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Abstract:
It is clear from a large number of electrophysiological studies in primates that complex patterns and objects are selectively represented in the firing of neurons in the visual cortex. However, less clear are the computational strategies by which the brain encodes and differentiates between highly similar patterns, such as that used in recognizing the identity of an individual face. One possibility is that such neural encoding proceeds in a parametric manner, exploiting the naturally occurring intra-category variability. We have recently demonstrated that adaptational aftereffects can systematically and precisely bias the perception of faces (Leopold et al., 2001). Our results suggest a privileged role for the prototype or central tendency face in the decoding of face identity. We speculated that the analysis of a complex sensory pattern may involve a comparison with a prototype representation implicitly stored in the sensory apparatus. The present study is a first step to investigating this hypothesis in alert, behaving, monkeys. Two monkeys were trained to indicate the identity of up to four individual faces by pressing one of four buttons. In the first experiment, they were required to identify briefly presented, identity-depleted faces. In the second experiment a 4-second adaptation to a different face preceded each test face presentation. Baseline discrimination thresholds as a function of face identity were similar to those of humans performing the same task. When the discrimination was preceded by a 4-second adaptation with a different face, perception was biased according to the structure of the adapting stimulus, again in a manner similar to humans. These results suggest that monkeys and humans employ similar strategies in the neural processing of human faces. Current studies are underway using multielectrode bundles implanted in the inferotemporal cortex of both monkeys to elucidate the role of the prototype in the neural representation of faces.
