The superior colliculus (SC) is part of a network of brain areas that directs saccadic eye movements, overtly shifting both gaze and attention from position to position, in space. Here, we seek direct evidence that the SC also contributes to the control of covert spatial attention, a process that focuses attention on a region of space different from the point of gaze. While requiring monkeys to keep their gaze fixed, we tested whether microstimulation of a specific location in the SC spatial map would enhance visual performance at the corresponding region of space, a diagnostic measure of covert attention. We find that microstimulation improves performance in a spatially selective manner: thresholds decrease at the location in visual space represented by the stimulated SC site, but not at a control location in the opposite hemifield. Our data provide direct evidence that the SC contributes to the control of covert spatial attention. discrimination ͉ psychophysics S everal lines of evidence suggest that eye movements and covert attention may be mediated by the same neural mechanisms (1-3). For example, a human subject can direct attention to a specific location in space, thereby gaining a measurable advantage in visual performance, even while maintaining fixation at an altogether different location (4). During a saccadic eye movement, however, the subject is unable to direct attention to any location other than the endpoint of that eye movement (5).In the monkey, electrophysiological experiments have increasingly implicated eye-movement planning structures in the control of covert spatial attention. Following the original observation by Goldberg and Wurtz (6) of attention-related neural activity in the superior colliculus (SC) (7, 8), single-unit recordings have detected attentional effects in other eye movementrelated areas of the brain, including the inferior parietal cortex (9-11) and the frontal eye fields (FEF) (12)(13)(14). Recent studies by Bisley and Goldberg (15) in the lateral intraparietal area and by Ignashchenkova et al. (16) in the SC were particularly incisive because the neural effects correlated parametrically with variations in the strength and timing of attentional effects in the behavioral data. Electrophysiological evidence, however, is necessarily correlative and cannot demonstrate that neural activity causes behavior (17).Remarkable studies published recently by Moore and Fallah (18,19) bridged this gap. Using visual threshold measurements as a behavioral metric of attention, they showed that electrical microstimulation of the FEF improved psychophysical performance by facilitating the deployment of attention to the location of the visual stimulus. The effect was spatially localized to the region of the visual field encoded at the stimulation site and thus could not be attributed to a general increase in arousal or vigilance. This was a landmark study first because of its implications concerning the neural substrate of visuo-spatial attention, but more importantly because it is the only d...