Visual cortex in a reptile, the turtle ( Pseudem ys scripta and Chrysem ys picta)

Abstract

The afferent and efferent connections of general cortex were studied in two species of turtle, Pseudemys scripta and Chrysemys picta. The thalamic distribution of labeled cells following cortical applications of horseradish peroxidase (HRP) indicated that at least 3 dorsal thalamic nuclei, the dorsal lateral geniculate nucleus, nucleus ventralis and the nucleus dorsolateralis anterior, project to general cortex. When cortical applications of HRP were combined with intraocular injections of tritiated proline in the same animal, autoradiographically labeled retinal terminations were found among the dendrites of lateral geniculate neurons containing the HRP reaction product. These experiments demonstrated that the dorsal lateral geniculate nucleus not only projects to general cortex, but also receives retinal input. Thus, gereral cortex in the turtle is a target of a visual pathway which relays in the dorsal thalamus. Cortical lesions produced anterograde degeneration in the same thalamic nuclei which the HRP experiments demonstrated project to cortex, thereby indicating that the dorsal thalamus and general cortex have reciprocal connections in the turtle. These same experiments with cortical lesions demonstrated that general cortex also sends projections to the optic tectum and tegmentum of the midbrain. These afferent and efferent connections of general cortex in the turtle are compared with the connections of general cortex in other reptilian groups and with those of neocortex in mammals.