Functional mapping of human sensorimotor cortex with electrocorticographicspectral analysis - II. Event-related synchronization in the gamma band

It has been shown in animals that neuronal activity in the 'gamma band' (>3 0 Hz) is associated with cortical activation and may play a role in multi-r egional and multi-modal integration of cortical processing. Studies of gamm a activity in human scalp EEG have typically focused on event-related synch ronization (ERS) in the 40 Hz band. To assess further the gamma band ERS fu rther, as an index of cortical activation and as a tool for human functiona l brain mapping, we recorded subdural electrocorticographic (ECoG) signals in five clinical subjects while they performed visual-motor decision tasks designed to activate the representations of different body parts in sensori motor cortex. ECoG spectral analysis utilized a mixed-effects analysis of v ariance model in which within-trial temporal dependencies were accounted fo r. Taking an exploratory approach, we studied gamma ERS in 10-Hz-wide bands (overlapping by 5 Hz) ranging from 30 to 100 Hz, and compared these findin gs with changes in the alpha (8-13 Hz) and beta (15-25 Hz) bands. Gamma ERS (observed in three out of subjects) occurred in two broad bands-'low gamma ' included the 35-45 and 40-50 Hz bands, and 'high gamma' the 75-85, 80-90, 85-95 and 90-100 Hz bands. The temporal and spatial characteristics of low and high gamma ERS were distinct, suggesting relatively independent neurop hysiological mechanisms. Low gamma ERS often began after onset of the motor response and was sustained through much of it, in parallel with event-rela ted desynchronization (ERD) in the alpha band. High gamma ERS often began d uring, or slightly before, the motor response and was transient, ending wel l before completion of the motor response. These temporal differences in lo w and high gamma suggest different functional associations with motor perfo rmance. Compared with alpha and beta ERD, the topographical patterns of low and high gamma ERS were more discrete and somatotopically specific and onl y occurred over contralateral sensorimotor cortex during unilateral limb mo vements (alpha and beta ERD were also observed ipsilaterally). Maps of sens orimotor function inferred from gamma ERS were consistent with maps generat ed by cortical electrical stimulation for clinical purposes. In addition, d ifferent task conditions in one subject produced consistent differences in both motor response latencies and onset latency of gamma ERS, particularly high gamma ERS. Compared with alpha and beta ERD, the topography of gamma E RS is more consistent with traditional maps of sensorimotor functional anat omy. In addition, gamma ERS may provide complementary information about cor tical neurophysiology that is useful for mapping brain function in humans.