Ellie Rodgers¹ (), Antonio Vitor Reis Goncalves Mello, Brad Duchaine; ¹Dartmouth College, ²Dartmouth College, ³Dartmouth College

Prosopometamorphopsia (PMO) is a rare neurological condition in which individuals experience perceptual distortions when they view faces. Although these distortions were first documented more than a century ago, the processes underlying them are poorly understood. Here, we present a detailed case study of a participant with PMO to investigate whether asymmetry between the left and right halves of the face drives distortions. A.S. is a 58-year-old woman who has experienced intense, dynamic distortions affecting facial feature shape and position since 2010. In Experiment 1, A.S. rated distortion intensity (scale: 0-6) of 40 faces. Ratings for symmetric faces (mean) were significantly lower than ratings for natural faces (mean) (p < 0.001), indicating that left–right facial asymmetry contributes to distortions. In Experiment 2, we further assessed the effect of asymmetry by asking A.S. to rate distortions in naturally asymmetric faces presented with different parts visible. The stimuli were either full faces (asymmetry present), top and bottom halves (asymmetry present), left and right halves (asymmetry absent), or isolated facial features. Distortions were strongest for the full faces, present but reduced in intensity for the top halves and pairs of eyes, and mostly absent for the bottom halves, left halves, right halves, and individual features (p < 0.001). In Experiment 3, we contrasted frontal views of faces (asymmetry present) with full profiles (asymmetry absent); A.S. saw distortions to all of the frontal views and no distortions to faces presented in profile. Together, these findings suggest that the visual system calculates the asymmetry between the left and right halves of the face every time a face is viewed, and therefore, some cases of PMO depend critically on facial symmetry. Disrupting left-right symmetry substantially reduces perceived distortions, providing insight into the perceptual mechanisms underlying this condition.

Acknowledgements: Dartmouth College Department of Psychological and Brain Sciences