Interictal Infraslow Activity in Stereoelectroencephalography: From Focus to Network.
OBJECTIVE
Infraslow activity (ISA) occurring during the interictal state in focal epilepsy is largely unstudied. In this exploratory analysis, the authors aimed to characterize features of interictal ISA in a cohort of patients studied by stereoelectroencephography.


METHODS
The interictal stereoelectroencephography records for 15 consecutive adult patients were retrospectively analyzed, after application of both conventional (1.6-70 Hz) and infraslow (0.01-0.1 Hz) bandpass filters. Visual analysis was complemented by time-frequency analysis to quantify the change in ISA power over hours. Linear correlation coefficient (R) calculations were used to map interictal connectivity in the infraslow band.


RESULTS
Interictal ISA background fluctuations were present throughout the interictal state in all patients, manifesting as recurrent and stereotyped oscillations. These oscillations had an apparent modulatory effect on conventional-band activities and spikes ("spike-crested oscillations"). In the infraslow band, the correlations between electrode contacts were shown to have a stable structure over time.


CONCLUSIONS
Infraslow activity exists as a fundamental component of wideband cortical dynamics in focal epilepsy, with features suggestive of scale-free (1/f) dynamics: evidence of phase-amplitude coupling and functional connectivity in the infraslow band. Rather than viewed as a focal paroxysmal activity, interictal ISA may be better understood as a network process, although this requires further study.
*Department of Neurology, University of Texas Health Science Center, Houston, Texas, U.S.A.; and †Epilepsy Center, Neurological Institute, Cleveland Clinic, Cleveland, Ohio, U.S.A.
Stephen A
SA
Thompson
Balu
B
Krishnan
Jorge
J
Gonzalez-Martinez
Juan
J
Bulacio
Lara
L
Jehi
John
J
Mosher
Andreas
A
Alexopoulos
Richard C
RC
Burgess
D000328:Adult; D001921:Brain / Q000503:physiopathology*; D004569:Electroencephalography / Q000379:methods*; D004828:Epilepsies, Partial / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D012189:Retrospective Studies