An objective index of individual face discrimination in the right occipito-temporal cortex by means of fast periodic oddball stimulation

We introduce an approach based on fast periodic oddball stimulation that provides objective, high signal-to-noise ratio (SNR), and behavior-free measures of the human brain's discriminative response to complex visual patterns. High-density electroencephalogram (EEG) was recorded for human observers presented with 60 s sequences containing a base-face (A) sinusoidally contrast-modulated at a frequency of 5.88 Hz (F), with face size varying every cycle. Different oddball-faces (B, C, D…) were introduced at fixed intervals (every 4 stimuli=F/5=1.18 Hz: AAAABAAAACAAAAD…). Individual face discrimination was indexed by responses at this 1.18 Hz oddball frequency. Following only 4 min of recording, significant responses emerged at exactly 1.18 Hz and its harmonics (e.g., 2F/5=2.35 Hz, 3F/5=3.53 Hz…), with up to a 300% signal increase over the right occipito-temporal cortex. This response was present in all participants, for both color and greyscale faces, providing a robust implicit neural measure of individual face discrimination. Face inversion or contrast-reversal did not affect the basic 5.88 Hz periodic response over medial occipital channels. However, these manipulations substantially reduced the 1.18 Hz oddball discrimination response over the right occipito-temporal region, indicating that this response reflects high-level processes that are partly face-specific. These observations indicate that fast periodic oddball stimulation can be used to rapidly and objectively characterize the discrimination of visual patterns and may become invaluable in characterizing this process in typical adult, developmental, and neuropsychological patient populations.