The sources of the nigrotectal pathway.

The nigrotectal pathway plays a role in the generation of saccade related responses by cells in the deep layers of the superior colliculus. By using a retrograde horseradish peroxidase technique that homogeneously fills neurons, the present experiments demonstrate that the source of the nigrotectal projection to the intermediate gray layer of the grey squirrel (Sciurus carolinensis) is a heterogeneous population of neurons whose somas and dendrites are concentrated in the rostral pole of pars reticulata. This region of pars reticulata receives projections from the posterior caudate, which in turn is a target of both the pulvinar and visual cortex. In addition, these experiments reveal the presence of a second, distinct set of neurons projecting to the midbrain tectum that are located in pars lateralis of the substantia nigra. These neurons can be distinguished from those in pars reticulata by their homogeneity and by their prominent basal dendrites. Furthermore, pars lateralis of the squirrel substantia nigra is, on cytoarchitectonic and immunocytochemical grounds, a distinct subdivision that does not receive projections from the posterior caudate. We conclude that both pars reticulata and lateralis are sources of the nigrotectal pathway. In addition, our results suggest, on connectional grounds, that the rostral pole of pars reticulata may be specialized to subserve the visual guidance of orienting movements.