Anuk Dias^*1 (), Vandita Gupta^*2, Abigail West¹, Charisse Pickron²; ¹Department of Psychology, University of Minnesota, ²Institute of Child Development, University of Minnesota

Human adults are better at differentiating among human faces relative to non-human primates faces. This robust finding has been demonstrated by the inversion effect eliciting larger, more negative neural responses to inverted human faces. However, there is a research gap in comparing the human perception of different species of non-human primate faces. Differences in facial recognition could arise from evolutionary impulses or featural differences (presence of visible sclera or spacing). In this on-going EEG study, we examined differences in time-locked face-sensitive neural responses (P1 and N170 components) to humans, chimpanzees, and macaques faces across 38 adult participants using an ERP inversion effect task. Participants viewed 600 randomized grayscale, front-facing, luminance-matched upright (n=300) and inverted (n=300) images. ERP component amplitude and latency were separately analyzed in a 2x3x2 repeated measures ANOVA with face orientation (upright, inverted), face species (human, chimpanzee, macaque), and hemisphere (left, right) as within-subjects variables. For the P1, a statistically significant main effect of species was found (F=23.68, p<0.001, ɳ p2= 0.39) such that chimpanzee faces elicited the highest amplitude (p<0.001) relative to both human and macaque faces. For the N170, a statistically significant interaction between species and orientation ( F=13.13, p<0.001, ɳ p2=0.27) was found. A pairwise comparison revealed a larger, more negative, N170 amplitude for human inverted faces compared to upright faces (p<001). No differences were found for either chimpanzees or macaque faces. These preliminary findings indicate a replication of previously reported face inversion effect. This effect was not present for chimpanzee or macaque faces, suggesting differential engagement of face-sensitive processing mechanisms. Chimpanzees had an unexpectedly higher P1 amplitude compared to humans or macaques. This may indicate that chimpanzee faces are perceived distinctly from animal and human face categories.