Projection of precentral motor cortex upon nucleus medialis dorsalis thalami in the monkey.

The efferent connections of the lateral aspects of the precentral, premotor and granular frontal cortex were reexamined in 12 monkeys (Macaca fascicularis) by means of anterograde labelling techniques using radioactive proline and leucine. A distinct projection of the precentral gyrus upon the paralamellar portion of the nucleus medialis dorsalis (MD) was observed. The somatotopic arrangement within this projection is less well defined than within the ventrolateral thalamic complex. The facial division of area 4 has not only a marked ipsilateral but in contrast to the body representation also a distinct if much weaker contralateral projection. No label was found in MD after injections in the postcentral gyrus. Thus, MD represents a thalamic link between agranular (motor) and granular (association) cortex of the frontal lobe.