Movements resembling orientation or avoidance elicited by electrical stimulation of the superior colliculus in rats.

Some studies have reported that stimulation of the superior colliculus in rats produces orienting responses, as it does in a number of species. However, other studies have reported movements resembling avoidance and escape, which are not characteristic of collicular stimulation in other mammals. This apparent discrepancy was investigated by systematically recording the effects on head and body movements of electrical stimulation at a large number of sites throughout the superior colliculus (SC) and surrounding structures. It was found that the nature of the movements observed depended on the location of the stimulating electrode. Contralateral head and body movements resembling orienting and approach were obtained from sites in the intermediate and deep layers in rostral colliculus, the intermediate white layer and immediately surrounding tissue in central colliculus, and in all layers except deep white in caudal colliculus. At the remaining responsive sites, movements resembling avoidance and escape were obtained. The most common response was an ipsilateral cringelike movement of the body that developed into ipsilateral locomotion, followed by running and jumping as the current was increased. These movements were obtained from sites in the superficial and intermediate layers rostrally; from the intermediate gray and the medial superficial and deep layers in central colliculus; and from the deep layers and underlying tegmentum caudally. The distributions of sites, together with evidence from other studies, suggested the following conclusions: Within the superficial layers, avoidance responses were obtained from a region of the superior colliculus that appeared to represent the upper visual field, whereas orienting responses were obtained from a region apparently representing the lower visual field. Stimulation of the area containing the cells of origin of the predorsal bundle produced orientation and approach movements, whereas the avoidance and escape movements were probably mediated by parts of the ipsilateral descending pathway. The stimulation-induced avoidance and escape may reflect the importance of such responses to visual "events," particularly in the upper part of the visual field, in animals, like rats, with many predators.