Overlapping Corticostriatal Projections from the Rodent Vibrissal Representations in Primary and Secondary Somatosensory Cortex

To determine whether the neostriatum receives overlapping projections from two somatosensory cortical areas, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into the whisker representations of primary (SI) and secondary (SII) somatosensory cortex. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to compare the extent of overlapping projections to both subcortical structures. Corticostriatal projections from focal sites in both somatosensory areas exhibited substantial amounts of divergence within the dorsolateral neostriatum. Most of the labeled terminals were concentrated in densely packed arborizations that occupied lamellar-shaped regions along the dorsolateral edge of the neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the thalamus, especially in the ventrobasal complex (VB) and in the medial part of the posterior (POm) nucleus. Because these thalamic regions are topographically organized and have reciprocal connections with corresponding representations in both SI and SII, the amount of labeled overlap in the thalamus was used to indicate the degree of somatotopic correspondence at the SI and SII injection sites. We found that the proportion of overlapping projections to the neostriatum was moderately correlated with the amount of overlap observed in the thalamus. This result strongly indicates that specific sites in the dorsolateral neostriatum receive convergent projections from corresponding somatotopic representations in SI and SII, but also suggests that some of the corticostriatal divergence may reflect neostriatal integration of somatosensory information from noncorresponding representations in SI and SII.