Electrophysiologic Recordings Directly from the Surface of the Human Brain:
Lateral Coherence Patterns
Vernon L. Towle, Department of Neurology, The University of Chicago Hospitals
Electrophysiologic recordings recorded directly from the cortex of patients with chronically implanted subdural electrodes make it possible to map the functional organization of human cortex. Functional mapping is possible using direct cortical stimulation, sensory evoked potentials, and electrocorticography (ECoG). The analysis of ECoG coherence patterns may be helpful in identifying the epileptogenic regions in cortical multifocal epilepsy. Analysis of interictal ECoG coherence can reveal cortical areas that are functionally distinct from patent cortex.
Subdural ECoGs have been analyzed from 30 medically refractive pediatric epileptic patients as part of their routine surgical work-up. Recording arrays were implanted over the frontal, parietal, occipital or temporal lobes for 4-10 days, depending on the patient's seizure semiology and imaging studies. Segments of interictal ECoG ranging in duration from 5 s to 45 min were examined for areas of increased power and increased local coherence. Ictal records were examined to identify the stages and spread of the seizures.
When computed over relatively long periods (45 min) coherence patterns were found to be highly stable (r = 0.97, p < .001), and only changed gradually over days. On the other hand, when calculated over short periods of time (5 sec) coherence patterns were highly dynamic. Coherence patterns revealed a rich topography, with reduced coherence across sulci and major fissures. As has been reported by others, coherence increased during the ictal and post-ictal periods, especially within the epileptic regions.
Our findings suggest that analysis of coherence patterns can supplement visual inspection of conventional records to help identify pathological regions of cortex. With further study, it is possible that analysis of coherence patterns, combined with nonlinear analyses may allow us to determine the boundaries of epileptic foci based on interictal recordings, possibly obviating the need for extended monitoring.
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