Characterising the frequency response of impedance changes during evoked physiological activity in the rat brain

Abstract

Objective: Electrical impedance tomography (EIT) can image impedance changes associated with evoked physiological activity in the cerebral cortex using an array of epicortical electrodes. An impedance change is observed as the externally applied current, normally confined to the extracellular space is admitted into the conducting intracellular space during neuronal depolarisation. The response is largest at DC and decreases at higher frequencies due to capacitative transfer of current across the membrane. Biophysical modelling has shown that this effect becomes significant above 100 Hz. Recordings at DC, however, are contaminated by physiological endogenous evoked potentials. By moving to 1.7 kHz, images of somatosensory evoked responses have been produced down to 2 mm with a resolution of 2 ms and 200 μm. Hardware limitations have so far restricted impedance measurements to frequencies <2 kHz. The purpose of this work was to establish the optimal frequency for extending EIT to image throughout the brain and to characterise the response at frequencies >2 kHz using improved hardware. Approach: Impedance changes were recorded during forepaw somatosensory stimulation in both cerebral cortex and the VPL nucleus of the thalamus in anaesthetised rats using applied currents of 1 kHz to 10 kHz. Main results: In the cortex, impedance changed by –0.04 ± 0.02 % at 1 kHz, reached a peak of –0.13 ± 0.05 % at 1475 Hz and decreased to –0.05 ± 0.02 % at 10 kHz. At these frequencies, changes in the thalamus were –0.26 ± 0.1%, –0.4 ± 0.15 % and –0.08 ± 0.03 % respectively. The signal-to-noise ratio was also highest at 1475 Hz with values of –29.5 ± 8 and –31.6 ±10 recorded from the cortex and thalamus respectively. Signficance: This indicates that the optimal frequency for imaging cortical and thalamic evoked activity using fast neural EIT is 1475 Hz.