Pretectal and tectal projections to the homologue of the dorsal lateral geniculate nucleus in the pigeon: an anterograde and retrograde tracing study with cholera toxin conjugated to horseradish peroxidase.

The possibility of a projection from the pretectal nucleus, lentiformis mesencephali (LM), to the principal optic nuclei (OPT) of the dorsolateral thalamus was investigated using cholera toxin conjugated to horseradish peroxidase (CT-HRP). Injections of CT-HRP into LM produced massive anterograde labeling of the pars lateralis of the nucleus dorsolateralis anterior thalami (DLL), and sparse labeling of the pars magnocellularis (DLAmc). Injections of CT-HRP into OPT in turn produced massive retrograde labeling of both parvocellular and magnocellular divisions of LM. These results relate to possible neural mechanisms underlying optokinetic nystagmus. OPT injections also retrogradely labeled small neurons throughout the rostrocaudal extent of the tectum which were confined to superficial laminae.