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On distributed cell assemblies, high frequencies, and the significance of EEG/MEG recordings
In: Time and the brain, Harwood Academic Publishers, R Miller (Ed), Amsterdam, 241 - 249
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According to recent proposals, coherent high-frequency activity in cortical neurons may be a specific indicator of cognitive processes in the brain, including those related to the perception of objects, the programming of actions and the processing of language. One may therefore speculate that cell assemblies involved in higher cortical functions primarily generate high-frequency rhythms. In contrast, it has been argued that such neuronal assemblies may as well generate equally strong spectral activity in various frequency bands, high and low, when active. It is proposed here that the controversy can be decided on the basis of knowledge about axonal conduction times inferred from fiber calibers measured in the human cortex. These data suggest that conduction times of most long axons are around 5-10 m/s, and even loops between neurons in distant areas can be travelled in a few hundredths of a second. Reverberation of excitation in assemblies would thus primarily lead to high-frequency cortical activity above 20 Hz. This leads to an explanation of recent findings from large-scale neurophysiological recordings during cognitive processing in humans.