Human Brain Mapping Abstract (June 9-15, 2006)

Co-modulation of spectral power between maximally independent brain sources

Julie Onton and Scott Makeig

EEG data recorded at scalp electrodes is highly correlated between channels. This phenomenon is due to single sources projecting to multiple electrodes, as well as some smearing of the electric field by the skull and scalp. Using independent component analysis (ICA), these mixed signals can be separated into EEG sources whose locations in the brain can be approximated. The resulting signals are mostly uncorrelated and the scalp projections unique. As a novel exploration of the spectral modulation of these signal over time, we decomposed overlapping 1-sec epochs from continuous data collected during an emotional imagery task using a fourier transform. These data were then submitted to ICA which returned, for each subject, 15 spectral modulation factors or patterns, which included a power spectrum template for each IC included in the analysis. These spectral co-modulation factors revealed 4 major patterns that were reasonably preserved across subjects: 1) alpha and harmonic modulation at the peak frequency, 2) a shift up in alpha peak frequency, 3) a shift down in alpha peak frequency and 4) a broad shift in 20-40 Hz activity. The alpha modulations and co-modulations were largely posterior, with peak frequency modulations being mostly confined to the occipito-parietal junction. The alpha shifts occurred in occipito-parietal as well as somato-motor regions. High frequency power shifts were the most widespread with extensive co-modulation between all major regions of the brain including occipito-parietal, somato-motor and frontal areas. The results indicate that independent brain sources modulate oscillatory power and frequency in non-phase-locked unison with other components, suggesting an influence of modulator neurotransmitters projecting to specific combinations of brain regions simultaneously. The release of particular neurotransmitters in conjunction with current synaptic input, and likely myriad other factors, may cause predictable shifts in local field oscillations, thus drawing physically separated regions of the brain into relative unison. The advantage of spectral co-modulation can only be speculated at this point, but may contribute to enhanced synaptic communication between the co-modulated regions.