Sources of adaptation of inferior temporal cortical responses.

Neurons of different brain regions change their response when a stimulus is repeated. In inferior temporal cortex (IT), stimulus repetition typically reduces the responses of single neurons, i.e., IT neurons show repetition suppression. Single unit recordings in IT showed that individual neurons vary in their degree of adaptation effects, ranging from strong suppression to slight enhancement of the response to the repeated stimulus. The suppression is maximal after the peak of the response and then reduces during the further course of the response. Repetition suppression in IT is still present for interstimulus intervals of at least 900 msec. I discuss the contribution of mechanisms that have been proposed to explain adaptation effects of IT responses. Firing-rate dependent response fatigue, e.g., a prolonged hyperpolarization, intrinsic to the recorded neuron cannot explain the stimulus specificity of the adaptation effect. The latter can be explained by synaptic depression or an adapted input from other IT neurons. We observed repetition suppression of IT neurons when adapter and test stimuli were presented at locations that differed by 8 degree of visual angle, suggesting that at least part of the adaptation effect is not inherited from retinotopic visual areas with small receptive fields. We observed no effect of repetition probability on repetition suppression in macaque IT using images of various categories, suggesting a dissociation between top-down expectation effects and repetition suppression. Together, our data agree with the hypothesis that adaptation in IT serves to reduce the saliency of recently seen stimuli, highlighting stimuli that differ from recently presented ones.