Simultaneous head tissue conductivity and EEG source location estimation

• Published in: NeuroImage (Orlando, Fla.) Vol. 124; no. Pt A; pp. 168 - 180
• Main Authors: Akalin Acar, Zeynep, Acar, Can E., Makeig, Scott
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.01.2016; Elsevier Limited
• Subjects: Adult; Algorithms; Cerebral Cortex - physiology; Conductivity; Data Interpretation, Statistical; EEG; Electric Conductivity; Electrodes; Electroencephalography - methods; FEM; Finite Element Method; Four-layer realistic head modeling; Humans; Image Processing, Computer-Assisted; Localization; Male; Models, Neurological; Scalp - physiology; Sensitivity of EEG to skull conductivity; Skull - physiology; Skull conductivity estimation; Source localization; Tomography; Young Adult; Finite Element Method; Source localization; EEG; Sensitivity of EEG to skull conductivity; Skull conductivity estimation; Four-layer realistic head modeling; FEM
• ISSN: 1053-8119; 1095-9572; 1095-9572
• DOI: 10.1016/j.neuroimage.2015.08.032

Accurate electroencephalographic (EEG) source localization requires an electrical head model incorporating accurate geometries and conductivity values for the major head tissues. While consistent conductivity values have been reported for scalp, brain, and cerebrospinal fluid, measured brain-to-skull conductivity ratio (BSCR) estimates have varied between 8 and 80, likely reflecting both inter-subject and measurement method differences. In simulations, mis-estimation of skull conductivity can produce source localization errors as large as 3cm. Here, we describe an iterative gradient-based approach to Simultaneous tissue Conductivity And source Location Estimation (SCALE). The scalp projection maps used by SCALE are obtained from near-dipolar effective EEG sources found by adequate independent component analysis (ICA) decomposition of sufficient high-density EEG data. We applied SCALE to simulated scalp projections of 15cm2-scale cortical patch sources in an MR image-based electrical head model with simulated BSCR of 30. Initialized either with a BSCR of 80 or 20, SCALE estimated BSCR as 32.6. In Adaptive Mixture ICA (AMICA) decompositions of (45-min, 128-channel) EEG data from two young adults we identified sets of 13 independent components having near-dipolar scalp maps compatible with a single cortical source patch. Again initialized with either BSCR 80 or 25, SCALE gave BSCR estimates of 34 and 54 for the two subjects respectively. The ability to accurately estimate skull conductivity non-invasively from any well-recorded EEG data in combination with a stable and non-invasively acquired MR imaging-derived electrical head model could remove a critical barrier to using EEG as a sub-cm2-scale accurate 3-D functional cortical imaging modality. •We propose Simultaneous Conductivity And Location Estimation (SCALE) for EEG data.•SCALE uses only EEG independent component sources and a subject head MR image.•In simulation studies, SCALE accurately estimated skull conductivity.•Applied to EEG from two subjects, SCALE converged regardless of initial condition.•SCALE may make noninvasive, high-resolution EEG source imaging feasible.