The topographic organization of the connections between the mediodorsal nucleus and the prefrontal cortex in the rhesus monkey

The organization of the connections between the mediodorsal nucleus of the thalamus and the prefrontal cortex have been studied by several investigators. While most agree on the particular connections, topographic interpretations of these connections vary. Despite a number of studies, the current understanding of MD connections lacks a convincing organizational principle with which clinical and behavioral data might be interpreted more meaningfully.;The MD consists of three medial-to-lateral divisions: MDmc, MDpc, and MDmf. Architectonic analyses of PFC have shown that it can be subdivided into a ventral region consisting of orbital and ventrolateral prefrontal areas, and a dorsal region comprised of medial and dorsolateral areas. Both of these regions display a trend of increasing laminar differentiation beginning in simple, unlaminated cortex, passing through intermediate stages leading to architectonically well differentiated isocortices. It is possible that the topography of PFC-MD connections may be organized according to PFC architectonic patterns. This possibility was investigated in the present study by demonstrating patterns of corticothalamic and thalamocortical connections between the three subdivisions of MD and the PFC with anterograde and retrograde pathway tracers.;The results show that architectonically less well differentiated PFC is reciprocally connected with MDmc, and the most differentiated areas with MDmf. Furthermore, prefrontal regions with intermediate architectonic characteristics are connected with MDpc. Additionally, dorsal and ventral portions of MD have topographically organized connections with dorsal and ventral PFC regions respectively. This correlation between connection patterns and architectonics is also reflected by variation in the laminar distribution of the origins and terminations of these connections in the various prefrontal areas. Thalamocortical projections to less differentiated PFC areas terminate in layers I, III, and V, whereas, projections to better differentiated areas terminate in layers III and IV. Corticothalamic projections from less differentiated areas arise mainly from layers V and VI, whereas those from progressively better differentiated areas arise mainly from layer V and less from layer VI.;The differential laminar distribution and the ordered topographic connectivity between MD and PFC suggests that behaviorally distinct regions of PFC and their respective affiliates in MD may share common behavioral functional characteristics.