The influence of familiarity on the neural coding of face sex

In behaviour, humans have been shown to represent the sex of faces categorically when the faces are familiar to them. This leads to them judging faces crossing the category boundary (i.e. from male to female) as more different than faces that are within the same category. In this study, we investigated how faces of different sexes are encoded in the brain, and how familiarity changes the neural coding of sex. We recorded participants brain activity using fMRI while they viewed both familiar and unfamiliar faces that were morphed in their sex characteristics (i.e. between male and female). Participants viewed pairs of faces that were either identical, or differed in their sex morph level, with or without a categorical change in perceived sex (i.e. crossing the perceived male/female category boundary). This allowed us to disentangle physical and categorical neural coding of face sex, and to investigate if neural coding of face categories was enhanced by face familiarity. Our results show that the sex of familiar, but not unfamiliar, faces was encoded categorically in the medial prefrontal and orbitofrontal cortex as well as in the right intraparietal sulcus. In contrast, the fusiform face area showed a sensitivity to the physical changes in the sex of faces that was unaffected by face familiarity. The occipital face area showed its highest responses to faces towards the ends of the sex morph continuum (i.e. the most male or most female faces), and these responses were also unaffected by face familiarity. These results suggest that there is a dissociation between the brain regions encoding physical and categorical representations of face sex, with occipital and fusiform face regions encoding physical face sex properties and frontal and parietal regions encoding high-level categorical face sex representations that are linked to face identity.