The neural correlates of parallel and serial search

Abstract

For decades, target–distractor similarity has been known to induce distinct visual search modes. A highly salient target can pop out, suggesting parallel processing of all items irrespective of set size. By contrast, high similarity among items requires item-by-item assessment, a characteristic of serial search. Despite this long-standing distinction, search modes remain poorly defined due to confounding of behavioural measures by differences in local contrasts and display density just as neural correlates are confounded by distinct displays used to prompt different search modes. Here, we biased search mode by manipulating target–distractor similarity in inducer trials, while embedded test trials afforded both modes, allowing us to isolate neural signatures of serial and parallel search under visually identical displays. Behavioral results from 24 participants (21 female, 3 male) confirmed successful induction of distinct search modes. EEG decoding reliably discriminated search modes and generalized across inducer and test trials. Attentional deployment toward the target differed across search modes, revealing topographical differences in target location representations. The representation of target location was associated with response times, indicating when subjects swapped search mode from parallel to serial if target was not detected quickly. Moreover, the representation of search target diverged between search modes: A temporally stable pattern emerged during serial search, suggesting the maintenance of conjunction item in working memory, whereas the representations were dynamic in parallel search, likely reflecting the relevant feature. These findings demonstrate that search history shapes search mode, giving rise to clearly distinct neural dynamics even under visually identical stimulation. Significance statement Serial and parallel search distinction is recognized half a century ago. However, search modes remain poorly defined. This is because visual displays used to prompt distinct search modes confound behavioural measures and neural correlates of search modes. Here, we biased search modes in subsequent blocks by manipulating target-distractor similarity in a set of inducer trials. Embedded among inducer trials, test trials afforded both search modes, allowing us to isolate neural signatures of serial and parallel search under visually identical displays. We used EEG decoding to distinguish neural correlates of search modes and their influence on the modulation of target location and the representation of the target itself.