Direct visual input to the limbic system: crossed retinal projections to the nucleus anterodorsalis thalami in the tree shrew.

Evidence for a direct projection from the retina to the nucleus anterodorsalis thalami in the insectivore, Tupaia glis, is presented. 100 muCi tritiated amino acid or amino acid/monosaccharide cocktail were administered as two separate intravitreal injections of 50 muCi each. Fibers were traced using thawmount autoradiography in which tissue is frozen in liquid propane, cryostat-cut, and mounted on photographic emulsion precoated slides (Conrad and Stumpf, 1974). From the dorsolateral geniculate body, contralateral retinal fibers continue as a dorsomedial extension of the optic tract. When traced rostrally from this point, the fibers form a thin fascicle coursing medially under the third ventricle. In the anterior thalamus the bundle arborizes within the n. anterodorsalis, infiltrating its caudal pole completely but only encapsulating the nucleus anteriorly. This new retino-anterodorsal thalamic projection, together with the known anterior thalamic-retrosplenial projection, represents a third visual pathway in Tupaia glis distinct from the retino-geniculo-striatal and retino-tecto-pulvinar-peristriatal systems. It is significant that each of these thalamic relay nuclei projects to an architectonically different cortical region, with the nucleus anterodorsalis sending afferents to the phylogenetically most primitive of the visual cortices, the retrosplenial proisocortex or prostriata (Vitzthum and Sanides, 1967). It is proposed that the retino-anterior thalamic-retrosplenial circuit forms an anatomical substrate by which light cues may affect emotional behavior and corresponding neurovisceral responses.