Parabigeminal projections to the superior colliculus in the cat

Small amounts of horseradish peroxidase were injected into the superior colliculus of the cat and the distribution of labeled neurons in the parabigeminal nuclei was mapped. After injections placed dorsal to the stratum opticum in the superior colliculus, the parabigeminal nucleus is the only mesencephalic and/or rhombencephalic structure in which labeled neurons are observed. The number of labeled neurons in the parabigeminal nucleus increases after injections that include both the superficial and the deep layers of the superior colliculus. Each part of the superior colliculus receives projections from wide areas of both parabigeminal nuclei, although it also receives more abundant projections from one or more restricted parts of these nuclei. The anterior third of the parabigeminal nuclei is the part which sends the fewest projections to the superior colliculus. These projections terminate principally in the central and intermediate part of the contralateral colliculus, while a smaller number of fibers terminate in the lateral and rostral part of the ipsilateral colliculus. The intermediate third of the parabigeminal nuclei sends projections to all parts of the ipsilateral colliculus, but the greatest number of these goes to the contralateral colliculus. These contralateral projections terminate principally in the lateral parts of the contralateral colliculus, and in lesser number in its central and rostral, and medial and rostral areas. The posterior third of the parabigeminal nucleus sends scant efferents to wide areas of the ipsilateral and contralateral colliculi, and a dense projection to the medial and intermediate, medial and caudal and central and intermediate parts of the ipsilateral colliculus. There are also consistent projections from the posterior third of the parabigeminal nucleus to the central and rostral and medial and rostral parts of this ipsilateral colliculus. These results demonstrate a topographical organization of the parabigemino-tectal projections in the cat, as a pathway that facilitates the integration in the colliculus of visual impulses of different origin in the retina. This organization permits the modulation of the superior colliculus in its participation in both the extrageniculate visual system and in the regulation of eye and head orientation movements through the parabigeminal projections to the superficial and deep layers of the colliculus, respectively.