Looking for a face in the crowd: Fixation-related potentials in an eye-movement visual search task

Kaunitz, Lisandro; Kamienkowski, Juan E.; Varatharajah, Alexander; Sigman, Mariano; Quiroga, Rodrigo Quian; Ison, Matias J. · 2013 · OpenAlex-citations

DOI: 10.1016/j.neuroimage.2013.12.006

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Summary

This study addresses the challenge of measuring event-related potentials (ERPs) during naturalistic visual exploration, a task historically avoided due to eye-movement artifacts in EEG recordings. The authors aimed to determine whether classical cognitive ERP signatures, specifically those associated with target detection, could be reliably recorded during unconstrained visual search tasks involving natural scenes. To achieve this, they designed a paradigm allowing the concurrent recording of EEG and eye movements while subjects searched for hidden target faces within crowded stadium scenes. The experimental design involved twelve participants performing two tasks: a free-viewing visual search task and a fixed-gaze visual oddball task. In the search task, subjects viewed crowd images and were trained to maintain fixations longer than 500 ms to minimize contamination from saccadic artifacts. In the control oddball task, faces were flashed at fixation in a pseudo-random sequence. EEG data were recorded using a 64-channel system, aligned to fixation onset, and filtered. The researchers analyzed fixation-related potentials (fERPs) for both target and distractor faces, comparing them across tasks and employing single-trial classification using Support Vector Machines to discriminate between target and distractor fixations. The results demonstrated that robust sensory and cognitive components emerged during the visual search task. Early potentials (up to 250 ms) were similar for targets and distractors, including a P1 component modulated by the amplitude of the preceding saccade. However, fixations on target faces elicited a distinct, sustained P3 component starting around 250 ms with a centro-parietal topography, which was absent for distractors. Comparing the two tasks revealed qualitative similarities but notable differences: the P1 occurred earlier in the search task, and the P3 in the oddball task showed a biphasic structure with earlier fronto-central activity and later centro-parietal activity compared to the single centro-parietal P3 in the search task. Furthermore, single-trial classification successfully discriminated target from distractor fixations above chance levels in 11 of 12 subjects, with the most discriminative features originating from centro-parietal channels around 450 ms. The significance of these findings lies in demonstrating that EEG signatures related to cognitive processing, such as the P3 component, develop reliably during spatially unconstrained exploration of natural scenes. This validates the use of concurrent EEG and eye-tracking for studying human vision in ecological environments. The study provides a methodological foundation for understanding the neural mechanisms of target detection during natural search, bridging the gap between controlled laboratory paradigms and real-world visual behavior.

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• visual search