Relationships of precentral premotor and prefrontal cortex to the mediodorsal and intralaminar nuclei of the monkey thalamus.

The projections of precentral, premotor and prefrontal cortical areas to the mediodorsal and intralaminar thalamic nuclei have been reexamined by means of the anterograde labelling technique in 21 macaque monkeys. Three principles which have been already mentioned in previous reports could be confirmed within limits: (i) The dichotomy of agranular and granular frontal cortices related to lateral and medial thalamic nuclear division respectively, (ii) the reciprocity of fronto-thalamic connections and (iii) the thalamic matrix consisting of longitudinally arranged cell columns (Kievit and Kuypers 1977) from which the cortical connections originate and where they end. One main exception to these principles involves the intralaminar and paralaminar nuclear continuum which seems to disrupt the nearly parallel system of thalamically relayed information channels by forming complex patterns of converging and diverging connectivities with cortical, subcortical and possibly intrathalamic regions. Another exception concerns the partial bilaterality of fronto-thalamic projections which is not reciprocated as far as we know by the thalamo-cortical counterparts. Major emphasis is laid upon the fact that agranular frontal cortex, i.e., precentral and premotor (including supplementary motor) areas not only project to the ventrolateral nuclear complex and to intralaminar nuclei but in addition to MD (paralaminar zone) whose efferent connections are known to be directed mainly towards the frontal eyefield and the caudate nucleus and to a lesser degree to the agranular frontal cortex.