Scalp EEG is not a Blur: It Can See High Frequency Oscillations Although Their Generators are Small
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High frequency oscillations (HFOs) are emerging as biomarkers of epileptogenicity. They have been shown to originate from small brain regions. Surprisingly, spontaneous HFOs can be recorded from the scalp. To understand how is it possible to observe these small events on the scalp, one avenue is the analysis of the cortical correlates at the time of scalp HFOs. Using simultaneous scalp and intracranial recordings of 11 patients, we studied the spatial distribution of scalp events on the cortical surface. For typical interictal epileptiform discharges the subdural distributions were, as expected, spatially extended. On the contrary, for scalp HFOs the subdural maps corresponded to focal sources, consisting of one or a few small spatial extent activations. These topographies suggest that small cortical areas generated the HFOs seen on the scalp. Similar scalp distributions corresponded to distinct distributions on a standard 1 cm subdural grid and averaging similar scalp HFOs resulted in focal subdural maps. The assumption that a subdural grid “sees” everything that contributes to the potential of nearby scalp contacts was not valid for HFOs. The results suggest that these small extent events are spatially undersampled with standard scalp and grid inter-electrode distances. High-density scalp electrode distributions seem necessary to obtain a solid sampling of HFOs on the scalp. A better understanding of the influence of spatial sampling on the observation of high frequency brain activity on the scalp is important for their clinical use as biomarkers of epilepsy.
KeywordsHigh frequency oscillations Scalp HFO Ripple Simultaneous scalp and subdural EEG Spatial sampling
We thank Dr. Federico Melani for his help in the identification of interictal epileptiform discharges. We thank Francois Tadel and Dr. Sylvain Baillet for their suggestions on how to implement brainstorm for our simulations. We are grateful to Dr. Matthias Dümpelmann for his help in obtaining the data. This project was founded in part by DAAD short-term scholarship, the Savoy Foundation, and grant MOP-102,710 from the Canadian Institutes of Health Research. JJ was supported by the German Research Foundation (grant JA 1,725/2-1).
- Brodbeck V, Spinelli L, Lascano AM, Wissmeier M, Vargas MI, Vulliemoz S, Pollo C, Schaller K, Michel CM, Seeck M (2011) Electroencephalographic source imaging: a prospective study of 152 operated epileptic patients. Brain 134(Pt 10):2887–2897. doi: 10.1093/brain/awr243 PubMedCentralPubMedCrossRefGoogle Scholar
- Gambardella A, Palmini A, Andermann F, Dubeau F, Da Costa JC, Quesney LF, Andermann E, Olivier A (1996) Usefulness of focal rhythmic discharges on scalp EEG of patients with focal cortical dysplasia and intractable epilepsy. Electroencephalogr Clin Neurophysiol 98(4):243–249PubMedCrossRefGoogle Scholar
- Lai Y, van Drongelen W, Ding L, Hecox KE, Towle VL, Frim DM, He B (2005) Estimation of in vivo human brain-to-skull conductivity ratio from simultaneous extra- and intra-cranial electrical potential recordings. Clin Neurophysiol 116(2):456–465. doi: 10.1016/j.clinph.2004.08.017 PubMedCrossRefGoogle Scholar