The cellular basis of rapid human face categorization.

Humans readily identify sensory patterns as faces in the environment, yet the neuronal mechanisms underlying rapid face categorization remain unclear. We report the recording of spiking activity inside the face-selective midfusiform gyrus ('Fusiform Face Area', FFA) of 8 human participants to natural images of faces inserted every five stimuli in a fast (6Hz) train of non-face objects. Objective identification of periodic firing rates reveals 81% of visually responsive frequency-tagged neurons (N = 375), with 85% showing significant differential activity between faces and objects (i.e., 6Hz/5 = 1.2Hz). About half of these face-selective neurons are face-exclusive, firing only to faces (i.e., no 6Hz response). Face-selectivity is expressed both as spike increases and decreases of roughly similar proportions, but fewer face-exclusive neurons showing decreases. Cellular face-selective activity shows consistent time-courses across neurons, emerging shortly after 100 ms and lasting for about 210 ms, defining the minimal neuronal discharge time for full face categorization. These findings provide unique insights into the neural basis of rapid human face categorization, highlighting its diverse cellular manifestations.