MATHEMATICAL BIOLOGY SEMINAR

        According to the "cell assembly" hypothesis (D.O. Hebb, "The Organization of Behavior", 1949), information is represented in the brain by groups of anatomically distributed neurons which come together briefly in synchronous activity, and whose activity underlies both processing of external sensory input, and internal cognitive processes. Accordingly, neuronal populations should show an arrangement into synchronous groups, and the firing patterns of these groups should show coordination beyond that predicted by common modulation by sensory input. While such possibilities cannot be tested using traditional single unit recording methods, modern electrophysiological technology now allows for simultaneous monitoring of very large populations of neurons, allowing these theories to be investigated experimentally.
        Using parallel recordings in the behaving rat, we found it was possible to predict the exact spike times of hippocampal pyramidal neurons from the spike times of simultaneously recorded neurons, better than from the animal' s trajectory in space, or from a spatially-dependent theta phase modulation (O'Keefe and Recce, Hippocampus, 1993; Harris et al, Nature, 2002). This suggests that hippocampal neurons are organized into assemblies whose activity is not controlled strictly by the time-course of sensory input. The time window within which spike times were best predicted from simultaneous peer activity was 20-30ms, suggesting that this is the timescale at which cell assemblies are synchronized. Because this temporal window matches the membrane time constant of pyramidal neurons, the period of the hippocampal gamma oscillation, and the time window for synaptic plasticity, we suggest that cooperative activity at this timescale is optimal for information transmission and storage in cortical circuits.

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