Cosyne 2007 Workshops
February 26-27, 2007
The Canyons, Utah
Elizabeth A. Buffalo Emory University School of Medicine Yerkes National Primate Research Center
Hippocampal Synchronization and Memory Formation
Medial temporal lobe (MTL) structures, including the hippocampus and surrounding cortical regions, play a critical role in memory formation. Memory formation is presumed to require coordinated activity within these MTL structures, and interactions of this complex with neocortex. However, our understanding of the neuronal mechanisms that support these interactions remains unclear. We have begun to address this issue in my laboratory using multi-electrode recordings of spiking activity and local field potentials (LFPs) in medial temporal lobe structures of behaving monkeys. Our central hypothesis is that memory formation is associated with enhanced neuronal synchronization within and among MTL structures. Here, I will present data from our recent study investigating the role of hippocampal synchronization in recognition memory performance. In this study, we recorded from four electrodes positioned in the hippocampus of a monkey performing the Visual Preferential Looking task. This task has been shown previously to depend upon the integrity of the hippocampus in both monkeys and humans. Using multi-taper methods of spectral analysis, we found an increase in gamma-band (30-90 Hz) synchronization, measured as spike-field coherence, during stimulus encoding relative to pre-stimulus baseline. Furthermore, the degree of coherence predicted subsequent recognition memory performance. We found significantly increased gamma-band coherence during the encoding of stimuli that are subsequently well-remembered relative to poorly-remembered stimuli. These data suggest that successful encoding incorporates transient oscillatory synchronization among hippocampal neurons. Phase synchronization of spikes in the gamma-band may be particularly relevant for memory formation, because synchronization in this range would ensure that spikes arrive at downstream targets within ~10 ms of one another, i.e., within one-half of a gamma cycle. Such close temporal spike alignment may lead to enhanced synaptic efficacy, which is considered to be one of the primary information storage principles in the brain.
