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Oscillatory and Intermittent Synchrony in the Hippocampus: Relevance to Memory Trace Formation

  • G. Buzsáki
  • A. Bragin
  • J. J. Chrobak
  • Z. Nádasdy
  • A. Sik
  • M. Hsu
  • A. Ylinen
Part of the Research and Perspectives in Neurosciences book series (NEUROSCIENCE)

Summary

The different cell populations of the hippocampus are involved in behaviorregulated intermittent population bursts (sharp-waves, (SPW) and dentate spikes) and oscillations (theta, gamma and 200 Hz). SPW occur during consummatory behaviors and slow wave sleep. This field potential reflects summated EPSPs in the apical dendrites of CA1 pyramidal neurons as a result of population synchrony in the CA3 recurrent network. During SPW bursts, CA1 pyramidal cells display highly coherent transient network oscillations (200 Hz). Participating pyramidal cells discharge at a significantly lower rate than the frequency of the population oscillation and their action potentials are phase-locked to the simultaneously recorded oscillatory field potentials. Antagonistic to these intermittent events are the theta (6–10 Hz) and associated gamma (40–100 Hz) oscillations during exploratory behavior and REM sleep. In the intact rat the gamma pattern has its largest amplitude in the hilus, is modulated by theta and is entrained by the entorhinal input. The theta pattern is generated by several spatially distinct but highly coherent dipoles and requires an external pacemaker (septum).

The cooperative neuronal discharges underlying the various oscillatory and intermittent synchronous patterns may support neuronal mechanisms underlying memory trace formation in the hippocampus. We hypothesize that such a process requires two stages. 1) During theta-associated exploration, activation vectors from the entorhinal cortex code mnemonic representations in subsets of CA 3 pyramidal cells, where information is temporarily held. 2) At the end of exploration, CA3 cells are disinhibited and discharge in bursts, the most recently and hence most strongly excited ones first (burst initiators). Excitation is spread to less excitable pyramidal cells by the extensive recurrent CA3 collateral system. In essence, the information gathered during the exploratory stage is “replayed” during the SPW bursts. The most important aspect of the model is that the sequential recruitment of individual cells into population bursts (dynamic hierarchy) is based on representations acquired during exploration. In this two-stage model both activation patterns (theta and gamma) and intermittent patterns (SPW) subserve to process specific information.

Keywords

Granule Cell Dentate Gyrus Pyramidal Cell Entorhinal Cortex Mossy Fiber 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag Berlin Heidelberg 1994

Authors and Affiliations

  • G. Buzsáki
  • A. Bragin
  • J. J. Chrobak
  • Z. Nádasdy
  • A. Sik
  • M. Hsu
  • A. Ylinen

There are no affiliations available

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