Variation in reported human head tissue electrical conductivity values

Abstract

Electromagnetic source characterisation requires accurate volume conductor models representing head geometry and the electrical conductivity field. Head tissue conductivity is often assumed from previous literature, however, despite extensive research, measurements are inconsistent. A meta-analysis of reported human head electrical conductivity values was therefore conducted to determine significant variation and subsequent influential factors. Of 3,121 identified publications spanning three databases, 56 papers were included in data extraction. Conductivity values were categorised according to tissue type, and recorded alongside methodology, measurement condition, current frequency, tissue temperature, participant pathology and age. We found variation in electrical conductivity of the whole-skull, the spongiform layer of the skull, isotropic, perpendicularly-and parallelly-oriented white matter (WM) and the brain-to-skull-conductivity ratio (BSCR) could be significantly attributed to a combination of differences in methodology and demographics. Specifically, whole-skull conductivity varied significantly with the employed method, whilst the spongiform layer differed with measurement condition. Grey matter conductivity fluctuated according to method and pathology, whilst isotropic WM differed according to all of the independent variables. Finally, the BSCR significantly varied with participants age. The reported results provided evidence for head conductivity inconsistencies being dependent on methodology and demographics, supporting the hypothesis that conductivity values should not be assumed from the literature.