Projections of the nucleus of the basal optic root in pigeons (Columba livia): a comparison of the morphology and distribution of neurons with different efferent projections.

The avian nucleus of the basal optic root (nBOR) is a visual structure involved in the optokinetic response. nBOR consists of several morphologically distinct cell types, and in the present study, we sought to determine if these different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of nBOR neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, the pretectal nucleus lentiformis mesencephali (LM), the contralateral nBOR, the oculomotor complex (OMC) and a group of structures along the midline of the mesencephalon. The retrogradely labeled neurons fell into two broad categories: large neurons, most of which were multipolar rather than fusiform and small neurons, which were either fusiform or multipolar. From injections into the IO, LM, contralateral nBOR, and structures along the midline-mesencephalon small nBOR neurons were labeled. Although there were no differences with respect to the size of the labeled neurons from these injections, there were some differences with the respect to the distribution of labeled neurons and the proportion of multipolar vs. fusiform neurons. From injections into the VbC, the large multipolar cells were labeled throughout nBOR. The only other cases in which these large neurons were labeled were contralateral OMC injections. To investigate if single neurons project to multiple targets we used paired injections of red and green fluorescent retrograde tracers into different targets. Double-labeled neurons were never observed indicating that nBOR neurons do not project to multiple targets. We conclude that individual nBOR neurons have unique projections, which may have differential roles in processing optic flow and controlling the optokinetic response.