activated, leading to small eye movements. For saccades, neurons are activated in a region that represents the point to which the saccade will be directed. Just prior to a saccade, activity rapidly builds up at the target location and decreases in other parts of the SC. The coding is rather broad, so that for any given saccade the activity profile forms a "hill" that encompasses a substantial fraction of the collicular map: The location of the peak of this "hill" represents the saccade target. The SC encodes the target of a gaze shift, but it does not seem to specify the precise movements needed to get there. The decomposition of a gaze shift into head and eye movements and the precise trajectory of the eye during a saccade depend on integration of collicular and non-collicular signals by downstream motor areas, in ways that are not yet well understood. Regardless of how the movement is evoked or performed, the SC encodes it in "retinotopic" coordinates: that is, the location of the SC 'hill" corresponds to a fixed location on the retina. This seems to contradict the observation that stimulation of a single point on the SC can result in different gaze shift directions, depending on initial eye orientation. However, it has been shown that this is because the retinal location of a stimulus is a non-linear function of target location, eye orientation, and the spherical geometry of the eye. There has been some controversy about whether the SC merely commands eye movements, and leaves the execution to other structures, or whether it actively participates in the performance of a saccade. In 1991, Munoz et al., on the basis of data they collected, argued that, during a saccade, the "hill" of activity in the SC moves gradually, to reflect the changing offset of the eye from the target location while the saccade is progressing. At present, the predominant view is that, although the "hill" does shift slightly during a saccade, it does not shift in the steady and proportionate way that the "moving hill" hypothesis predicts. However, moving hills may play another role in the superior colliculus; more recent experiments have demonstrated a continuously moving hill of visual memory activity when the eyes move slowly while a separate saccade target is retained. The output from the motor sector of the SC goes to a set of midbrain and brainstem nuclei, which transform