In birds, the nucleus of the basal optic root (nBOR) of the accessory
optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM)
are involved in the analysis of optic flow and the generation of the
optokinetic response. In several species, it has been shown that the AOS
and pretectum receive input from visual areas of the telencephalon.
Previous studies in pigeons using anterograde tracers have shown that both
nBOR and LM receive input from the visual Wulst, the putative homolog of
mammalian primary visual cortex. In the present study, we used retrograde
and anterograde tracing techniques to further characterize these
projections in pigeons. After injections of the retrograde tracer cholera
toxin subunit B (CTB) into either LM or nBOR, retrograde labeling in the
telencephalon was restricted to the hyperpallium apicale (HA) of the
Wulst. From the LM injections, retrograde labeling appeared as a discrete
band of cells restricted to the lateral edge of HA. From the nBOR
injections, the retrograde labeling was more distributed in HA, generally
dorsal and dorso-medial to the LM-projecting neurons. In the anterograde
experiments, biotinylated dextran amine (BDA) was injected into HA and
individual axons were reconstructed to terminal fields in the LM and nBOR.
Those fibers projecting to the nBOR also innervated the adjacent ventral
tegmental area. However, tracing of BDA-labeled axons revealed no evidence
that individual neurons project to both LM and nBOR. In summary, our
results suggest that the nBOR and LM receive input from different areas of
the Wulst. We discuss how these projections may transmit visual and/or
somatosensory information to the nBOR and LM.