Prospects for mobile, high-definition brain imaging: EEG spectral modulations during 3-D reaching

Scott Makeig¹, Julie Onton¹, Terrence J. Sejnowski^1,2, and Howard Poizner¹

¹Institute for Neural Computation, University of California San Diego

²Howard Hughes Medical Institute

INTRODUCTION: Unlike other neuroimaging modalities, subjects in electroencephalographic (EEG) experiments are not required to remain rigidly still during the recording. Using currently available recording methods, subjects may make considerable movements without negatively impacting the resulting EEG data. The present experiment was designed to discover the EEG dynamics associated with specific directed movements in 3-D space.

METHODS: Subjects were seated in the dark facing three small LEDs lights, one aligned with the right shoulder, one with the right sternum and the third with the left shoulder. When one of the three LEDs brightened, subjects reached out to touch it with the tip of their right index finger. Between reaches they withdrew their hand to one of two starting positions, either resting on their right thigh (horizontal) or near their right shoulder (vertical). Arm movements were tracked by motion capture using infrared LEDs attached to their fingertip, wrist, elbow and shoulder. EEG data were analyzed using a novel single-trial analysis that revealed independent log spectral modulations (IMs) across multiple EEG source domains. IM decomposition identified separate portions of trial log spectrograms that co-varied across trials.

RESULTS: IMs whose trial weights differed according to reach direction and/or starting position included tonic shifts in gamma activity in frontal and parietal EEG sources up to the Nyquist frequency (125 Hz) as well as early low-frequency bursts (~3 Hz) in pre-motor areas differentiated between horizontal and vertical starting position. Reaches to right as opposed to left targets was defined by activity in parietal reach regions and/or inferior frontal cortex comprising (~3-Hz) low-frequency bursts or changes across broad bands of spectral power (5-100 Hz). Some EEG activity patterns were specifically time-locked to movement onsets and offsets, suggesting that some EEG source activities reflect moment-to-moment changes in cortical motor planning and decision-making.

DISCUSSION: These results suggest that mobile, high-definition, high-bandwidth dynamic brain imaging of physically active subjects is now possible using EEG, allowing studies of previously unexplored brain mechanisms of motor function and active cognition.