This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Recognition memory, the ability to perceive recently encountered items as familiar, relies on structures in the medial temporal lobe, including the hippocampus. Studies have shown the visual paired comparison, or visual preferential looking task (VPLT), to be a test of recognition memory that is sensitive to hippocampal damage in monkey and humans. Accordingly, to examine possible recognition memory signals in the hippocampus, we recorded hippocampal activity in rhesus monkeys as they performed the VPLT. Hippocampal neurons responded significantly to stimulus presentation relative to the baseline pre-stimulus period, and a substantial proportion of these visually-responsive neurons showed significant firing rate modulations that reflected whether stimuli were novel or familiar. Additionally, these firing rate modulations were correlated with recognition memory performance on the VPLT such that larger modulations by stimulus novelty were associated with better performance. We also observed an increase in gamma-band neuronal synchronization across the hippocampus during encoding that predicted the strength of subsequent recognition. Finally, theta-band oscillations in hippocampal LFPs were strongly coupled to eye movements, undergoing a phase resetting with each new fixation. The phase of the network theta oscillation at fixation onset and the degree of theta-band spike-field phase synchronization across the trial was correlated with the strength of stimulus encoding. The amplitude of hippocampal gamma-band oscillations was also modulated at theta frequency. Taken together, these findings suggest that neuronal activity in the hippocampus is organized at multiple levels, is related to the strength of memory formation, and is intimately connected to behavior.
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