Cortical Substrates of Scalp EEG Epileptiform Discharges

Abstract

Scalp EEG is an essential component of epilepsy presurgical evaluation during the lateralization and localization of epileptogenic focus. Scalp EEG epileptiform discharges may either guide direct surgical intervention or provide necessary information to further localize the epileptic focus with intracranial EEG recording. Despite the importance and widespread use of scalp EEG epileptiform discharges, the cortical EEG substrates underlying these spikes and seizure discharges are mostly speculative. Misconceptions are therefore prevalent regarding the necessary cortical area, synchrony, and amplitude required to generate those that are recordable at the scalp. Using contemporary EEG recording techniques such as simultaneous scalp and intracranial EEG recording, the authors' recent studies have shown that the cortical area of epileptiform discharges required for the scalp recording is considerably larger than commonly thought. A cortical area of 10 to 20 cm is often required to generate a scalp recognizable interictal spike or ictal rhythm. Sufficient cortical source area and synchrony are mandatory factors for the corresponding scalp EEG epileptiform recording. The amplitude is primarily dependent on source area and synchrony; therefore it is a less important factor. The authors review the previous literatures in conjunction with their recent investigations on this topic.

fNIRS-EEG study of focal interictal epileptiform discharges

“…As expected, the EEG-fNIRS approach encountered difficulties with MTLE cases. Even if temporal IEDs detected on scalp EEG meant that IEDs from mesial structures had projected to  6-10 cm 2 of temporal neocortex (Cooper et al, 1965;Tao et al, 2007), we did not detect significant and specific temporal neocortical activations in most cases.…”

Automatic detection of fast oscillations (40–200Hz) in scalp EEG recordings

“…A much larger area of synchronized cortex (at least 6-9 cm 2 ) has been reported to be required for interictal spikes to be visible on the scalp (Tao et al, 2007). It is important to note that the skull and other tissues between the brain and the scalp do not attenuate the frequencies up to several KHz any more than the standard EEG frequencies below 40 Hz (Hämäläinen et al, 1993, Oostendorp et al, 2000.…”

Remission of benign epilepsy with rolandic spikes: An EEG-based connectivity study at the onset of the disease and at remission

“…Sharp waves generated within small cortical areas are not detectable by scalp EEG (Tao et al, 2007) but may alter EEG dynamics. Furthermore, we are going to learn that "EEG background activity without epileptiform events" is perhaps a fiction at the submillimeter scale (Schevon et al, 2010).…”