In contrast, when low frequencies dominate, cortical neurons can be hyperpolarized or depolarized and cortical areas and circuits can be silent or active for hundreds of milliseconds. The key difference between the desynchronized states and states when delta dominates the EEG is that in the desynchronized states cortical circuits are constantly active and cortical neurons are persistently depolarized. The LFP, EEG, and multiunit activity reflect this activity through large-amplitude, low-frequency synchronized fluctuations ( Steriade et al. In contrast, during slow-wave sleep cortical neurons and circuits are in a different operating regime and the membrane potential of individual neurons fluctuates by 10–20 mV ( Steriade et al. For example, when cortical circuits are in a desynchronized state (evident in the EEG Adrian and Matthews 1934 Berger 1929 Jasper and Carmichael 1935), individual cortical neurons are persistently depolarized close to threshold for action potentials, the local field potential (LFP) and EEG show low-amplitude, high-frequency components, and multiunit activity is maintained at a sustained level ( Steriade et al. These differences in activity create different operating regimes for cortical neurons. A fundamental property of cortical circuits in vivo is that these circuits and the individual neurons that constitute them are spontaneously active and the levels and patterns of activity vary with cortical state.
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