The overall objective of the proposed research is to identify and validate electrophysiological signatures of human cortical processing and to use them to study the neural mechanisms of motor, sensory, arid language functions. Previous EEG and MEG studies have linked regional cortical activation with event-related changes in the energy of alpha (8-13 Hz) and gamma (40 Hz) oscillations. Using electrocorticographic (ECoG) recordings from subdural electrodes implanted for clinical purposes, we have confirmed these observations and discovered an event-related broad-band augmentation of power in high gamma frequencies (80-100 Hz). This novel index of function cortical activation is more discretely localized in space and time, and more consistent with functional-anatomic and neurophysiological (ERP) correlates, than previously described spectral changes. Gamma oscillations have been associated with the synchronization of neuronal assemblies during cortical processing. In contrast, suppression of alpha oscillations is thought to arise from the activation of thalamocortical circuits that otherwise gate cortical processing. We therefore propose to test the following hypotheses: (1) that broad-band """"""""high"""""""" gamma ERS indexes task-specific cortical processing, and that (2) alpha desynchronization facilitates this processing via thalamocortical circuits.
Our specific aims are directed toward testing the (A) spatial and (B) temporal correspondence between these spectral indices of cortical activation and the following benchmarks: (i) general knowledge of the functional anatomy of perceptual, motor, and language tasks, (ii) specific functional-anatomic information derived from cortical stimulation mapping for clinical purposes, (iii) the latency and duration of perceptual stimuli and motor responses, and (iv) spatiotemporal properties of event-related potentials. The generalizability of our conclusions from these ECoG studies will be tested in a group of normal subjects using high density scalp EEG. Information derived from this research will facilitate future investigations of the neurophysiological correlates of human brain activation, including studies of the timing and topography of normal and disordered cognitive/neuronal operations.
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