Involvement of single unit activity in inferotemporal and perirhinal cortices in recognition memory of visual objects in the macaque.

Recognizing objects from the past is a vitally important ability for everyday live. The studies of brain mechanisms responsible for visual recognition memory suggest that the modulation of single unit activity in the inferotemporal and perirhinal cortices could be an important part of the neuronal substrate of recognition memory. In this review, I will describe Stimulus Specific Adaptation (SSA)--the reduction in neuronal response to previously viewed objects. The experimental tasks in which SSA is observed will be presented, along with the possibility that SSA may be enhanced by saccadic exploration of visual scene. Next, I will demonstrate that under special circumstances (partially split-brain preparation) monkeys could recognize the re-presentation of visual images without the concomitant appearance of SSA. The most promising alternative candidate for neuronal mechanism involved in recognition memory is delay activity--an increased frequency of cell firing in the time between the initial presentation of an image and its subsequent re-presentation. In order to determine if delay activity is important for recognition we have started to investigate the effects on recognition memory of disrupting delay activity by electrical stimulation. Preliminary results indicate a positive correlation between a reduction in delay activity and a decrease in recognition performance.