Transthalamic conduction of visual impulses to the cortex and subcortical forebrain structures in turtles

Abstract

The thalamic relays for the conduction of impulses arising during photic stimulation of the eyes and electrical stimulation of the tectum in the general cortex, hyperstriatum (the dorsal ventricular ridge), and the striatum proper were studied in the turtleEmys orbicularis. Acute experiments on immobilized animals showed that anodal polarization temporarily and destruction of n. rotundus irreversibly suppress the main negative wave of the responses to tectal stimulation and to flashes in the hyperstriatum, whereas the corresponding responses in the general cortex still persist. Polarization and destruction of the lateral thalamic region, including the lateral geniculate body, have the opposite effect: responses in the hyperstriatum to photic and tectal stimulation are virtually unchanged whereas those in the general cortex disappear, except their late components. Preceding single stimulation of the tectum or n. rotundus depresses responses in the hyperstriatum evoked by flashes. However, during stimulation of the lateral thalamic region, combined potentials and single unit responses appear in the hyperstriatum and interact with responses evoked by tectal stimulation. It is concluded that the main pathways in turtles which supply visual information to the general cortex and hyperstriatum differ: the former relay in the lateral thalamic region, the latter in n. rotundus, although some overlapping of their projections in the hyperstriatum and striatum is possible.