  Objective: Identifying abnormalities in interictal intracranial EEG, by
comparing patient data to a normative map, has shown promise for the
localisation of epileptogenic tissue and prediction of outcome. The approach
typically uses short interictal segments of around one minute. However, the
temporal stability of findings has not been established.
  Methods: Here, we generated a normative map of iEEG in non-pathological brain
tissue from 249 patients. We computed regional band power abnormalities in a
separate cohort of 39 patients for the duration of their monitoring period
(0.92-8.62 days of iEEG data, mean 4.58 days per patient, over 4,800 hours
recording). To assess the localising value of band power abnormality, we
computed DRS - a measure of how different the surgically resected and spared
tissue were in terms of band power abnormalities - over time.
  Results: In each patient, band power abnormality was relatively consistent
over time. The median DRS of the entire recording period separated seizure free
(ILAE = 1) and not seizure free (ILAE > 1) patients well (AUC = 0.69). This
effect was similar interictally (AUC = 0.69) and peri-ictally (AUC = 0.71).
  Significance: Our results suggest that band power abnormality DRS, as a
predictor of outcomes from epilepsy surgery, is a relatively robust metric over
time. These findings add further support for abnormality mapping of
neurophysiology data during presurgical evaluation.
