Functional topography of the human mu rhythm

Abstract

Topographic and functional testing was conducted to determine if the Rolandic mu rhythm (rhytme en arceau) could be reliably detected from the human scalp, and to describe its spatiotemporal and functional characteristics in relation to the feline sensorimotor rhythm (SMR) and the human occipital alpha rhythm. Mu activity was rarely visualized clearly in referential recordings or from the major 10–20 system locations, but was detected by power spectral analysis and functional testing in 7 of 14 subjects studied. It was best recorded in closely spaced bipolar derivations from central areas posterior to the central sulcus as shown in topographic analysis of its occurrence at rest and its reactivity to contralateral hand movement. Mu activity was attenuated by voluntary movement and somatosensory stimulation but was minimally affected by visual stimulation. Its frequency ranged within the alpha band (mean of 10.1 c/sec), but was separate from the occipital alpha rhythm and stable in frequency across days. The anatomical and functional similarities between human mu activity and the feline SMR suggested that they are functionally analogous resting rhythms which are disrupted primarily by input to the somatosensory system. The concept of sensory-specific alpha rhythms was discussed, in which the mu rhythm was conceptualized as a central “somatosensory alpha rhythm” in contrast to the occipital “visual alpha rhythm”.