Perceptual categorization occurs rapidly under natural viewing conditions. Yet, the neural spatio-temporal dynamics of category-selective processes to single-glanced, natural (i.e., unsegmented) images in a rapidly changing presentation stream remain unknown. We presented human observers with natural images of objects at a fast periodic rate of 12.5Hz, i.e., every 80ms. Images of faces were inserted every 3, 5, 7, 9, or 11 stimuli, defining stimulus-onset-asynchronies (SOAs) between 240-880ms, i.e., presentation frequencies (Fs) between 4.17-1.14Hz. Robust face-selective responses were objectively identified and quantified at F and its harmonics (2F, 3F, etc.) for every condition in the electroencephalogram (EEG). The summed-harmonic face-selective response was significantly reduced by 25% at the lowest face SOA, i.e. 240ms between two faces, but remained stable from 400ms SOA onward. This high-level, right lateralized face-selective response emerged at about 100ms post-stimulus onset and progressed spatially throughout four successive time-windows (i.e., P1-face, N1-face, P2-face, P3-face) from posterior to anterior occipito-temporal electrode sites. The total duration of a category-selective response to a briefly presented face stimulus in a rapid sequence of objects was estimated to be 420ms. Uncovering the neural spatio-temporal dynamics of category-selectivity in a rapid stream of natural images goes well beyond previous evidence obtained from spatially and temporally isolated stimuli, opening an avenue for understanding human vision and its relationship to categorization behavior.
Keywords: Electroencephalography; Face; Frequency tagging; Humans; Visual evoked potentials.
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