Substantia innominata projection to caudate nucleus in macaque monkeys

The distribution of the cells of origin projecting from the substantia innominata to the caudate nucleus was studied in macaque monkeys by means of the retrograde axonal transport of horseradish peroxidase (HRP). The HRP was injected, after callosotomy, into the exposed head of the caudate nucleus. Labeled cells appeared in the ipsilateral substantia innominata, extending rostrally to the level of the optic chiasma.