Abstract
In a recent study, we showed that EEG power at a constant frequency of stimulation (3.5 Hz) was much larger when different individual faces were presented compared to when the same face was presented repeatedly (Boremanse et al., J Vis, 2010 10(7): 564). This adaptation of the steady-state visual evoked potential response was localized on occipito-temporal sites, in particular over the right hemisphere. Here, we defined the suitable stimulation frequency ranges of this effect. Four observers were submitted to 81 s sequences of faces presented at different rates (1 Hz to 16 Hz or faces/second; Figure 1) while high-density EEG (128 channels) was recorded. Fast-Fourier Transform (FFT) of EEG and computation of signal-to-noise ratio (SNR) showed a larger response to different faces than identical faces between 3 Hz and 9 Hz on occipito-temporal sites. In three observers, this difference had the form of a Gaussian curve that peaked at 6 Hz (Figure 2), while in one observer the peak of maximal difference was at 4 Hz. This observation indicates that individual faces are best discriminated perceptually when the face stimulation oscillates at 6 Hz. Such a fundamental frequency rate should be used by future studies that aim to tag the sensitivity to individual faces in the human brain. Interestingly, at 6 Hz it takes 160 ms for the sinusoidal stimulation to complete a full cycle, and this time value coincides with the latency of the earliest face identity adaptation effect as found on the face-sensitive N170 component after flash stimulation (Jacques et al., J Vis, 2007). Hence, while the exact nature of the relation between the N170 and the phenomenon reported here in the frequency domain is still to be determined, our results support the view that the human brain requires about 160 ms to process individual facial information efficiently.