Specific characteristic frequency bands are known to be modulated by activity in specific brain regions, but it is unknown whether this modulation arises from one or multiple physiological processes, from surface or deep structures, and what rules govern the spatial and anatomic constraint.

     In this study, we address the nature of the theta and beta rhythms on the lateral cortical surface using measured phase coherence in electrocorticographic (ECoG) potentials.

     ECoG recordings were measured from 20 human epilepsy patients while they performed simple cue-based movements of the hand.

     Pairwise phase-coherence was calculated between the most movement specific electrode (M0) and the rest of the array by band-passing the signal between 4-8Hz (theta) or 12-20Hz (beta), taking the Hilbert transform, and then calculating the phase coherence. The same was also done with seed electrodes chosen in frontal, parietal, and temporal cortex.

     Spatially continuous regions of rhythm coherence (designated "rhythmic units") are clearly produced, with boundaries that are determined more by skips in the phase, rather than the magnitude, of the coherence measure. These are sharply delineated along sulcal boundaries, with the movement specific unit in the beta range including the pre- and post-central gyri.

     This is a retrospective study.

     Coherent "rhythmic units" were identified on the cortical surface, which were sharply demarcated along sulcal boundaries.

     This implies two important and novel principles about rhythms in the brain: they represent a distributed cortical surface phenomenon and they are functionally specific within distinct spatial "rhythmic units".