Electrophysiological correlates of sleep delta waves1
Section snippets
Oscillations or waves?
One of the major sources of confusion stems from the undiscriminated use of terms such as oscillations or waves. From the spectral point of view there is no reliable border between the two, both contributing to the power spectrum. However, from the electrophysiological point of view, waves and oscillations may reflect different phenomena. The term oscillation may have various meanings according to the context. In its most general form, it is a variation of a parameter (e.g. current, voltage)
Slow cortical oscillation (<1 Hz) during sleep
A slow oscillation (<1 Hz) has been described in intracellular recordings of cortical neurons in anesthetized cats (Steriade et al., 1993d). This cellular oscillation is marked by a continuous alternation of the membrane potential between two voltage levels (Fig. 1): a depolarized and a hyperpolarized one. The membrane depolarization lasts for about 0.4–0.8 s, is due to synchronous synaptic activities in the cortical network (Amzica and Steriade, 1995b), and is made mainly of excitatory and
Slow (<1 Hz) glial activities
It has long been known that glial cells of the cerebral cortex exhibit electrical activity related to the physiological behavior of the brain. The contribution of glial cells to the genesis of delta waves has been repeatedly hypothesized (see Elul, 1972; Ball et al., 1977), but never demonstrated. The relationship between glial and neural activities has been mostly investigated through non-electric methods which did not disclose the basic mechanisms that underlie the communication between
Integration of activities below 4 Hz in corticothalamic networks
Several types of activities in the 0.5–4 Hz frequency band have been presented here. They have different sites and mechanisms of genesis, and may contribute together or separately to the delta activity recorded in the EEG. The slow (<1 Hz) oscillation has a distinct nature from other slow (1–4 Hz) rhythms. It should be therefore regarded as a distinct activity, not only because of its frequency range, but mostly because of its electrophysiologic features and because it has the ability to
Methodological issues for electroencephalographers
We believe that analyses of EEG data should take into consideration the actual aspect of waves and, if possible, their relationship with the state of the cellular aggregates of the corticothalamic network. Obviously this is not possible by merely a spectral approach. Fourier spectra are not able to discriminate between periodic phenomena and waves with a given shape, i.e. with a given spectral content. During quiet sleep, the slow oscillation provokes rhythmic KCs (see above) every 1 s or more.
Conclusions
Several electrophysiological phenomena are responsible for the genesis of <4 Hz activities (Fig. 9). Depending on the weight of synaptic linkages, on local circuit configurations and on the general behavioral state of the network, the slow oscillation, the thalamically generated clock-like delta oscillation and cortical delta waves may coalesce to produce the intricate electrographic pattern of slow wave sleep. The use of a more specific language (slow oscillation, thalamic clock-like delta,
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