Crossed thalamocortical connections in the Madagascan hedgehog tenrec: dissimilarities to erinaceous hedgehog, similarities to mammals with more differentiated brains.

The adult erinaceous hedgehog, unlike other mammals, has recently been shown to have prominent crossed projections from the thalamus to the motor cortex. There are suggestions relating this unique pattern of connectivity to the overall degree of brain differentiation and/or the poorly developed corpus callosum. The present tracing study demonstrates that the Madagascan lesser hedgehog tenrec, with its tiny corpus callosum and one of the lowest neocorticalization indices among insectivores, has extensive crossed cortico-thalamic projections, but essentially the same sparse thalamic projections to the contralateral cortex as have placental mammals with more differentiated brains. The implications of the findings and the relevance of extracallosal pathways are discussed.