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Abstract

Many neurophysiological studies have revealed single-unit responses to complex objects such as faces in the primate inferotemporal cortex. Nonetheless, the basic principles underlying the encoding of such stimuli remain a topic of active research. Of particular interest is the brain's ability to differentially process highly similar patterns, such as faces, that ultimately leads to the recognition of an individual. We have recently demonstrated using a high-level adaptational aftereffect that the perception of face identity is strongly influenced by an unseen central tendency or prototype face representation (Leopold et al, 2001). We thus speculated that the brain's analysis of complex patterns might entail the comparison of an incoming sensory pattern with a stored neural prototype representation. Here we investigate this hypothesis using a combined psychophysical and physiological approach. Two monkeys were trained to indicate the identity of four individual faces. Their psychophysical responses indicated that their perception of human face identity was similar to human subjects, with and without prior adaptation. Using chronically implanted electrodes in the anterior portion of area TE, we found that many neurons responded to the test faces. The response magnitude often varied monotonically as a function of the amount individual identity present in a face (i.e. the distance of a face from the average prototype). Five seconds of prior adaptation to a different face, a condition which produced consistent perceptual aftereffects, resulted in a diversity of effects on the neural responses, with some neurons showing a shift in their tuning that directly mirrored the perceptual distortions. These results, taken together, provide evidence that the neural analysis of identity for faces and perhaps other complex patterns, exploits a prototype-based strategy.