Spatial characterization of interictal high frequency oscillations in epileptic neocortex

Area of Science:

• Neuroscience
• Epileptology
• Biomarkers

Background:

• Interictal high frequency oscillations (HFOs), especially fast ripples (>200 Hz), are potential biomarkers for epileptic cortex and seizure generation.
• Understanding the spatial characteristics of HFOs is crucial for their accurate interpretation in epilepsy.

Purpose of the Study:

• To investigate the spatial extent, classification, and distribution of interictal high frequency oscillations (HFOs) in the human neocortex.
• To determine the relationship between HFOs and epileptiform discharges.
• To differentiate the role of focal versus large-scale HFOs.

Main Methods:

• Utilized a dense 2D microelectrode array (4x4 mm²) implanted in the neocortex of epilepsy surgery patients.
• Analyzed the spatial distribution and morphology of detected HFOs.
• Correlated HFO events with interictal epileptiform discharges and microdischarges.

Main Results:

• The vast majority (97%) of detected oscillations were fast ripples, concentrated in few recording sites.
• While most HFOs were focal (single channel), ~10% occurred on a larger spatial scale across multiple channels.
• Large-scale HFOs were frequently associated (80%) with interictal epileptiform discharges, unlike focal HFOs or microdischarges.

Conclusions:

• Large-scale HFOs, rather than focal events, are likely the substrate detected by clinical depth electrodes.
• Fast ripples may serve as markers of a secondary local response rather than initiators of epileptiform activity.
• HFOs are commonly associated with widespread interictal epileptiform discharges.