Statistical analysis of non-invasive low frequency EIT human measurements of euronal activity

Abstract

EIT has the potential to achieve non-invasive functional imaging of fast neuronal activity in the human brain, due to opening of ion channels during neuronal depolarization. Biophysical modelling suggests local brain changes of impedance are about 1% but peak changes are about 1000x smaller on the scalp. The purpose of this work was to determine if reproducible resistance changes from individual channels could be recorded with scalp electrodes during visual stimulation. Techniques from neuroimaging were employed to enhance the signal-to-noise ratio. Low frequency EIT (LFEIT) was recorded during 20 sessions in 7 subjects with a 1Hz square wave applied to occipital electrodes synchronized to visual evoked responses. Using an estimator derived from the covariance matrix C, and averaging across subjects, changes of 0.0016±0.0003% (0.33±0.01μV, mean±1SE), occurred with a Signal to Noise Ratio of 7.7 and were significant (p<0.05) in 75% of sessions. Modelling of the expected changes with an anatomically realistic FEM of the head suggested peak scalp changes of 0.004±0.003% (0.10±0.07μV). This work suggests, for the first time, that reproducible changes during LF-EIT can be recorded, and appear to be physiologically plausible as they agree with modelling. At present, it seems unlikely that they are robust enough to permit imaging with non-invasive recording in humans. Work in progress includes statistical analysis of a similar dataset acquired with magnetic recording of the impedance changes when applying 1Hz square wave or 125 or 225Hz sine wave currents and determination of the actual size of the changes locally in brain in physiological studies and patients with epilepsy who have intracranial electrodes. © Springer-Verlag 2007.