It is well known that we can attend to images in our visual periphery. Numerous experimental studies have demonstrated that visual perception is improved at the location of attention. Even though primates can easily separate the direction of attention from any motor response, it has been hypothesized that those brain processes that mediate the shift of attention also prepare the organism to make an eye movement to the attended location. To study the relationship between attention and eye movements, we have trained monkeys on a variety of attention and eye movement tasks. When monkeys are correctly cued, either symbolically or peripherally, to a location in space, they make eye movements to that location rapidly. When the cueing is invalid, their eye movements are slowed. While monkeys were performing these tasks, we electrically stimulated the superior colliculus. The direction of the eye movements evoked by collicular stimulation was shifted toward the attended location. The time course and magnitude of these effects varied, depending on the type of task, validity of cueing, and interval within the task. These animals were both shifting their attention and preparing to make saccadic eye movements. We also trained monkeys on variants of these tasks; here, their behavioral response was a hand movement. Under these conditions, stimulation of the superior colliculus produced the same effects. When the animals shifted their attention toward a spatial location, the eye movement evoked by collicular stimulation was deviated toward that location. The effects were fastest and largest with peripheral cueing. When the initial cue was invalid, the deviation initially was to the cue; it later shifted directions after the correct target was presented. These data show that shifts of attention, however they are generated, are associated with changes in the status of the oculomotor system. They are consistent with the hypothesis that each time there is a shift of attention, there is a preparation to make an eye movement to the attended location. These data suggest that within the brain, the attentional and saccadic systems are the same.
