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Laura Dugué, Philippe Marque, Rufin VanRullen; TMS reveals attentional feedback to area V1 during serial visual search. Journal of Vision 2011;11(11):1340. https://doi.org/10.1167/11.11.1340.
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Visual search tasks have been used to understand how, where and when attention influences visual processing. Current theories posit that when an observer searches for a target among an array of distractors, low-level processes first decompose the visual scene into elementary features, then a higher-level process involving a “saliency map” selects a candidate location to focus attentional resources. For a parallel (or “pop-out”) task, this location is always the target; but for a serial (or “difficult”) task, the system may cycle on a few distractors before finally focusing on the target. This implies that attentional effects upon early visual areas, involving feedback from higher areas, should be visible at longer latencies during serial rather than parallel visual search. A previous study (Juan & Walsh, 2003) had used Transcranial Magnetic Stimulation (TMS) to support this conclusion; however, only a few post-stimulus delays were compared, and no control TMS location was used. Here we applied TMS double-pulses (sub-threshold) to induce a transient inhibition of area V1 at every post-stimulus delay between 100ms and 500ms (50ms steps). The search array was presented either at the location affected by the TMS pulses (previously identified by applying several pulses at supra-threshold intensity to induce phosphene perception), or in the opposite hemifield which served as a retinotopically-defined control location. Two search tasks (n = 12) were used: a parallel (finding the symbol + among Ls) and a serial one (T among Ls). TMS specifically impaired the serial, but not the parallel search. We highlight an involvement of V1 in serial search 300 ms after the onset; conversely V1 did not contribute to parallel search at delays beyond 100 ms. This study supports the idea that serial search differs from parallel search by the presence of additional cycles of a select-and-focus iterative loop between V1 and higher-level areas.
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