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Anthony M Harris, Joshua O Eayrs, Nilli Lavie; The effect of perceptual load on gaze and EEG signals in multi-target visual search with free eye-movements. Journal of Vision 2019;19(10):273. doi: https://doi.org/10.1167/19.10.273.
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All search tasks are not created equal. The difficulty of search tasks varies considerably with the perceptual load of the task (Lavie & Cox, 1997; Roper et al., 2013). The neural correlates of perceptual load have typically been examined with tasks that require participants to maintain central fixation. Recently, however, there has been growing interest in investigating the neural correlates of attention in more naturalistic scenarios, such as free viewing, where gaze metrics and neural responses are measured simultaneously while participants explore a display with their eyes (e.g., Kamienkowski et al., 2012). Here we examined the neural correlates of perceptual load with eye-tracking and electroencephalography (EEG) while participants performed a free-viewing multi-target visual search task. Participants searched for either a shape feature (low load) or conjunctions of shape and colour (high load) and responded by holding fixation until the target disappeared (after 500ms), after which the search continued. Participants also detected an infrequent tone presented during fixation or saccade. Tone detection was significantly impaired under high (vs. low) load during both fixations and saccades, confirming the manipulation of perceptual load. Eye-tracking revealed that under high load participants fixated nontargets more often and for a significantly longer duration, and also made significantly shorter saccades, than under low load. EEG responses co-registered to the onset of nontarget fixations and matched for potentially confounding fixation parameters across load conditions, revealed a significantly larger P300 component under high than low load. High load search was also associated with significantly greater fixation-locked ~10 Hz oscillatory alpha power reduction than low load. These results demonstrate that the increased attentional requirements associated with high perceptual load (as reflected in the P300; Kok, 2001) recruit alpha-mediated attentional processes in a gaze-dependent manner under naturalistic search conditions.
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