"Own-species" bias in the categorical representation of a human/monkey continuum in the human and non-human primate temporal lobe

While face categorization is a fundamental cognitive ability of human and non-human primates, its neural basis remain poorly understood. Using a new morphing technique, we created realistic three-dimensional morphed faces that linearly span the continuum between humans and monkeys (“species” continuum). Extensive categorization and discrimination experiments in human observers show that humans perceive the “species” continuum categorically. Moreover, the position of the categorical boundary is shifted from the center towards the human end of the continuum, suggesting a higher sensitivity to changes near the own-species prototype. We presented a subset of these faces to human subjects in a block-design fMRI experiment to record BOLD signals from the temporal lobe while participants performed an unrelated task at fixation. We applied a multivariate approach based on (Pearson) correlations to compute the difference between activity patterns elicited by faces along the continuum. Using this method, we looked for a categorical representation in face selective areas previously defined using an independent, standard "Face-localizer" experiment. Consistent with the psychophysical results, we found a categorical response with a bias towards the human end of the stimulus continuum in the activation patterns of the left human STS. In addition, activation in human ventral temporal cortex was most sensitive to deviations from the human prototype. To look for similar effects in monkeys, we applied an equivalent multivariate approach to analyze extracellular signals from a population of neurons recorded from the STS of two macaque monkeys while they fixated at the same type of faces. Additionally, the position of the perceptual category boundary was determined with a preferential-looking-time experiment. In both behavioral and neuronal monkey data, we found a categorical representation of the continuum, but in this case, with a bias towards the monkey end of the continuum. Our results demonstrate the neural basis of categorical representation of a facial attribute in the human and non-human primate brain. Together, our findings suggest that experience can lead to significant shifts in category boundary for face stimuli