Abstracts

Rationale: Sensorimotor tasks often elicit attenuation of beta activity along with augmentation of high-gamma activity on electrocorticography (ECoG); thus, one can hypothesize that event-related beta attenuation reflects in situ cortical activation. The potential advantage of measurement of beta attenuation over that of high-gamma augmentation, for the purpose of functional brain mapping, is its smaller susceptibility to artifacts of muscular origin on scalp EEG recording. Here, we generated a four-dimensional functional map of the visual cortex, based on beta amplitudes modulated by full-field visual stimulation, and determined if event-related beta activity can provide the spatial-temporal profiles of neural activation/deactivation as similar to those suggested by event-related high-gamma activity.  Methods: We studied a series of 63 patients with drug-resistant extra-occipital epilepsy. During chronic ECoG recording in a darkened room, a series of 50 full-field stroboscopic flash stimuli were given to each patient with a frequency of 0.5 Hz, at a distance of 30 cm from the closed eyes. At each electrode site, we measured the amplitude change at beta band (20-30 Hz) in steps of 10 Hz and 5 ms, relative to the mean amplitude in the baseline period between 200 and 100 ms prior to stimulus. Event-related potentials (ERPs) were also measured by averaging ECoG signals time-locked to stimulus onset. The percent change in beta amplitude was sequentially animated on the FreeSurfer standard surface image as a function of time. Also by calculating event-related beta amplitude after removing ERPs from each single trial time series, we delineated event-related beta modulation not attributable to the effect of ERPs.  Results: Intense beta augmentation diffusely involved bilateral lingual and cuneus gyri within 60 ms following stimulus onset, and also largely involved the lateral occipital and fusiform gyri. Beta augmentation in these gyri lasted for 600 ms or longer. Time-frequency analysis following removal of ERPs showed that bilateral lingual and cuneus gyri showed beta attenuation between 100 and 200 ms and augmentation between 300 and 600 ms after onset.  Conclusions: Measurement of beta activity modulated by full-field flash stimuli failed to delineate the spatial-temporal neural modulation profiles similar to those suggested by event-related high-gamma activity. ERP components contributed to a substantial proportion of early beta augmentation; therefore, it was not feasible to assume that the degree of beta attenuation is linearly correlated to the underlying activation of lower-order visual areas. Funding: Grant support: R01NS064033 to E. Asano.