Brain-stem afferents upon retinal projecting isthmo-optic and ectopic neurons of the pigeon centrifugal visual system demonstrated by retrograde transneuronal transport of rhodamine beta-isothiocyanate.

Brain-stem afferents to the n. isthmo-opticus (NIO) and ectopic neurons (EN) of the centrifugal visual system (CVS) were determined in the pigeon using the retrograde transneuronal transport of the fluorescent dye Rhodamine beta-isothiocyanate (RITC) after its intraocular injection. In other experiments, either RITC was injected into various periocular tissues (controls) or the retrograde tracer Fluoro-gold (FG) was injected stereotaxically in the NIO. Following intraocular injections, the RITC retrograde labeling of cell bodies was observed contralaterally in the NIO and EN and transneuronally in layers 9/10 of the optic tectum, area ventralis-Tsai, zona peri-NIII, mesencephalic and pontine reticular formation (PRF), n. linearis caudalis-raphe, and bilaterally within a region referred to as zona peri-n.NVI (Zp-n.NVI) immediately underlying the abducens nerve nucleus. None of the above structures were labeled after RITC periocular injections. The FG labeling revealed that the tectal efferent neurons were mainly medium-sized, multipolar cells whose dendrites extended superficially to retino-recipient tectal layers 6 and 5. Quantitative measurements of the distribution of layers 9/10 RITC-labeled neurons indicated the highest densities to be localized within the ventral tectum corresponding to the representation of the dorsal retina and inferior visual field. We suggest that visual and nonvisual brain-stem afferents upon NIO and EN may play a role in the proposed mechanism of the avian CVS in attention, ground-feeding behavior, and modulation of retinal sensitivity.