Using 3-D human-monkey morphs to explore the boundaries of species dependent face-categories in humans

Face perception has often been investigated with human faces differing in categories such as race or gender. Here, we investigate the perceptual border across species. We applied a method based on support vector machines to generate images of hybrid monkey - human faces (‘morphs‘) with different levels of human contribution. In the ‘explicit‘ experiment, we asked subjects to rate morphs at different morph levels as ‘humans‘ or ‘monkeys‘. We found that subjects rated the morphs as humans when they had a human contribution of at least 56±3. In the ‘implicit‘ experiment, we asked whether subjects could distinguish between successively presented morphs differing by ±10 morph level from a morph centre. By varying the morph centre value from 10 to 90, we were able to measure subjectlsqu o;s sensitivity to detect species differences along the human - monkey continuum. We found that the sensitivity of subjects to detect species differences was highest when morphs had a human contribution of 65±3. In summary, the human - monkey boundary does not lie at the midpoint of the human - monkey continuum, but tends to be shifted towards the human side. Our results reveal an asymmetry in the perception of human - monkey morphed faces, which may be species-specific and/or due to expertise