Multisensory response modulation in the superficial layers of the superior colliculus.
In mammals, the paired superior colliculi of the midbrain play a significant role in the generation and guidance of eye movements that enable an animal to orient to novel visual stimuli. In several species including monkey, cat and hamster, the paired colliculi are connected by a commissure. In the cat, many commissural axons arise from tectotectal neurons located in the deep layers in the rostral two-thirds of the colliculus. The role of these tectotectal neurons is unclear, but it is likely that they play some role in eye movement control. In this study, the neuroanatomical tracer Biocytin was used to make small, localized injections into the deep layers of the cat superior colliculus at a variety of different locations in nine animals. The distribution of tectotectal synaptic terminals in the opposite colliculus was then plotted. Regardless of which layers were included the injection site, labelled boutons were most dense in the deep layers in the contralateral colliculus. There was a striking point-to-point organization in the tectotectal projection such that terminals were concentrated at an almost mirror-symmetrical region to the injection site in the rostrocaudal plane. In the majority of cases, however, the focus of terminal boutons was shifted medially by 1-2 mm. These results suggest that tectotectal connections may influence select populations of neurons in the contralateral colliculus. By coupling specific groups of neurons in the two colliculi, their effectiveness in sensory motor processing could be enhanced. At this time it is not clear whether specific commissural terminals contain excitatory or inhibitory neurotransmitters, and our ongoing studies are addressing this question.