Functional anatomy of the thalamus in the blind mole rat Spalax ehrenbergi: an architectonic and electrophysiologically controlled tracing study.

The occipital cortex of the naturally blind mole rat, Spalax ehrenbergi, is occupied by an area of somatosensory representation. To date, no visual cortex has been identified electrophysiologically. In order to determine whether there are corresponding modifications in the thalamus, thalamocortical connections were studied with neuroanatomical tracing methods. Three different fluorescent tracers were injected under electrophysiological control into distinct cortical areas. Injections into the somatosensory head/face and hindlimb/trunk areas of representation revealed a posteromedial ventral nucleus and a posterolateral ventral nucleus, respectively. Additional somatotopic labeling was found in an area dorsomedial to the two ventral nuclei. This structure may be equivalent to the posterior nuclear complex in the laboratory rat. Injections into the auditory cortex of the mole rat resulted in labeling of the medial geniculate body. In contrast to the situation in the laboratory rat, in which a prominent dorsolateral geniculate body and a ventrolateral geniculate body assume dorsolateral positions, the somatosensory thalamus of the mole rat almost reaches the dorsolateral surface. This finding is corroborated by the results of the architectonic study, which failed to reveal a differentiated lateral geniculate body. Our observations suggest that the thalamocortical visual system in the mole rat is minute, whereas the somatosensory system is expanded. This situation fits the mode of life of this subterranean animal, for which touch is more important than vision.