Neonatal EEG at scalp is focal and implies high skull conductivity in realistic neonatal head models Maryam Odabaee a , Anton Tokariev b,g , Siamak Layeghy a , Mostefa Mesbah c,d , Paul B. Colditz a , Ceon Ramon e,f , Sampsa Vanhatalo a,b, ⁎ a The University of Queensland Centre for Clinical Research (UQCCR) and Perinatal Research Centre, Royal Brisbane & Women's Hospital, Queensland, Australia b Department Clinical Neurophysiology, Children's Hospital, Helsinki University Central Hospital and University of Helsinki, Helsinki, Finland c School of Computer Science and Software Engineering, The University of Western Australia, Crawley, Western Australia, Australia d Department of Electrical and Computer Engineering, Sultan Qaboos University, Muscat, Oman e Department of Electrical Engineering, University of Washington, Seattle, WA, USA f Institute of Biomedical and Neural Engineering, Reykjavik University, Reykjavik, Iceland g Department of Biosciences and Neuroscience Center, University of Helsinki, Helsinki, Finland abstract article info Article history: Accepted 4 April 2014 Available online 13 April 2014 Keywords: Neonatal EEG High density EEG Source localization Head model The potential improvements in spatial resolution of neonatal EEG used in source localization have been chal- lenged by the insufficiencies in realistic neonatal head models. Our present study aimed at using empirical methods to indirectly estimate skull conductivity; the model parameter that is known to significantly affect the behavior of newborn scalp EEG and cause it to be markedly different from that of an adult. To this end, we used 64 channel EEG recordings to study the spatial specificity of scalp EEG by assessing the spatial decays in focal transients using both amplitudes and between-c'hannels linear correlations. The findings showed that these amplitudes and correlations decay within few centimeters from the reference channel/electrode, and that the nature of the decay is independent of the scalp area. This decay in newborn infants was found to be ap- proximately three times faster than the corresponding decay in adult EEG analyzed from a set of 256 channel re- cordings. We then generated realistic head models using both finite and boundary element methods along with a manually segmented magnetic resonance images to study the spatial decays of scalp potentials produced by sin- gle dipole in the cortex. By comparing the spatial decays due to real and simulated EEG for different skull conduc- tivities (from 0.003 to 0.3 S/m), we showed that a close match between the empirical and simulated decays was obtained when the selected skull conductivity for newborn was around 0.06–0.2 S/m. This is over an order of magnitude higher than the currently used values in adult head modeling. The results also showed that the neonatal scalp EEG is less smeared than that of an adult and this characteristic is the same across the entire scalp, including the fontanel region. These results indicate that a focal cortical activity is generally only registered by electrodes within few centimeters from the source. Hence, the conventional 10 to 20 channel neonatal EEG acquisition systems give a significantly spatially under sampled scalp EEG and may, con- sequently, give distorted pictures of focal brain activities. Such spatial specificity can only be reconciled by appre- ciating the anatomy of the neonatal head, especially the still unossified skull structure that needs to be modeled with higher conductivities than conventionally used in the adults. © 2014 Elsevier Inc. All rights reserved. Introduction Recent advances in developmental neuroscience as well as in medi- cal care of preterm and ill infants have significantly increased the inter- est in functional brain assessment. Brain activity in babies is most reliably recorded with neonatal EEG. It is now known, however, that the conventional recording configuration with only 6–10 electrodes (André et al., 2010) does hardly suffice to distinguish brain lobes from each other, making its spatial information content severely compro- mised (Grieve et al., 2004; Odabaee et al., 2013 see also Zwiener et al., 1991). A better spatial parcellation has been recently attempted by de- vising various means to record high density EEG (hdEEG) from the neo- natal head in the laboratory environment (Fifer et al., 2006; Grieve et al., 2008; Odabaee et al., 2012; Roche-Labarbe et al., 2008), and even in the neonatal intensive care units (Stjerna et al., 2012; Vanhatalo et al., 2008; Welch et al., 2013). Increasing the number of recording electrodes leads to clear theoret- ical benefits, including recognition of cerebral activities that may, other- wise, go unnoticed or unlocalized. Most importantly, higher electrode number (ie. increased spatial sampling) opens a possibility for genuine NeuroImage 96 (2014) 73–80 ⁎ Corresponding author at: Department of Clinical Neurophysiology, Children's Hospital, Helsinki University Hospital, PO Box 280, FIN-00029 HUS, Finland. E-mail address: sampsa.vanhatalo@helsinki.fi (S. Vanhatalo). http://dx.doi.org/10.1016/j.neuroimage.2014.04.007 1053-8119/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/ynimg