Ictal infraslow activity in stereoelectroencephalography: Beyond the "DC shift"

Area of Science:

• Neuroscience
• Epileptology
• Clinical Neurophysiology

Background:

• The role of infraslow activity (ISA) in focal epilepsies remains unclear.
• Recent studies suggest ictal ISA is more widespread than previously thought.
• Stereoelectroencephalography (SEEG) analysis can elucidate the localizing value of ISA.

Purpose of the Study:

• To analyze the focal versus widespread expression of ISA using SEEG.
• To clarify the localizing value of ISA in focal epilepsies.
• To understand the spatiotemporal characteristics of ISA during seizures.

Main Methods:

• Retrospective analysis of ictal SEEG recordings from fifteen adult patients.
• Application of conventional (1.6-70 Hz) and infraslow (0.01-0.1 Hz) bandpass filters.
• Seizure averaging in the infraslow band and wavelet analysis for time-frequency quantification.

Main Results:

• All analyzed seizures exhibited invariant ISA.
• ISA displayed biphasic power peaks at ictal onset and offset, most prominent in conventionally implicated structures.
• ISA showed association with low voltage fast activity and a more restricted field than conventional activity.

Conclusions:

• ISA is both widespread anatomically and focal in its electric field, defining a spatiotemporal signature for seizures.
• ISA likely arises from network processes within wideband ictal dynamics.
• Further research is needed to relate ISA findings to the clinical definition of the epileptogenic zone.