Impedance Changes Induced in the Brain by Electric Stimulation

Abstract

The extensive application of electric stimulation of the brain, not only experimentally but also for therapeutic purposes as in the so-called electric shock treatment of certain psychoses, makes it desirable to get a more intimate knowledge of the effects of electric stimuli and the subsequent excitation upon the state of the cells of the central nervous system. It is known that excitation of peripheral nerves is associated with decrease in D.C. resistance (Hermann, 1 Ebbecke 2 ) and A.C. impedance (Lullies, 3 Cole and Curtis 4 ) respectively. Since such changes may be able to throw some light upon the state of the cellular surface films (Gildemeister), 5 it seemed of interest to ascertain whether similar effects are demonstrable in the central nervous system.

For the measurement of the impedance a Wheatstone bridge arrangement energized with A.C. of from 500 to 6000 cycles was used as previously described. (Spiegel and Spiegel-Adolf). 6 Quick determinations of the balance were facilitated by a 5-stage amplifier connected not only to a telephone receiver but also to an “electric eye” so that the minimum could be detected by visual as well as by acoustic observations.

In the first series of experiments on cats under superficial ether anesthesia the electrodes were platinum wires coated with platinum black, inserted into the brain substance as in previous experiments (Spiegel and Spiegel-Adolf) 6 ti or platinum-Ringer electrodes placed with the smallest possible pressure on the surface of the brain. The stimulating electrodes were placed on the motor cortex and the testing electrodes (connected to the Wheatstone bridge) on the parieto-occipital region of the same or of the opposite hemisphere.