Investigating the origins of the face inversion effect with an extraordinary participant

What factors produce the face inversion effect (FIE)? A purely experiential account suggests the FIE results solely from the greater experience people have with faces that match the orientation of their faces (upright). In contrast, a purely phylogenetic account proposes the FIE reflects the operation of evolved orientation-specific mechanisms. Deciding between these alternatives is challenging because almost everyone has more experience with faces that match their own face’s orientation and everyone’s ancestors were selected to process faces with matched orientations. Here we report results from an individual whose extraordinary perceptual experience allows investigation of the origin of the FIE. Claudio is a 42-year-old man with a congenital joint disorder that causes his head to rest upside-down between his shoulder blades. As a result, most faces Claudio has observed are mismatched (upright) to his face’s orientation. We assessed Claudio’s performance with matched and mismatched faces in three types of tasks and compared his results to 22 age-matched controls. On two Thatcher tasks, Claudio showed better performance with mismatched (mean accuracy= 91.5%) than matched faces (mean accuracy = 75.0%). These scores provide formal evidence that he has had more perceptually-relevant experience with mismatched than matched faces. On seven face detection tasks, Claudio’s matched and mismatched performance was comparable and he did better with mismatched faces than controls. On seven identity matching tasks, Claudio’s accuracy was better with matched than mismatched faces and his FIE was similar to controls’ FIE. Because Claudio has had more experience with mismatched faces than matched faces, his scores in detection and identity matching indicate the adult visual system includes mechanisms that evolved to process matched faces. Additionally, Claudio’s reduced FIE in detection tasks is inconsistent with a purely phylogenetic account. Together, our result suggests the FIE reflects the effects of both experiential and phylogenetic factors.