Contralateral thalamic projections predominantly reach transitional cortices in the rhesus monkey.

Connections between the thalamus and the cortex are generally regarded as ipsilateral, even though contralateral connections exist as well in several adult mammalian species. It is not known, however, whether contralateral thalamocortical projections reach particular cortices or whether they emanate from specific nuclei. In the rhesus monkey different types of cortices, ranging from transitional to eulaminate, vary in their cortical connectional pattern and may also differ in their thalamic connections. Because olfactory and transitional prefrontal cortices receive widespread projections, we investigated whether they are the target of projections from the contralateral thalamus as well. With the aid of retrograde tracers, we studied the thalamic projections of primary olfactory (olfactory tubercle and prepiriform cortex) and transitional orbital (areas PAII, Pro, 13) and medial (areas 25, 24, 32) areas, and of eulaminate (areas 11, 12, 9) cortices for comparison. To determine the prevalence of neurons in the contralateral thalamus, we compared them with the ipsilateral in each case. The pattern of ipsilateral thalamic projections differed somewhat among orbital, medial, and olfactory cortices. The mediodorsal nucleus was the predominant source of projections to orbital areas, midline nuclei included consistently about 25% of the thalamic neurons directed to medial transitional cortices, and primary olfactory areas were distinguished by receiving thalamic projections predominantly from neurons in midline and intralaminar nuclei. Notwithstanding some broad differences in the ipsilateral thalamofrontal projections, which appeared to depend on cortical location, the pattern of contralateral projections was consistent with cortical type rather than location. Labeled neurons in the contralateral thalamus were noted in midline, the magnocellular sector of the mediodorsal nucleus, the anterior medial and intralaminar nuclei, and ranged from 0 to 14% of the ipsilateral; they were directed primarily to olfactory and transitional orbital and medial cortices but rarely projected to eulaminate areas. Several thalamic nuclei projected from both sides to olfactory and transitional areas, but issued only ipsilateral projections to eulaminate areas. Though ipsilateral thalamocortical projections predominate in adult mammalian species, crossed projections are a common feature in development. The results suggest differences in the persistence of contralateral thalamocortical interactions between transitional and eulaminate cortices.