Uncovering the neural magnitude and spatio-temporal dynamics of natural image categorization in a fast visual stream

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 12.5 Hz, i.e., every 80 ms. Images of faces were inserted every 3, 5, 7, 9, or 11 stimuli, defining stimulus-onset-asynchronies (SOAs) between 240-880 ms, i.e., presentation frequencies (Fs) between 4.17-1.14 Hz. 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. 240 ms between two faces, but remained stable from 400 ms SOA onward. This high-level, right lateralized face-selective response emerged at about 100 ms 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 420 ms. 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.