Abstracts

Rationale: A deeper understanding of cortical mechanisms of language production is dependant upon a delineation of the pathways that connect it’s distributed substrates. Cortical stimulation with single pulses can evoke remote responses that in turn can be related to tractography data collected on the same individuals. Methods: Three patients with pharmacologically resistant epilepsy and Wada testing proven left hemispheric dominance, scheduled for left hemispheric SDE implantation were enrolled in the study. T1 anatomical MR images obtained on a 3T scanner was used to construct mesh models of the gray/white matter interface and pial surface (Dale et al., 1999). A CT scan obtained following electrode implantation was co-registered to the pre-MR with a rigid-body transformation algorithm using a mutual information cost function (Cox 1996). The non-linear deformation of the cortical surface post implant, was corrected by creating a smoothed envelope to fit the shape and size of the subject's brain. SDEs were realigned to this new surface by displacing them along a surface normal to the point on the envelope closest to their location on the deformed brain surface. Adjoining pairs of SDEs localized using cortical stimulation mapping (dysnomia) over Wernicke’s area were stimulated in a bipolar fashion using single pulses (10 mA, 500 μs, 1 Hz for 50 seconds) using a Grass Stimulator. Concurrent electro-corticography (ECoG) at 1000 Hz was carried out using NeuroFax (Nihon Kodon) hardware. After these recording sessions the data was exported for further analysis in MATLAB. All channels are referenced to an average of intra-cranial electrodes that did not demonstrate any evoked response. The selected time locked epochs were averaged and the maximum of the evoked response was graded for visualization on the pial mesh. Results: In all patients, stimulation of electrode pairs over Wernicke’s area resulted in CCEP responses noted in sensori motor cortex (M1 mouth and M1 larynx), the fusiform gyrus (coinciding with basal temporal language sites) and the inferior parietal lobule. In one case, CCEPs were noted over electrodes were placed over Broca’s area. Responses at distant sites coincide with the expected connections of Wernicke’s area and reveal a disseminated language network. Comparison of these CCEP data with DTI data also obtained in the same individuals shows broadly coinciding connectivity patterns. Conclusions: Cortical stimulation mapping (CSM) using artificially delivered currents at 50 Hz, between 1 and 15 mA intensity, is a routinely used strategy for localizing essential brain regions that are then preserved during resective procedures for epilepsy and brain tumors. An assumption made in using this technique is that the neuronal perturbation produced by these currents is spatially specific. These preliminary results suggest that CSM produces a widespread disruption, beyond the site of stimulation. They also suggest that the pattern of this disruption may be predictable based on the structural connectivity of the region being stimulated. Combined CCEP-DTI techniques provide insight into anatomico-physiologic connectivity of the language system.