Author's School

School of Engineering & Applied Science

Author's Department/Program

Biomedical Engineering

Language

English (en)

Date of Award

January 2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Eric Leuthardt

Abstract

High-gamma band: >60Hz) power changes in cortical electrophysiology are a reliable indicator of focal, event-related cortical activity. In spite of discoveries of oscillatory subthreshold and synchronous suprathreshold activity at the cellular level, there is an increasingly popular view that high-gamma band amplitude changes recorded from cellular ensembles are the result of asynchronous firing activity that yields wideband and uniform power increases. Others have demonstrated independence of power changes in the low- and high-gamma bands, but to date, no studies have shown evidence of any such independence above 60Hz. Based on non-uniformities in time-frequency analyses of electrocorticographic: ECoG) signals, we hypothesized that induced high-gamma band: 60-500Hz) power changes are more heterogeneous than currently understood. We quantified this spectral non-uniformity with two different approaches using single-word repetition tasks in human subjects. First, we showed that the functional responsiveness of different ECoG high-gamma sub-bands can discriminate cognitive tasks: e.g., hearing, reading, speaking) and cortical locations. Power changes in these sub-bands of the high-gamma range are consistently present within single trials and have statistically different time courses within the trial structure. Moreover, when consolidated across all subjects within three task-relevant anatomic regions: sensorimotor, Broca's area, and superior temporal gyrus), these behavior- and location- dependent power changes evidenced nonuniform trends across the population of subjects. Second, we studied the dynamics of multiple frequency bands in order to quantify the diversity present in the ECoG signals. Using a matched filter construct and receiver operating characteristic: ROC) analysis we show that power modulations correlated with phonemic content in spoken and heard words are represented diffusely in space, time and frequency. Correlating power modulation in multiple frequency bands above 60 Hz over broad cortical areas, with time varying envelopes significantly improved performed area under the ROC curve scores in phoneme prediction experiments. Finally we show preliminary evidence supporting our hypothesis in microarray ECoG data. Taken together, the nonuniformity of high frequency power changes and the information content captured in the spatio-temporal dynamics of those frequencies suggests that a new approach to evaluating high-gamma band cortical activity is necessary. These findings show that in addition to time and location, frequency is another fundamental dimension of high-gamma dynamics.

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Permanent URL: http://dx.doi.org/10.7936/K7W66HS5

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