Marsupial interhemispheric relation.

Abstract

AbstractThree different responses recorded from the opossum neocortex following a stimulus to the homotopical point on the contralateral hemisphere were: the interhemispheric early response (IER), the interhemispheric delayed response (IDR), and the rhythmic afterdischarge (RA). The IER was a short latency biphasic slow wave evoked potential which could be recorded from every homotopic point on the neopallium with the exception of a part of the striate cortex. Each active homotopic point had a limited heterotopic halo. The homotopic positive peak latency varied from 5.3 msec in the area parietalis to 14.7 msec in the area striata. Interhemispheric transfer was effected via the anterior commissure. The IDR was a late slow wave evoked potential and had an extensive heterotopic representation. This potential could be recorded in the contralateral hemisphere after sectioning the anterior commissure. The homotopic positive peak latency varied from 60.2 msec in the area parietalis to 76.1 msec in area striata. To abolish the IDR both the di‐mesencephalic junction and the interthalamic commissure had to be sectioned. The RA is a very labile late oscillatory potential which was very difficult to demonstrate without a computer of average transients. The pattern of its homotopic and heterotopic representation resembled very closely that of the IER. Under the conditions of our experiment it had a negative peak latency of 224–230 msec, a frequency of oscillation of three seconds and a maximum duration of 2.2 seconds. To abolish the RA both the di‐mesencephalic junction and the interthalamic commissure had to be sectioned. The dorsal commissure contained no neopallial intorhemispheric fibers. The anterior commissure carried these commissural connections. The two late responses reported in the opossum fell within the general context of mammalian late responses.