CHARACTERIZING THE LOCAL OSCILLATORY CONTENT OF SPONTANEOUS CORTICAL ACTIVITY DURING MENTAL-IMAGERY

We report on the determination of detailed spectra for simultaneously active sources of spontaneous neuronal activity in humans directly fro m data recorded with a whole-scalp 122-channel magnetometer array. Sub jects rested with eyes open and performed two contrasting mental image ry tasks: the imagination of the self-performance of a motor activity and the silent generation of a chain of words. A novel analysis techni que, frequency-domain signal-space projection (FDSSP) was utilized to determine the temporal and spectral characteristics of spontaneous bra in activity at specific cortical sites. Although intersubject differen ces were significant, spectra for individual subjects contained task-d ependent features which were reproducible over successive 20-s epochs. This result supports the concept of multiple sources of spontaneous c ortical activity and suggests that detailed spectra of localized oscil latory activity obtained non-invasively with magnetoencephalographic a rrays may provide a useful characterization of cortical involvement in mental imagery.