What is the stimulus presentation rate at which the human brain can discriminate each exemplar of a familiar visual category? We presented faces at 14 frequency rates (1.0-16.66 Hz) to human observers while recording high-density electroencephalogram (EEG). Different face exemplars elicited a larger steady-state visual evoked (ssVEP) response than when the same face was repeated, but only for stimulation frequencies between 4 and 8.33 Hz, with a maximal difference at 5.88 Hz (170 ms cycle). The effect was confined to the exact stimulation frequency and localized over the right occipito-temporal cortex. At high frequency rates (>10 Hz), the response to different and identical exemplars did not differ, suggesting that the fine-grained analysis needed for individual face discrimination cannot be completed before the next face interrupts, or competes, with the processed face. At low rates (<3 Hz), repetition suppression could not be identified at the stimulation frequency, suggesting that the neural response to an individual face is temporally dispersed and distributed over different processes. These observations indicate that at a temporal rate of 170 ms (6 faces/s) the face perception network is able to fully discriminate between each individual face presented, providing information about the temporal bottleneck of individual face discrimination in humans. These results also have important practical implications for optimizing paradigms that rely on repetition suppression, and open an avenue for investigating complex visual processes at an optimal range of stimulation frequency rates.
Keywords: 6Hz; Face perception; Frequency tuning; Identity adaptation; SSVEP; Theta range N170.
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