Intermediate and deep layers of the macaque superior colliculus: a Golgi study.

We studied the intermediate and deep layers of the macaque superior colliculus by means of the Golgi technique in an attempt to better understand the structural features of this important oculomotor center. For this study, we examined the optic (stratum opticum, SO), intermediate gray (stratum griseum intermedium, SGI), intermediate white (stratum album intermedium, SAI), and deep gray (stratum griseum profundum, SGP) layers. These are the four layers in which neurons having saccade-related activity are localized. We identified eight neuronal types on the basis of differences in somatic and dendritic morphologies: large multipolar neurons (Type I); large pyramidal neurons (Type II); large fusiform neurons (Type III); medium fusiform neurons with spiny, radially oriented dendrites (Type IV); medium round neurons with fan-shaped dendritic trees (Type V); medium stellate neurons with varicose dendrites (Type VI); medium multipolar neurons with robust, spiny dendrites (Type VII); and local interneurons (Type VIII). Most neuronal types possessed features that are homologous to presynaptic dendritic features in other brain centers. With the exception of the medium stellate neurons (Type VI), which are aspinous, and the local interneurons (Type VIII), which are sparsely spinous, all other types had a moderate number of spines on their dendrites. Dendrites that terminated in the optic layer had specializations not observed elsewhere, suggesting that these tips may sample a tectal afferent that is not present in the more ventral layers. These eight types comprise all the neuronal morphologies observed in a large number of Golgi-impregnated macaque brains (n = 50). We suggest that they represent the full range of neuron types in the saccade-related layers of the macaque tectum.