Midbrain connections of the olivary pretectal nucleus in the marmoset (Callithrix jacchus): implications for the pupil light reflex pathway.

Midbrain projections of the pretectal olivary nucleus (PON) were studied in the marmoset, a New World primate. The fluorescent retrograde tracers Fluoro-Gold (FG) and Fast Blue (FB) were injected into the Edinger-Westphal (EW) nucleus and the lateral terminal nucleus (LTN), respectively. EW nucleus injections resulted in retrograde labeling of significant numbers of FG-positive neurons of the PON as well as a small number of cells in the LTN. LTN injections led to labeling of a population of singly-labeled cells seen dispersed through the larger population of FG-labeled somata within the contralateral PON. The ipsilateral PON was devoid of FB-labeled somata, whereas the adjacent nucleus of the optic tract (NOT) contained FB-labeled cells. These findings show that a large number of PON neurons project directly to the oculomotor complex. Additionally, the study shows the presence of a separate population of PON neurons projecting to the contralateral LTN. This, combined with our earlier observation that LTN neurons project to the EW nucleus in the marmoset (see main text for reference), lends support to the presence of separate direct and indirect pupillary light reflex pathways from the PON to the nucleus of EW.