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

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 this effect. In contrast to previous studies which used filtering, the present studies 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. We further investigated the time course of horizontal tuning using event-related potentials (ERP). We observed that (i) upright faces were best discriminated when the horizontal structure was preserved (ie V-randomization) compared to H-randomization; (ii) this phase-randomization effect was eliminated by inversion, resulting in (iii) a smaller FIE for H-randomized than V-randomized faces. This pattern was still present but was less consistent when SF-AS was equalized across SF orientations, suggesting that SF-AS in horizontal orientation contributes to the horizontal tuning of face perception. 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. Altogether, our findings indicate that horizontal tuning is a robust property of face perception that arises early in high-level visual cortex.