Human face preference in gamma-frequency EEG activity

Abstract

Previous studies demonstrated that induced EEG activity in the gamma band (iGBA) plays an important role in object recognition and is modulated by stimulus familiarity and its compatibility with pre-existent representations. In the present study we investigated the modulation of iGBA by the degree of familiarity and perceptual expertise that observers have with stimuli from different categories. Specifically, we compared iGBA in response to human faces versus stimuli which subjects are not expert with (ape faces, human hands, buildings and watches). iGBA elicited by human faces was higher and peaked earlier than that elicited by all other categories, which did not differ significantly from each other. These findings can be accounted for by two characteristics of perceptual expertise. One is the activation of a richer, stronger and, therefore, more easily accessible mental representation of human faces. The second is the more detailed perceptual processing necessary for within-category distinctions, which is the hallmark of perceptual expertise. In addition, the sensitivity of iGBA to human but not ape faces was contrasted with the face-sensitive N170-effect, which was similar for human and ape faces. In concert with previous studies, this dissociation suggests a multi-level neuronal model of face recognition, manifested by these two electrophysiological measures, discussed in this paper.

Figure 1

The 64 sites from where EEG was recorded and the demarcation of the 9 regions where iGBA was analyzed. The gray labeled sites were included in the statistical analysis.

Figure 2

iGBA activity elicited by the five stimulus categories (A) Time-frequency plots from mid-posterior electrode Pz for human faces, ape faces, human hands, buildings and watches. The scalp distributions of iGBA are presented for all categories, at the latency of their respective maximal peak; note that whereas the same scale is used for the time-frequency plots, different scales are used for each distribution to allow visualization. (B) The time course of the iGBA amplitude (averaged over 20-80Hz) for each stimulus category. (C) Mean iGBA amplitudes for each stimulus category, as measured from the maximal peaks.

Figure 3

Distribution of the peak latencies of maximal iGBA activity for each stimulus category (A) Box plots of the latencies. The line in the middle of each box is the sample median, while the top and bottom of the box are the 25th and 75th percentiles of the samples, respectively. Lines extend from the box to show the furthest observations. (B) Distribution of latency values from all subjects for each category. The mean latency of iGBA peaks for human faces was earlier than for the other categories. In addition, the peak latencies of individual subjects in response to human faces are more concentrated around the mean while for the other categories there is more dispersion.

Figure 4

Average ERPs for human faces, ape faces, hands, buildings and watches over posterior lateral scalp sites in each hemisphere (P10 from the Right Hemisphere and P9 from the Left Hemisphere P9). Both human and ape faces exhibit higher N170 amplitudes than the non-face categories.