UC Berkeley

There is an increasing demand for source analysis of neonatal EEG, but currently there is inadequate knowl- edge about i) the spatial patterning of neonatal scalp EEG and hence ii) the number of electrodes needed to capture neonatal EEG in full spatial detail. This study addresses these issues by using a very high density (2.5 mm interelectrode spacing) linear electrode array to assess the spatial power spectrum, by using a high density (64 electrodes) EEG cap to assess the spatial extent of the common oscillatory bouts in the neo- natal EEG and by using a neonatal size spherical head model to assess the effects of source depth and skull conductivities on the spatial frequency spectrum.

The linear array recordings show that the spatial power spectrum decays rapidly until about 0.5–0.8 cycles per centimeter. The dense array EEG recordings show that the amplitude of oscillatory events decays within 4–6 cm to the level of global background activity, and that the higher frequencies (12–20 Hz) show the most rapid spatial decline in amplitude. Simulation with spherical head model showed that realistic variation in skull conductivity and source depths can both introduce orders of magnitude difference in the spatial fre- quency of the scalp EEG.

Calculation of spatial Nyquist frequencies from the spatial power spectra suggests that an interelectrode dis- tance of about 6–10 mm would suffice to capture the full spatial texture of the raw EEG signal at the neonatal scalp without spatial aliasing or under-sampling. The spatial decay of oscillatory events suggests that a full representation of their spatial characteristics requires an interelectrode distance of 10–20 mm.

The findings show that the conventional way of recording neonatal EEG with about 10 electrodes ignores most spatial EEG content, that increasing the electrode density is necessary to improve neonatal EEG source localization and information extraction, and that prospective source models will need to carefully consider the neonatally relevant ranges of tissue conductivities and source depths when source localizing cortical ac- tivity in neonates.