Color robustly affects the intensity of facial distortions in two cases of prosopometamorphopsia

Prosopometamorphopsia (PMO) is a striking perceptual disorder in which faces appear distorted. Mello et al. (2023) reported that viewing faces through color filters modulated the intensity of face distortions in case V.S., but it was unclear if this effect was independent of luminance and saturation. Here, we investigated this question in case V.S. (59-year-old man) and a new case, Ezri (32-year-old woman), using a replicated single-case experimental design (SCED). Each participant looked at two faces in real life through combinations of colored and neutral-density filters that allowed us to test the impact of hue while controlling for chroma and luminance. We tested perception through a neutral filter (control) and through filters of eight hues that evenly sampled the CIELAB color space at two lightness levels (L* = 53, 73) and two chroma levels (chroma = 32, 52). Each participant rated the intensity of the distortions on a scale from zero (“no distortions”) to six (“extreme distortions”) across 270 randomized trials (30 per hue and control). Results from multiple regression analyses tailored for SCEDs (Manolov & Onghena, 2018) showed that, in both cases, specific hues significantly modulated the distortions even when considering effects of luminance, chroma, and face identity (V.S.’s adjusted R2 = 0.75, Ezri’s = 0.38; both p < .001). Surprisingly, cool and warm colors had opposite effects for the participants. For V.S., green and cyan decreased distortions (p < .001), and lavender, pink, and red increased distortions (p < .001); for Ezri, green, cyan, and light blue increased distortions (p < .001), and pink decreased distortions (p < .05). These results demonstrate that hue can robustly modulate PMO distortions and that the interaction of color and face distortion can show substantial individual differences. In addition, they suggest color may be tightly linked with face perception in typical visual systems.