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Guillaume Rousselet, Gilman Hannah, Robin Ince, Philippe Schyns; The N170 is mostly sensitive to pixels in the contralateral eye area. Journal of Vision 2015;15(12):687. doi: https://doi.org/10.1167/15.12.687.
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In humans, the N170 event-related potential (ERP) is an integrated measure of cortical activity that varies in amplitude and latency across trials. Recently, we quantified the coding function of the N170 single-trial variability in possibly the simplest socially relevant task: face detection (Rousselet et al. Journal of Vision 2014). On each experimental trial, observers saw face and noise pictures sparsely sampled with Gaussian apertures (“bubbles”; Gosselin & Schyns, Vision Research 2001). Using reverse correlation and mutual information, we found that the presence of pixels around the eye contralateral to the recording electrode modulated single-trial ERPs at lateral-occipital electrodes, and most strongly so during the rising part of the N170. This result is consistent with previous studies using different face sets and tasks (e.g. Smith et al. Psychological Science 2004, Schyns et al. Current Biology 2007), and suggests that the first processing step indexed by the N170 is the coding of the contralateral eye area. Here, in a series of control experiments, we show that this result holds for faces of different sizes, faces expressing different facial expressions of emotions, and after contrast normalisation. However, the N170 contralateral eye sensitivity was absent for very small faces, and was delayed and strongly reduced by contrast reversal. Because contrast reversal preserves local edges, and eye saliency, but affects the distribution of contrasts across the face, the lower brain sensitivity to eye pixels in that condition suggests that it reflects some form of feature processing in the context of the face. The feature in question could be the iris/pupil area, which had the strongest effect on the N170 to large faces. We conclude that the N170 is a critical time-window in human face processing mechanisms, reflecting predominantly, in a face detection task, the encoding of a single feature: the contralateral eye.
Meeting abstract presented at VSS 2015
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