Thalamocortical theta coherence in neurological patients at rest and during a working memory task.

We simultaneously recorded the local field potential (LFP) in the thalamus and the electroencephalogram (EEG) on the scalp of 5 patients suffering from neurogenic pain, epilepsy and movement disorders. In an earlier study [], we have investigated the slowing of EEG and the high thalamocortical coherence in the framework of thalamocortical dysrhythmia, the common underlying pathophysiology. The current study focuses on the effects of different cognitive conditions. When patients rested with eyes closed, a theta peak dominated the EEG spectra. The peak height was reduced upon opening the eyes, reminiscent of the classical alpha blocking. This peak reduction also appeared in the thalamic LFP recording. When patients activated their working memory by counting backwards, the theta peak increased in scalp EEG or in the LFP recorded in thalamic nuclei VA/VL. The coherence estimates between EEG and LFP ranged between 21% and 76% for different patients and cognitive conditions (mean: 50%). The involvement of both cortex and thalamus in working memory and the high thalamocortical coherence underline, in addition to cortico-cortical interactions, the importance of thalamocortical modules in the generation of higher cognitive functions.

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 alone; rather sensory information has to be integrated with various other internal constraints such as expectations, recent memories, planned actions, etc. The question is how large scale integration over many remote and size-varying processes might be performed by the brain. We have conducted a series of EEG recordings during processes thought to involve neuronal assemblies of varying complexity. While local synchronization during visual processing 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-parietal interactions during working memory retention and mental imagery evolved in the theta and alpha (4-8 Hz, 8-12 Hz) frequency range. Thus, a relationship seems to exist between the extent of functional integration and the synchronization-frequency. In particular, long-range interactions in the alpha and theta ranges seem specifically involved in processing of internal mental context, i.e. for top-down processing. We propose that large scale integration is performed by synchronization among neurons and neuronal assemblies evolving in different frequency ranges.