Abstracts

Rationale: Understanding topography of basic human brain functions is vital to performing safe cortical resections for medically refractory epilepsy and tumors. Direct electrocortical stimulation mapping (ESM) is the gold standard in defining eloquent cortex. Direct ESM has been most systematically documented intraoperatively. Intraoperative identification of eloquent cortex is time-limited, can be hampered by after discharges or seizures, and the ability to test beyond craniotomy borders is limited. Extraoperative ESM, the standard of care to identify eloquent cortex in multi-stage epilepsy surgeries, gives us insight through data acquired over an extended time period from sampling multiple sites. Despite this, systematic mapping on normalized coordinates of functional behavioral responses obtained extraoperatively has not been done. We report the first practical atlas for definition of eloquent cortex based on extraoperative ESM in normalized Montreal Neurological Institute (MNI) space and within subject gyral anatomy. Methods: Bedside ESM was performed on 100 subjects who underwent clinically-indicated continuous intracranial EEG monitoring for seizure localization. Each subject had extraoperative ESM to identify eloquent (motor, language) cortex. ESM was conducted using a NicoletOne Cortical Stimulator (constant current output, pulse width 500 microseconds, pulse frequency 50 Hertz, maximum train duration 5 seconds), for which stimulus was applied to electrode pairs and current was manually controlled with maximal threshold of 12 mA for subdural electrodes and 7 mA for depth electrodes. Subjects performed motor and sensory tasks, counting (continuous speech), visual and auditory naming, and auditory comprehension. Stimulation-elicited positive and negative findings were recorded and localized to MRI brain coordinates within subject gyral anatomy (freesurfer segmentation), then transformed to standard MNI coordinates. Language mapping results were subdivided into Speech Arrest, Comprehension, and Anomia. Motor mapping was subdivided into Face/Head, Mouth, and Upper Extremity. Data points across patients were discretized on the cortical surface using MNI coordinates to create surface probability maps for basic motor, sensory and language functions. Finally, probabilities were calculated for each gyral segment according to the within subject parcellation. Results: Speech Arrest was found in the Inferior Frontal Gyrus (IFG), pre and postcentral gyri, and throughout the Superior Temporal Gyri (STG; caudal, medial, and rostral). Comprehension was observed in the Supramarginal Gyri and Mesial Temporal Gyri (MTG; caudal, medial, rostral). Anomia, while more broadly distributed, showed a clear predilection for the rostral STG and MTG, greater than medial and caudal STG/MTG. Similar to intraoperative reporting, speech arrest occurred with stimulation of the IFG, but was also observed throughout the STG, showing individual variability between patients. Intraoperative literature identified anomia broadly in the regions surrounding the caudal STG; however, we localized it more to the rostral STG and MTG using more data points, including more posterior structures to a lesser extent. Unsurprisingly, Face/Head was observed primarily in the precentral and secondarily in the postcentral gyri, whereas both Mouth and Upper Extremity hits were more commonly seen in postcentral over precentral gyri. While these findings are in line with intraoperative literature, our subjects were tested over a larger cortical area including more tasks. Conclusions: This analysis provides the first practical atlas based on extraoperative ESM of motor and language functions in both normal MNI coordinates and within subject gyral anatomy. The authors provide an expected probability of inducing a motor or language positive finding with the goal of being able to reliably identify and outline essential cortex within subject anatomical percolation. The results provide a bedside rather than intraoperative based approach with differing results than previously reported in the intraoperative literature. We believe these results will aid clinicians in preoperative planning of eloquent cortex resection and interpretation of ESM findings. Funding: No funding