Monkey superior colliculus activity during short-term saccadic adaptation

This article concerns the neural mechanisms that underlie short-term saccadic adaptation in the rhesus monkey. By means of a consistent intrasaccadic target displacement, the relation between visual input and motor output was gradually changed in three monkeys, such that they made hypometric saccades. During this process, the activity of saccade-related burst neurons in the intermediate and deep layers of the Superior Colliculus (SC) was recorded in two of the monkeys. Our findings show that, like in humans, only saccades evoked within a restricted field around the adaptation target were adapted. However, unlike in humans, the kinematic properties of adapted saccades also changed systematically during the adaptation process. Typically, adapted saccades were slower and had a longer duration than would be expected on the basis of the main sequence for nonadapted visually guided movements. During adaptation, saccade-related activity of units in the SC remained appropriate for the saccade that was required to foveate the initial target, rather than for the saccade that was actually made. This means that adaptation caused a dissociation between SC activity and the ensuing saccade. Thus, the activity of the colliculus was better described in "required eye displacement coordinates" than in "actual eye displacement coordinates." Our data provide further evidence for the hypothesis that short-term saccadic adaptation acts at a level downstream from the SC, presumably at a stage that determines the kinematics of saccadic eye movements.