Abstracts

Chronic intermittent cortical stimulation to abort seizures in humans is under investigation. Ideal electrode placement and stimulating parameters are unknown, largely due to a poor understanding of the volume of tissue affected by a stimulus, and the duration and nature of its effect on cortical activity. To help answer these questions, we have analyzed changes in ECoG activity adjacent to stimulated electrodes during and after stimulation in patients undergoing functional cortical mapping with subdural electrodes. Two patients undergoing intracranial electrode monitoring for seizure localization were chosen for analysis. During routine stimulation for mapping of language and motor function, one strip was chosen for bipolar stimulation at sequentially larger distance intervals between 1 and 10 cm between electrode contacts. ECoG was recorded from contacts not stimulated. Stimulation parameters were identical to those used for functional mapping: 50 hz, 0.2 ms pulse width, 2-10 mA, with 5 sec. duration. Average Teager energy was calculated for a baseline period 5 sec. prior to stimulation, a 3-5 sec. period during stimulation, and for 5 sec. intervals after the stimulus ended. Average energy was also calculated for delta, theta, alpha, beta, and gamma frequency bands. Teager energy was considerably increased in adjacent electrode contacts during stimulation. This increase fell as a function of distance from the stimulus. Average Teager energy was also increased (230-570%) compared to baseline during the first 5 second interval after stimulation (4, 6, 8, and 10 mA) in electrode contacts immediately adjacent to a stimulation site. The increases seen 1 cm from stimulated contacts were significantly higher than those at contacts 2 cm from stimulation (p[le]0.05). There was no difference between the response of contacts within the stimulated pair of contacts and those outside the bipolar pair. Bipolar stimulation of the human cortex at 50 Hz resulted in elevated ECoG energy measurements persisting [ge] 5 seconds after stimulus termination in adjacent electrodes. Contacts more than 1 cm from a bipolar stimulation electrode contact, inside or outside the bipolar pair, showed significantly smaller responses. Separating bipolar stimulating contacts [gt] 2 cm on the cortical surface may be of questionable benefit at common stimulation intensities (2-10 mA). Local changes in ECoG activity should also be measured at stimulation frequencies currently used in therapeutic trials to confirm cortical response profiles at those frequencies. (Supported by NIH R01 NS044102)