Elsevier

Vision Research

Volume 51, Issue 12, 21 June 2011, Pages 1297-1311
Vision Research

ERP evidence for the speed of face categorization in the human brain: Disentangling the contribution of low-level visual cues from face perception

https://doi.org/10.1016/j.visres.2011.04.003Get rights and content
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Abstract

How fast are visual stimuli categorized as faces by the human brain? Because of their high temporal resolution and the possibility to record simultaneously from the whole brain, electromagnetic scalp measurements should be the ideal method to clarify this issue. However, this question remains debated, with studies reporting face-sensitive responses varying from 50 ms to 200 ms following stimulus onset. Here we disentangle the contribution of the information associated with the phenomenological experience of a face (phase) from low-level visual cues (amplitude spectrum, color) in accounting for early face-sensitivity in the human brain. Pictures of faces and of a category of familiar objects (cars), as well as their phase-scrambled versions, were presented to fifteen human participants tested with high-density (128 channels) EEG. We replicated an early face-sensitivity – larger response to pictures of faces than cars – at the level of the occipital event-related potential (ERP) P1 (80–100 ms). However, a similar larger P1 to phase-scrambled faces than phase-scrambled cars was also found. In contrast, the occipito-temporal N170 was much larger in amplitude for pictures of intact faces than cars, especially in the right hemisphere, while the small N170 elicited by phase-scrambled stimuli did not differ for faces and cars. These findings show that sensitivity to faces on the visual evoked potentials P1 and N1 (N170) is functionally dissociated: the P1 face-sensitivity is driven by low-level visual cues while the N1 (or N170) face-sensitivity reflects the perception of a face. Altogether, these observations indicate that the earliest access to a high-level face representation, that is, a face percept, does not precede the N170 onset in the human brain. Furthermore, they allow resolving apparent discrepancies between the timing of rapid human saccades towards faces and the early activation of high-level facial representations as shown by electrophysiological studies in the primate brain. More generally, they put strong constraints on the interpretation of early (before 100 ms) face-sensitive effects in the human brain.

Highlights

► The larger P1 response for faces than cars is also found for phase-scrambled versions of the stimuli. ► The N170 is larger for faces than cars but of equally small amplitude to phase-scrambled stimuli. ► Sensitivity to faces on the P1 and N1 (N170) is functionally dissociated. ► The P1 face-sensitivity is driven by low-level visual cues. ► The N170 face-sensitivity is associated with the perception of a face.

Keywords

ERP
N170
P1
Face perception
Object processing
Low and high vision
Amplitude spectrum
Low-level visual cues

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