Local spatial effect of 50 Hz cortical stimulation in humans

Cortical stimulation to abort seizures is under human investigation. Ideal electrode placement and stimulating parameters are unknown with poor understanding of tissue volume affected by stimulation or duration and nature of its effect on cortical activity. To help characterize this effect, we analyzed electrocorticography (ECoG) recorded adjacent to stimulated contacts during and after bipolar stimulation in patients undergoing functional cortical mapping with subdural electrodes.

We analyzed four functional mapping procedures in three patients. One row of contacts was chosen for bipolar stimulation at sequential distances. Stimulation parameters were those used for functional mapping. Pooled Teager energy (TE) and band power were calculated for: (1) baseline, (2) 5 s during stimulation, and (3) 5-15 s after the stimulus.

Average TE increased during stimulation, falling with distance from the stimulus. Average poststimulus TE increased (284-905%) compared to baseline. Increased TE was observed: (1) up to 10 s after stimulation, (2) stimulation amplitudes of 4 mA or greater, and (3) up to 2 cm from the stimulus. There was no difference in poststimulus TE between the stimulated pair of contacts and outside the pair. Greatest increase in poststimulus signal power occurred in beta and gamma bands.

Human cortical stimulation of 50 Hz resulted in elevated ECoG energy measurements up to 10 s poststimulation. Contacts >2 cm from stimulated electrodes did not show significant response to stimulation. Separating contacts >2 cm on the cortical surface may not result in efficacious treatment of seizure activity using common stimulation amplitudes (2-10 mA).