IMAGING OF PHYSIOLOGICALLY EVOKED-RESPONSES BY ELECTRICAL-IMPEDANCE TOMOGRAPHY WITH CORTICAL ELECTRODES IN THE ANESTHETIZED RABBIT

The purpose of this study was to determine if electrical impedance tom ography (EIT) could be used to image impedance changes of several per cent over tens of seconds, known to occur during evoked activity of th e cerebral cortex. A ring of 16 electrodes was placed on the exposed s uperior surface of the brain of anaesthetized rabbits. EIT images were acquired every 15 s using a Sheffield Mark 1 EIT system. During perio ds of 2.5-3 min of intense photic stimulation of both eyes or electric al stimulation of a forepaw, reproducible impedance decreases of 4.5 /- 2.7% and 2.7 +/- 2.4% (mean +/- SD) respectively occurred in approp riate cortical areas, with a time course similar to the period of stim ulation. They were accompanied by adjacent smaller impedance increases . The decreases are probably due to increased blood flow and temperatu re; the cause of the adjacent increases may be a shadowing artefact of the reconstruction algorithm or due to physiological shrinkage of the extracellular space. This demonstrates, for the first time, that such small changes may be imaged under optimal conditions. These results a re encouraging to the prospect that EIT may eventually be suitable for imaging evoked responses or epilepsy in human subjects.