Brain impedance measurements by the use of small bipolar needle electrodes.

ments by the use of small bipolar needle electrodes. J. Appl. Physiol. 2 * (4) : 1237-1242. 1g66.-The use of small bipolar needle electrodes for the measurement of the capacitive properties of the brain tissue was found to be unsatisfactory. Electrode polarization effects, acting as a parasitaric phenomena, are the main obstacle of this method; thus, the values of capacitance obtained do not represent the capacitive properties of the tissue. In order to overcome the above difficulties, a comparison method was employed by which the electrical parameters measured in brain tissue were compared with the values ob-tained from an electrolyte solution of the same resistance. electrode polarization phenomena; mirror-image curves of re-sistance and capacitance; comparison method T HE ELECTRICAL IMPEDANCE of brain tissue, in differ-ent anatomical structures and in various physiological and pathological conditions, is the subject of extensive research. This parameter is thought to be an important factor involved in many basic biological phenomena oc-curring in the brain, although its relation to those proc-esses and the significance of its changes are so far, in most cases, poorly understood. The measurement of electrical impedance in brain tissue has been performed in various research centers by different techniques. Usually the resistive component of the impedance has been investigated, using bipolar needle electrodes and a Wheatstone bridge supplied by alternating current of a single frequency (I, 2, 7, 1 g, 24, 25, 28, 29). Tetrapolar electrodes technique has been used by others (8) for the measurements of cortical re-sistance. Lately, several articles have appeared dealing also with the capacitive component of brain impedance. Ranck (16-18) measured both parameters in the rabbit brain cortex at various frequencies of the measuring