Functional mapping of human sensorimotor cortex with electrocorticographicspectral analysis - I. Alpha and beta event-related desynchronization

Human scalp EEG studies have shown that event-related desynchronization (ER D) in the alpha (8-13 Hz) and beta (15-25 Hz) bands may be used to detect f unctional activation of sensorimotor cortex. However, in most previous stud ies somatotopy has not been examined in detail and brief, self-paced moveme nts, focusing on the planning of motor output, have been used. We recorded electrocorticographic (ECoG) signals in five clinical subjects during a vis ual-motor decision task that was designed to activate the representations o f different body parts in sensorimotor cortex. To focus more on execution o f motor output than on its planning, subjects were instructed to make susta ined isometric muscle contractions in different body parts (tongue protrusi on, fist-clenching or foot dorsiflexion) in response to randomized visual s timuli depicting each action. ECoG spectral analysis utilized a mixed-effec ts analysis of variance model in which within-trial temporal dependencies w ere taken into account, and the magnitude and statistical significance of a lpha and beta ERDs were mapped onto a surface rendering of each subject's b rain MRI. Cortical electrical stimulation was performed in all subjects for clinical purposes, and the resulting maps of sensorimotor function were co mpared with those generated by ECoG spectral analysis. During the early pha ses of the motor responses, alpha ERD commonly occurred in a diffuse spatia l pattern that was not somatotopically specific. During the late phases, th e spatial pattern of alpha ERD usually became more focused and somatotopica lly specific. Maps of alpha ERD were closer to cortical stimulation maps wh en alpha ERD was sustained throughout the late phases of the motor response s. Thus, the topography of alpha ERD more resembled traditional somatotopy when its temporal profile approximated that of the motor response. The topo graphy of beta ERD was often more discrete and somatotopically specific tha n that of alpha ERD, but beta ERD was often transient and sometimes absent. Sometimes, unilateral limb movement produced sustained alpha and beta ERD over bilateral sensorimotor cortices, with overlapping patterns for differe nt body parts. The topographical spread of alpha ERD beyond expected functi onal-anatomical boundaries during early (and sometimes late) phases of moto r responses invites re-examination of traditional assumptions about sensori motor functional neuroanatomy, as well as the role of alpha ERD as an index of cortical activation. We agree with others that the somatotopic represen tations of different body parts overlap more than previously thought. Also, unilateral limb movements may be associated with both contralateral and ip silateral activation of sensorimotor cortex. We conjecture that alpha ERD m ay reflect activity within a broad synaptic network with distributed cortic al representations.