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Abstract:
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.
