A cable theory based biophysical model of resistance change in crab peripheral nerve and human cerebral cortex during neuronal depolarisation: implications for electrical impedance tomography of fast neural activity in the brain.

Electrical impedance tomography (EIT) is a medical imaging method with the potential to image resistance changes which occur during neuronal depolarisation in the brain with a resolution of milliseconds and millimetres. Most biomedical EIT is conducted with applied current over 10 kHz, as this reduces electrode impedance and so instrumentation artefact. However, impedance changes during neuronal depolarization are negligible at such frequencies. In order to estimate optimal recording frequency and specify instrumentation requirements, we have modelled their amplitude and frequency dependence during evoked activity using cable theory. Published values were used for the electrical properties and geometry of cell processes. The model was adjusted for the filtering effect of membrane capacitance and proportion of active neurons. At DC, resistance decreases by 2.8 % in crab nerve during the compound action potential and 0.6 % (range 0.06-1.7 %) locally in cerebral cortex during evoked physiological activity. Both predictions correlate well with independent experimental data. This encourages the view that true tomographic imaging of fast neural activity in the brain is possible, at least with epicortical electrodes in the first instance. It is essential to undertake this at low frequencies below about 100 Hz as above 1 kHz the signal becomes vanishingly small.