Investigation of face representation in monkeys using adaptational aftereffects

Adaptational aftereffects have often been described as the “psychophysicist’s electrode” because of their ability to isolate specific populations of neurons related to perception. We have recently demonstrated that adaptational aftereffects can systematically and precisely bias the perception of complex patterns such as faces (Leopold et al., 2001). These results suggested a privileged role for the prototype or ‘central tendency’ of an object category in the representation of faces, which may be expressed in the selective responses of neurons in the inferotemporal cortex. Specifically, 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 shown brief presentations of faces whose identity was modulated between the mean face and each individual, and required to identify the face. In the second experiment a 4-second adaptation to a different face preceded each test face presentation. Without adaptation, thresholds for discriminating between the memorized faces were evaluated in both monkeys, and were very similar to those of humans performing the same task. Following adaptation, perception was biased according to the structure of the adapting stimulus. The nature and magnitude of the adaptation effects were very similar to that observed in humans. These results suggest that mechanisms underlying face recognition in the monkey are similar to those present in humans, even when it is across species. 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