Subcortical crossed axonal projections to the caudate nucleus of the cat: a double-labelling study

The anatomical organization of the interhemispheric projections of subcortical caudate nucleus (Cd) input neurons in the cat was assessed by the retrograde axonal transport of multiple marker substances. These double-labelling methods indicated the existence of two types of subcortical afferents to the Cd. (1) Uncrossed projections terminating in the ipsilateral Cd (but not the contralateral Cd) originated from the globus pallidus, thalamus, substantia nigra and midbrain raphe nuclei. The uncrossed axons provided the vast bulk of the subcortical Cd inputs. (2) Crossed projections terminating in the contralateral Cd (but not the ipsilateral Cd) originated from the substantia nigra and raphe nuclei. The crossed projections from the midbrain provided a very small Cd input compared to the crossed and divergent corticocaudate projections. Therefore, interhemispheric connections of the Cds may be subserved primarily by arrangement of corticocaudate projections. Monosynaptic interhemispheric subcortical inputs to the Cds are minor. Multisynaptic pathways could provide alternative, but less tightly coupled, interhemispheric linkages of the Cds.