Ensemble recording of novelty effects on unit activity in the rhesus macaque temporal lobe.
1. Calif Ntl Primate Rsch Ctr., Univ. California/Davis, Davis, CA;
2. ARL NSMA;
3. Evelyn F. McKnight Brain Inst., Univ. Arizona, Tucson, AZ
Unit activity was recorded, using chronically implanted 12-tetrode "hyperdrives", from four rhesus macaques performing a "Variable Novelty" task, in which they received juice reward for viewing images displayed on a computer monitor. For each session, three sets of images were used: a "familiar" set of 10 that were the same every day; a "novel" set of 10 that were changed for each recording session but used through the full session; and a "more novel" set of 30 that were used in groups of 10 during the first, second, and third parts of the session. The task was purely passive in that the only requirement was to maintain fixation during the 2 sec in which an image was visible. Units were categorized on the basis of post-recording histology as belonging to the perirhinal, entorhinal, or hippocampal regions. Responses were analyzed for three types of novelty effects: long term (different responses to "familiar" and "novel"); intermediate term (different responses to "novel" and "more novel"); and short term (different responses to the first and second presentation). In the perirhinal area, responses to all types of novelty were observed, with long-term effects being the most common and most robust. In the hippocampus, responses to long-term and intermediate-term novelty were observed in some units. Strikingly, there were many cases in which a hippocampal unit lost or gained a response partway through a session. Such changes were much less common for perirhinal units. In the posterior entorhinal area, very few units were significantly affected by any type of novelty; however, many units responded to the delivery of juice reward. These observations suggest that perirhinal responses to novelty are robust and relatively automatic, as has been concluded previously (e.g., Brown and Aggleton, Nat Rev Neurosci 2:51); that hippocampal units can respond to novelty, but such responses are affected by factors that change during the course of a session; and that posterior entorhinal responses are minimally affected by novelty but strongly affected by reward.
Grant/Other Support: AG003376; McKnight Brain Research Foundation
Keyword (Complete): hippocampus; perirhinal cortex; entorhinal cortex