  Identifying abnormal electroencephalographic activity is crucial in diagnosis
and treatment of epilepsy. Recent studies showed that decomposing brain
activity into periodic (oscillatory) and aperiodic (trend across all
frequencies) components may illuminate drivers of changes in spectral activity.
  Using iEEG data from 234 subjects, we constructed a normative map and
compared this with a separate cohort of 63 patients with refractory focal
epilepsy being considered for neurosurgery. The normative map was computed
using three approaches: (i) relative complete band power, (ii) relative band
power with the aperiodic component removed (iii) the aperiodic exponent.
Corresponding abnormalities were also calculated for each approach in the
separate patient cohort. We investigated the spatial profiles of the three
approaches, assessed their localizing ability, and replicated our findings in a
separate modality using MEG.
  The normative maps of relative complete band power and relative periodic band
power had similar spatial profiles. In the aperiodic normative map, exponent
values were highest in the temporal lobe. Abnormality estimated through the
complete band power robustly distinguished between good and bad outcome
patients. Neither periodic band power nor aperiodic exponent abnormalities
distinguished seizure outcome groups. Combining periodic and aperiodic
abnormalities improved performance, similar to the complete band power
approach.
  Our findings suggest that sparing cerebral tissue that generates
abnormalities in either periodic or aperiodic activity may lead to a poor
surgical outcome. Both periodic and aperiodic abnormalities are necessary to
distinguish patient outcomes, with neither sufficient in isolation. Future
studies could investigate whether periodic or aperiodic abnormalities are
affected by the cerebral location or pathology.
