Cerebral Cortex Advance Access originally published online on September 30, 2004
Cerebral Cortex 2005 15(7):877-884; doi:10.1093/cercor/bhh192
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Cerebral Cortex V 15 N 7 © Oxford University Press 2004; all rights reserved
Feature Article |
Spectral Power Time-courses of Human Sleep EEG Reveal a Striking Discontinuity at 
18 Hz Marking the Division between NREM-specific and Wake/REM-specific Fast Frequency Activity
1 Laboratoire de Sommeil et de Neurophysiologie, Hôpitaux Universitaires de Genève, Belle Idée, 1225 Chêne-Bourg, Geneva, Switzerland and 2 CERN European Organisation for Nuclear Research, 1211 Geneva 23, Switzerland
Address correspondence to Helli Merica, Hôpitaux Universitaires de Genève, Belle Idée, Laboratoire de Sommeil et de Neurophysiologie, 2 Chemin du Petit Bel-Air, 1225 Chêne-Bourg, Geneva, Switzerland. Email: helli.merica{at}hcuge.ch.
Spectral power time-courses over the ultradian cycle of the sleep electroencephalogram (EEG) provide a useful window for exploring the temporal correlation between cortical EEG and sub-cortical neuronal activities. Precision in the measurement of these time-courses is thus important, but it is hampered by lacunae in the definition of the frequency band limits that are in the main based on wake EEG conventions. A frequently seen discordance between the shape of the beta power time-course across the ultradian cycle and that reported for the sequential mean firing rate of brainstem–thalamic activating neurons invites a closer examination of these band limits, especially since the sleep EEG literature indicates in several studies an intriguing non-uniformity of time-course comportment across the traditional beta band frequencies. We ascribe this tentatively to the sharp reversal of slope we have seen at 
18 Hz in our data and that of others. Here, therefore, using data for the first four ultradian cycles from 18 healthy subjects, we apply several criteria based on changes in time-course comportment in order to examine this non-uniformity as we move in 1 Hz bins through the frequency range 14–30 Hz. The results confirm and describe in detail the striking discontinuity of shape at around 18 Hz, with only the upper range (18–30 Hz) displaying a time-course similar to that of the firing-rate changes measured in brainstem activating neurons and acknowledged to engender states of brain activation. Fast frequencies in the lower range (15–18 Hz), on the other hand, are shown to be specific to non-rapid-eye-movement sleep. Splitting the beta band at 18 Hz therefore permits a significant improvement in EEG measurement and a more precise correlation with cellular activity.
Key Words: beta band limits • brainstem–thalamic activating neurons • neuronal transition probability model • spectral analysis • spindles
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