Author:

Keywords:

FACIAL, VISUAL PERCEPTION, SINGLE UNITS

Abstract:

Functional magnetic resonance imaging (fMRI) has revealed multiple regions in the monkey superior temporal sulcus (STS) that respond more to faces than other objects. A large number of behavioral and neuroimaging studies performed on humans converge with electrophysiological recordings in monkeys to suggest that the representations built in these regions are sensitive to inversion. In light of recent evidence implying that the eye region dominates processing in the posterior face patches, here we investigated the possibility that the effect of inversion that has been reported at the single cell level was driven primarily by the orientation of the eye region. To test this hypothesis, we first localized the face-selective middle face patch in two monkeys with a block design fMRI localizer. Then we recorded single units in this patch for upright and inverted human faces that had the eyes in a congruent or incongruent orientation with the whole face. The monkeys were required to fixate on a small red target for 300 ms, continuing fixation for another 600 ms after stimulus onset which lasted 300 ms. In both monkeys, we found a substantial reduction of spiking activity for inverted faces, together with a null effect of congruency, indicating that, on average the units in this region prefer upright faces over inverted faces, regardless of the local orientation of the eyes. However, in one monkey, we also found an interaction between global and local orientation in the late response window indicating that that local inversion of the eyes had reduced the response in upright conditions but not inverted conditions. This observation is consistent with the notion that local inversion affects the processing of upright faces, but not inverted faces, a phenomenon often referred to as the Thatcher illusion.