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Neuroscience

What makes the brain's tickers tock

Nature volume 394, pages 132–133 (1998)Cite this article

Seventy-five years ago, Berger placed electrodes on his son's scalp and made the first observation of brain waves. The frequency of these waves was ten cycles per second (10 Hz), but since then oscillations at other frequencies (5, 40 and 200 Hz) have been detected. These oscillations involve the synchronized firing of large groups of neurons, suggesting a role in clocking, yet remarkably little is known about their function or mechanism. Oscillations do not normally occur in the brain slices that are routinely used to investigate mechanism. But recent studies — including reports by Fisahn et al.1 and Draguhn et al.2 on pages 186and of this issue — have succeeded in inducing 40- and 200-Hz oscillations, respectively, in hippocampal slices. These studies provide new insights into the mechanisms of oscillations, and these have interesting implications for function.

Fisahn et al.1 provide the strongest evidence to date that 40-Hz oscillations are due to a feedback-inhibition mechanism3 — the firing of pyramidal cells excites inhibitory interneurons, which then inhibit the pyramidal cells. After inhibition decays, the pyramidal cells fire again, initiating the next cycle. The key new evidence is that oscillations are prevented if the fast excitation of interneurons by pyramidal cells is blocked. This observation was made in the CA3 region of the hippocampal slice, but an entirely different result has been obtained by others in the CA1 region4, where the oscillation persists when fast excitation is prevented. This persisting oscillation is due to an interneuron network in which a steady depolarization causes interneurons to fire and mutually inhibit one another. The next cycle of firing then occurs after this inhibition decays4,5. In vivo studies should tell us whether either or both of these mechanisms operate under particular physiological conditions.

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Figure 1: Binding and segmentation of 40-Hz oscillations.
Figure 2: Mechanism by which a negative-feedback version of 40-Hz oscillation could serially activate memories in sequential 40-Hz cycles.

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  1. the Department of Biology, Brandeis University, Waltham, 02254, Massachusetts, USA

    John Lisman

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Lisman, J. What makes the brain's tickers tock. Nature 394, 132–133 (1998). https://doi.org/10.1038/28061

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