Research fellow Children’s Hospital of Michigan Novi, Michigan
Rationale: Invasive presurgical evaluation for patients with drug-resistant epilepsy includes the assessment of inter-regional effective connectivity, as rated by the latency and amplitude of neuronal responses to single-pulse electrical stimulation (SPES) via subdural or depth electrodes. Early components of cortico-cortical evoked potentials/spectral responses (CCEPs/CCSRs) are suggested to be useful measures to quantify the magnitude of direct effective connectivity. The present study initially determined the optimal depth electrode montages for the assessment of effective connectivity based on single-pulse electrical stimulation (SPES). We then investigated the effect of SPES locations on the extent of resulting neuronal propagations. We finally generated the four-dimensional tractography to animate the SPES-based neuronal propagations via an interhemispheric pathway. Methods: We initially reviewed the SPES literature published in 1989-2019 and identified the association between electrode configuration and montage. We studied the empirical intracranial EEG data from 14 epilepsy patients who underwent invasive monitoring with depth electrodes and measurement of cortico-cortical evoked potentials and spectral responses (CCEPs and CCSRs). We determined the effects of electrode montage and stimulus sites on the CCEP/CCSR amplitudes. Using the CCSR peak latency, we estimated the dynamics of SPES-related neuronal propagation via an interhemispheric pathway delineated on diffusion-weighted imaging (DWI) tractography. Results: Previous subdural SPES studies preferentially used referential montage, whereas those using depth electrodes preferred bipolar montage. Bipolar and Laplacian montages effectively reduced the degree of SPES-related signal deflections at extra-cortical levels, including outside the brain, while maintaining those at the cortical level. SPES of structures more proximal to the deep white matter, compared to the cortical surface, elicited greater CCEPs and CCSRs. The four-dimensional tractography successfully animated the dynamics of SPES-related neuronal propagations via the corpus callosum as well as the anterior commissure. Conclusions: On depth electrode recording, bipolar and Laplacian montages are suitable for measurement of near-field CCEPs and CCSRs. SPES of the white matter axons underlying the cortex may induce neuronal propagations extensively. The four-dimensional tractography may be useful to visualize the network dynamics supporting physiological function as well as those responsible for seizure propagation. Funding: Please list any funding that was received in support of this abstract.: NIH grant NS064033
