Associative learning signals in the monkey medial temporal lobe
Wendy A. Suzuki
New York University , New York , USA
A major goal in my laboratory is to understand how the hippocampus and related medial temporal lobe structures participate in the formation of new associative memories. To address this question, we recorded the activity of individual neurons throughout the hippocampus as monkeys learned novel associations between visual scenes and particular rewarded target locations (location-scene association task). Previous studies have shown that learning in this task is dependent on the medial temporal lobe 1 . We found that hippocampal neurons signal new learning with dramatic changes in their stimulus-selective response properties and this changing neural activity was significantly correlated with the animal’s behavioral learning curve 2 . We call these cells “changing” cells. The changing cells signaled learning either before, at the same time or after behavioral learning was expressed suggesting that these cells may play a role in both the early formation of new associations as well as in the strengthening of the newly formed associations in memory. The next question of interest was defining how other interconnected medial temporal lobe areas may contribute to new associative learning. We focused on the perirhinal cortex a memory-related brain area that is strongly interconnected with the hippocampus via the entorhinal cortex. We found that perirhinal cortex contains a similar proportion of changing cells as the hippocampus and the timing of these changing neural signals relative to behavioral learning is also similar across the two areas 3 . The major difference between the learning signals in these two areas was observed when we asked when the learning signals occurred within the trial. Early in the trial during the visual stimulus period, perirhinal cortex, but not hippocampus provided a strong learning signal. During the delay interval of the task, the learning signal was strong in the hippocampus, but not in the perirhinal cortex and both areas provided a strong learning signal during the final eye movement response period of the task. These findings suggest that hippocampus and perirhinal cortex work together to signal the formation of new associative memories.