Scrambling horizontal face structure: behavioral and electrophysiogical evidence for a tuning of visual face processing to horizontal information

Recent psychophysical evidence indicates that a central feature of human face processing is its tuning to horizontally-oriented information. Specifically, filtering faces to remove all but the horizontal information largely preserves behavioral signatures of face-specific processing, including the face inversion effect (FIE). Conversely, preserving only vertical information abolishes these effects. The purpose of the present experiments was twofold. First, in contrast to previous studies which used filtering, we manipulated orientation content of face images by randomizing Fourier phase spectrum in a narrow horizontal orientation band (H-randomization) or vertical orientation band (V-randomization). Phase-randomization was performed on face images in which spatial frequency amplitude spectrum (SF-AS) was either left unaltered or equalized across all SF orientations. Second, we investigated the time course of tuning to horizontal information using event-related potentials (ERP). Picture-plane inversion was used to evaluate whether the effects of orientation of phase-randomization arise due to inherent stimulus properties or to face-specific perceptual biases. In two psychophysics experiments, we observed that (1) upright faces were best discriminated when the horizontal structure was preserved (i.e. V-randomization) compared to H-randomization. (2) This phase-randomization effect was eliminated by inversion, resulting in (3) a smaller FIE for H-randomized than V-randomized faces. Although this pattern was still present when SF-AS was equalized across SF orientations, it was less consistent, suggesting that SF-AS in horizontal orientation contributes to the horizontal tuning of face perception. Consistent ERP evidence of horizontal tuning for upright face processing was observed in the N170 time-window, a well-known face-sensitive electrophysiological component. The N170 was delayed for H-randomized compared to V-randomized faces. Additionally, and in line with behavioural data, face inversion increased N170 latency to a smaller extent for H-randomized compared to V-randomized faces. Altogether, our findings indicate that horizontal tuning is a robust property of face perception that arises early in high-level visual cortex.