In eight SEEG patients, responses were about 9% larger near the second-named contact when contact distances differed by at least 2 mm

We used the archived per-channel results of publication 10df599c (OpenNeuro ds005448, SEEG patients STREEF06 to STREEF13, single pulses given in both current directions at the same contact pair). For each responsive bipolar recording channel (1262 channels, 100 pairs), we compared log10(RMS s1-s2 / RMS s2-s1), 15-100 ms, with the distance difference to the two stimulated contacts. We removed the gain change that is the same for all channels of a pair. Result: the response was larger when the nearer contact was the second contact in the site label. Spearman r was +0.149 (7 of 8 patients > 0, exact sign-flip p = 0.016). For channels where the two contacts are at least 2 mm different in distance, the RMS ratio was 0.92 (8 of 8 patients, p = 0.0078). The effect was at 0-20 mm (p = 0.023) and not at more than 20 mm. We did 7 tests without correction, and no single test passes a Bonferroni limit; the result is the consistent sign. The dataset does not state which contact is the cathode. If the second contact is the cathode, the result agrees with lower cathodal thresholds. A slow artifact tail and block drift can cause part of the effect. For BCI stimulation, polarity selection gives a small (approximately 10%) and local steering of the response.

SEEG · CCEP · intracranial stimulation · stimulation polarity · brain-computer interface · epilepsy · evoked response · contact distance