Different frequencies for different scales of cortical integration: from local gamma to long range alpha/theta synchronization

Cortical activity and perception are not driven by the external stimulus al one; rather sensory information has to be integrated with various other int ernal constraints such as expectations, recent memories, planned actions, e tc. The question is how large scale integration over many remote and size-v arying processes might be performed by the brain. We have conducted a serie s of EEG recordings during processes thought to involve neuronal assemblies of varying complexity. While local synchronization during visual processin g evolved in the gamma frequency range, synchronization between neighboring temporal and parietal cortex during multimodal semantic processing evolved in a lower, the beta1 (12-18 Hz) frequency range, and long range fronto-pa rietal interactions during working memory retention and mental imagery evol ved in the theta and alpha (4-8 Hz, 8-12 Hz) frequency range. Thus, a relat ionship seems to exist between the extent of functional integration and the synchronization-frequency. In particular, long-range interactions in the a lpha and theta ranges seem specifically involved in processing of internal mental context, i.e, for top-down processing. We propose that large scale i ntegration is performed by synchronization among neurons and neuronal assem blies evolving in different frequency ranges. (C) 2000 Elsevier Science B.V . All rights reserved.