Interhemispheric organization of corticocaudate projections in the cat: a retrograde double-labelling study.

The organization of interhemispheric corticocaudate projections in the cat was assessed by the retrograde axonal transport of different marker substances. These double-labelling methods indicated the existence of three types of neocortical efferents terminating in the caudate nucleus (Cd): (1) uncrossed axons projecting only to the ipsilateral Cd; (2) crossed axons projecting strictly to the contralateral Cd; and, (3) divergent axons projecting to both Cds. In all cases, the corticocaudate projections originated from small-to-medium sized pyramidal neurons situated in layers III, IV and V of the rostral neocortex. Interhemispheric inputs to the Cd may account, to some extent, for the functional coupling of the right and left basal ganglia.