[Transthalamic conduction of visual impulses into the cortex and subcortical portions of the turtle forebrain].

Thalamic structures relaying impulses originating from optic stimulation of the retina and electric stimulation of the tectum on the way to the general cortex, hyperstriatum dorsal ventricular ridge and striatum were studied in turtles Emys orbicularis. Anodal polarization of the n. rotundus temporarily, and the lesion of this nucleus irreversibly, suppressed the main negative component of responses evoked by tectal stimulation and light flashes in the dorsal ventricular ridge without affecting them in the general cortex. Polarization and lesions of the lateral thalamic region, including nucleus geniculatum lateralis produced an opposite effect. A conditioning single stimulation of the tectum or n. rotundus suppressed flash-evoked potentials in the hyperstriatum. However evoked potentials and single unit responses appeared following stimulation of the lateral thalamic region. They exhibited blocking interactions with the tectum-evoked responses. A conclusion is made that in the turtle the main pathways conducting visual information to the cortex and hyperstriatum are different: in the former case it is relayed in the lateral geniculatum region in the latter--in n. rotundus, although an overlap of the projections in dorsal ventricular ridge and striatum can be suggested.