Anatomical identification of a telencephalic visual area in crocodiles: ascending connections of nucleus rotundus in Caiman crocodilus.

Abstract

Nucleus rotundus receives a major input from the optic tectum in crocodiles, Caiman crocodilus. Telencephalic projections of nucleus rotundus were studied in Caiman by means by the Fink-Heimer procedure after anodal, stereotaxic lesions. Efferent axons of nucleus rotundus assemble on the ventromedial aspect of this nucleus and swing ventrolaterally to enter the dorsal peduncle of the lateral forebrain bundle. These ascending fibers continue rostrally in the dorsal peduncle of the lateral forebrain bundle to enter the telencephalon where they remain restricted to a lateral portion of the lateral forebrain bundle. At more anterior levels, these fascicles turn dorsally, pass through the ventrolateral area, and terminate massively in a lateral part of the rostral dorsolateral area. The results of this experiment are compared with similar studies on thalamotelencephalic connections of diencephalic visual areas in other amniotes. Parallels in fiber connections of thalamic auditory and visual areas and the segregation of these modalities in the telencephalon of Caiman are discussed. These similarities in neural circuitry and synaptic elements of auditory and visual systems that synapse in the midbrain of Caiman form the basis for a different interpretation of sensory system organization in amniotes.