Topographic representation of vestibular and somatosensory signals in the anuran thalamus.

In the isolated brain of the fire-bellied toad, Bombina orientalis, the spatial distribution of vestibular and somatosensory responses in thalamic nuclei was studied following electrical activation of the Vth nerve, the ramus anterior of the VIIIth nerve and of the dorsal roots of spinal nerves 3 and 8. Responses were systematically mapped in frontal planes through the diencephalon at four rostro-caudal levels. The calculated activity maps were superimposed on the outlines of diencephalic nuclei, and those nuclei that received particularly large inputs from the stimulated sensory nerve roots were indicated. Maximal response amplitudes coincided with ventral, central, and posterior thalamic areas and exhibited a topography that differed for each sensory nerve root. Maximal responses evoked from the Vth nerve were largely separated from those from spinal dorsal roots 3 and 8, whereas maximal vestibular responses partly overlapped with those from the other somatosensory nerve roots. Our findings indicate that within the amphibian thalamus sensory signals originating from different nerve roots are largely represented in separate areas as is the case in the thalamus of amniotes. However, the anterior dorsal thalamus which is the only origin of ascending pathways to the medial and dorsal pallium (assumed homologues of the mammalian hippocampus and neocortex, respectively) receives only minor vestibular and somatosensory input. This corroborates the view that amphibians lack a direct sensory thalamo-cortical, or "lemnothalamic," pathway typical of mammals and birds.