As a measure of dynamical structure, shortterm fluctuations of coherence between 0.3 and 100 Hz in the electroencephalogram (EEG) of humans were studied from recordings made by chronic subdural macroelectrodes 5-10 mm apart, on temporal, frontal, and parietal lobes, and from intracranial probes deep in the temporal lobe, including the hippocampus, during sleep, alert, and seizure states. The time series of coherence between adjacent sites calculated every second or less often varies widely in stability over time; sometimes it is stable for half a minute or more. Within 2-min samples, coherence commonly fluctuates by a factor up to 2-3, in all bands, within the time scale of seconds to tens of seconds. The power spectrum of the time series of these fluctuations is broad, extending to 0.02 Hz or slower, and is weighted toward the slower frequencies; little power is faster than 0. The present report is one of a series aiming at some insight into the fine structure in space and time of the dynamical signs provided in the compound field potentials, as recorded directly on or in the brain (1-9). The main goal is to test the hypothesis that one measure of cooperativity at each frequency-namely, coherence-varies in time on the scale of seconds or fractions of a second, with evidence of more than stochastic structure, and that the fluctuation is different for closely spaced loci. We will show in addition that a wide range of frequency components of the EEG tend to covary in coherence, contrary to the usual assumption of independent oscillators. MATERIALS AND METHODS Subdural EEG recordings from four patients were supplied by V.J.I.-M. from the University of California, San Diego, Epilepsy Center. Neurosurgeon J. F. Alksne had implanted plastic subdural strips bilaterally on frontal, temporal, and parietal lobes, each strip carrying a row of eight electrodes at 10-mm intervals, for long-term recording for localization of seizure foci. We took 2-to 4-min samples of EEG during slow-wave (stage 2-3) sleep, during alert interictal periods, and during electrical seizures. Some samples were taken in the operating room when the patient was sedated (nitrous oxide, Fentanyl, and Forane) but awake, able to respond to commands and questions, because recording arrays with 5-mm spacing were used.Depth
