Do retinal and spinal projections overlap within the turtle thalamus?

The spinal and retinal projections to the turtle thalamus were studied using the autoradiographic tracing technique. Particular attention is given to the regions receiving both spinal and retinal projections: the ovalis complex and a perirotundal stripe. Spinal and retinal projections do not overlap significantly in either of these regions. In the perirotundal stripe there appears to be little likelihood of convergence of these projections on single neurons. In the ovalis complex, the most densely and the most consistently innervated thalamic region, however, the respective spinal and retinal target areas are located immediately adjacent to each other and low order somatosensory and visual projections may converge on single neurons with dendrites extending into both the spinal and retinal terminal arborizations. The ovalis complex in the turtle may be compared with the ventral part of the mammalian lateral geniculate nucleus. Both these nuclei receive spinal and retinal projections and both, reportedly, do not project to the telencephalon. The findings are discussed in relation to the possible evolution of the specific thalamic sensory nuclei in higher mammals as well as to the prosencephalic processing of somatosensory and visual information in turtles and mammals.