AN ANALYSIS OF CORTICAL POTENTIALS MEDIATED BY THE CORPUS CALLOSUM

Abstract

In cats under barbiturate anesthesia single electrical shocks applied to the cortex of one cerebral hemisphere evoke potentials at one or more points on the cortex of the other hemisphere. These responses are mediated by the corpus callosum. The potential wave is typically diphasic; it is composed of an initial surface positive component lasting about 15 msec, and a surface negative component lasting about 75 msec. If a convulsant drug (picrotoxin) is applied to the surface of the pia under the pick-up electrode the negative component is greatly increased in magnitude and the positive component is increased slightly. If an anesthetic drug (nembutal) is applied, the negative component is com-pletly obliterated but the positive component undergoes no change. By inserting micro-eledtrodes to various depths in the cortex and underlying white matter during the action of convulsant and narcotic drugs it has been possible to gain some knowledge of the origin and course of the impulses which give rise to the potential changes recorded from the pial surface. The ascending fibers of the callosum apparently ramify in the upper layers of the cortex and end in the 1st layer where they make synaptic connections with descending interneurons which lead to the deeper cortical layers. The ascending fibers give rise to the surface positive component of the wave, the descending internuncial fibers to the surface negative component.