In 36 epilepsy patients, monopolar far-contact SEEG correlation was positive, while bipolar near pairs were not more correlated than far pairs

This analysis examined zero-lag Pearson correlation between stereo-EEG (SEEG) contacts in 36 patients with epilepsy, using one 20-second interictal window (seconds 60–80) per patient. It compared the recorded monopolar reference (MONO), a common average reference (CAR), and adjacent-contact bipolar derivations (BIP) across delta, theta, alpha, beta, gamma, and high-frequency activity (HFA, 70–150 Hz). Distances were measured in fsaverage template space. Far pairs were on different shafts and more than 40 mm apart; near pairs were on the same shaft and 3–12 mm apart. In all six bands, MONO far-pair correlation exceeded 0.1 (Holm-adjusted sign-flip test p = 3×10⁻⁵); median far r ranged from 0.219 in beta to 0.528 in HFA, and was positive in every subject with a result. CAR far-pair correlation was negative and had lower absolute magnitude than MONO in all six bands (mean CAR far r −0.05 to −0.10; Holm-adjusted p ≤ 0.009). BIP reduced absolute far-pair correlation relative to MONO in all six bands (mean difference 0.26–0.53; Holm-adjusted p = 3×10⁻⁵), with median far r near zero. The hypothesis that BIP near pairs were more correlated than far pairs was not supported: the difference was about zero in delta and theta (p = 0.84 and 0.96) and negative in beta, gamma, and HFA (mean difference −0.03 to −0.08; Holm-adjusted p ≤ 6×10⁻⁵). Exploratory delta-band MONO far correlation remained about 0.33–0.36 from 60 to 100 mm; the MONO far-to-near ratio was highest in HFA (mean 0.63). These results are consistent with a shared reference component in far MONO correlations and show that, in these data and at the tested spacing, BIP did not make near pairs more correlated than far pairs. Limits include one short window per patient, epilepsy patients, no exclusion of seizure-onset-zone contacts, unknown hardware references, template-space distances, and zero-lag correlations.

stereo-EEG · SEEG · reference scheme · monopolar reference · common average reference · bipolar derivation · zero-lag correlation · interictal · epilepsy · signal correlation