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Abstract

When perceiving a face, we can easily decide whether it belongs to a human or non-human primate. It is thought that face information is represented by neurons in the macaque temporal cortex. However, the precise encoding mechanisms used by these neurons remain unclear. Here we use face stimuli of humans, monkeys and monkey-human hybrids (morphs) to gain a better understanding of these mechanisms, in particular of the categorization of faces into different species, and how learning affects representation of these stimuli. We perform single cell and local field potential (LFP) recordings in the inferior-temporal (IT) cortex of the macaque brain during a fixation task. To investigate the perceptual effects of our stimuli and possible relations
to the neural data, we conduct in parallel psychophysical experiments with human subjects. On preliminary results of 75 recorded cells in one animal, we found 66 visual responsive neurons. From them, 12 were tuned to faces ('face-cells') and 9 to other test objects (like a hand, clock, fruits, etc.). Six 'face-cells' prefer monkeys while just two prefer humans. Considering the population activity, monkey faces elicited in general higher firing rates on the population of neurons (independent of its category) than human faces. Additionally, these
firing rates change gradually according to the human/monkey ratio of the morphed stimuli. After measuring the perceptual category boundary between monkeys and humans faces in our human subjects, we founded that it is shifted to the human side, independent of the method we use to measure it. Our preliminary cell recordings suggest that neural responses (firing rates) of some cells differentiate between monkey and human faces. Besides, the tuning curves of some neurons and the population correlate with the human-ratio of the morphed stimuli. Our psychophysical experiments confirm, on the one hand, the perceptual effect of our stimuli in which we manipulate the human-monkey ratio and, on the other hand showed a tendency of our subjects to set the category boundary between humans and monkeys closer to the human side. All these findings point to different mechanisms used by the brain to encode human and monkey faces, which
seem to be clearly represented by neurons in the inferior-temporal cortex of the money brain.