Bilateral projections from the parabigeminal nucleus to the superior colliculus in monkey.

We examined the distribution of labeled neurons in the parabigeminal nucleus of the monkey following injections of retrograde fluorescent tracers into the superior colliculus. The extent of the visual field representation included in the injection site was assessed from the location of labeled cells in striate cortex. The results suggest a rough topographic organization of the parabigeminal nucleus, with the lower quadrant represented anteriorly and the upper quadrant posteriorly. We also found bilateral projections from the parabigeminal nucleus to both superior colliculi, but the crossed projection appeared to terminate only in that part of the colliculus where the vertical meridian is represented. Parabigeminal cells with a crossed projection were larger than those projecting to the ipsilateral colliculus. The results suggest that the organization of the monkey's parabigemino-tectal system is fundamentally similar to that of many other vertebrates.