound interpretation. Here, we contrasted the neural correlates of serial and parallel search under visually identical displays. Across distinct blocks, we biased 24 participants (21 female) toward parallel or serial search by varying target-distractor similarity, thereby directing attentional focus toward a single feature or conjunction. Embedded among inducer trials, test trials were visually identical across all blocks and afforded both parallel and serial search. Behavioral analyses confirmed successful induction of distinct search modes during test trials. EEG decoding reliably discriminated search modes and these neural patterns generalized across inducer and test trials. Attentional deployment toward the target differed across search modes, revealing topographical differences in target location representations. The strength of target-location representations correlated with response times, indicating that during parallel test trials participants switched search strategies when the target was not detected early. Moreover, target representations diverged between search modes: a temporally stable pattern emerged during serial search, suggesting reliance on working memory, whereas parallel search was characterized by more dynamic representations, likely reflecting prioritization of the relevant feature. These findings demonstrate that search history shapes search mode, giving rise to clearly distinct neural dynamics even under visually identical stimulation.