Mosaic architecture of the somatic sensory-recipient sector of the cat's striatum.

The striatum is known to have a compartmental organization in which histochemically defined zones called striosomes form branched 3-dimensional labyrinths embedded within the surrounding matrix. We explored how fiber projections from cortical somatic sensory areas representing cutaneous and deep-receptor inputs are organized in relation to this striatal architecture. Areas SI and 3a were mapped electrophysiologically, and distinguishable anterograde tracers (wheat germ agglutinin-HRP and 35S-methionine) were injected into physiologically identified loci. Primary somatic sensory corticostriatal projections were confined to a small, well-defined sector in the dorsolateral corner of the ipsilateral striatum. The somatic sensory afferents were arranged according to a coherent global body map in which rostral body parts were represented more laterally than caudal body parts. Single cortical loci innervated branched and clustered striatal zones that were reminiscent of the striosomes in their range of sizes and shapes yet lay strictly within the extrastriosomal matrix. In contrast to the global orderliness of the striatal body map, there were clear examples of locally complex patterns in which functionally distinct inputs interdigitated with each other. These patterns were often, but not always, produced when corticostriatal afferents carrying different submodality types were labeled. These findings demonstrate the existence of striosome-like striatal compartments within the seemingly uniform extrastriosomal matrix. The principle of mosaic organization thus holds throughout the tissue of the somatic sensory striatum. The striatal architecture delineated here could provide the anatomical substrate for computations requiring cross-modality comparisons within the framework of an overall somatotopy. If a similar multicompartmental architecture also characterizes other striatal regions, as seems likely, it may set general constraints on the nature of associative processing within the striatum as a whole.