Brain Research, 156 (1978) 33-49 33 © Elsevier/North-Holland Biomedical Press LOCALIZATION AND DETECTION OF VISUAL STIMULI FOLLOWING SUPERIOR COLLICULUS LESIONS IN RHESUS MONKEYS C. M. BUTTER, C. WEINSTEIN, D. B. BENDER and C. G. GROSS Department of Psychology and Neuroscience Laboratory, University of Michigan, Ann Arbor, Mich. 48109 and Department of Psychology, Princeton University, Princeton, N. J. 08540 (U.S.A.) (Accepted February 23rd, 1978) SUMMARY Rhesus monkeys were trained to fixate a central stimulus and to detect and localize a 50 msec light flash presented 6-80 ° on either side of the central stimulus. Following large lesions of the superior colliculus, they showed persistent deficits in localizing flashes presented 43-80 ° from the fixation stimulus. However, they were not consistently impaired when the flashes were presented more centrally, and their performance with peripheral stimuli improved when the stimulus duration was 1 sec. Thus, the superior colliculus appears to be necessary for the localization of brief visual stimuli in the far periphery. INTRODUCTION Electrical stimulation, recording and ablation experiments have demonstrated a crucial role for the superior colliculus in visual orientation and localization in a variety of subprimate species15,1G. The situation is less clear in primates. On the one hand, the elegant recording and stimulation studies of Schiller, Wurtz and Robinson and their colleagues have provided a detailed account of how the colliculus may serve to direct the eyes and perhaps the animal's attention to peripheral stimuliS,6,12-14,17. On the other hand, there is little direct evidence that removal of the colliculus in primates markedly impairs visual localization and orientation toward peripheral stimuli. Reflexly induced eye movements are unaffected by colliculus removaP 1, and the deficits in voluntary saccades reported after colliculus lesions in monkeys are usually slight and transientlO, 18. While visual learning impairments have been reported in monkeys with collicular lesions l, orientation was not observed in that study. A tectal role in orientation and localization is more directly suggested by the findings that accuracy of reaching to the site of a brief visual stimulus is impaired after colliculus lesions s,9. However, it is not clear whether and to what extent these deficits were due to