Adaptive changes in saccade amplitude: oculocentric or orbitocentric mapping?

The saccadic system rapidly adjusts the amplitude of refixation movements to visual targets when abnormal postsaccadic errors occur. This is called rapid saccadic adaptation. It is not yet clear whether this form of adaptation produces changes related to oculocentric mechanisms, such as retinal error or motor error, or orbitocentric mechanisms, such as eye or gaze position. These experiments were designed to test whether rapid saccadic adaptation was orbitocentric, oculocentric, or both by creating a precise sensory motor mismatch between the visual target and the required saccade. Measurements were made to determine adaptive changes as function of (1) saccade direction; (2) eye position; and (3) saccade amplitude. Changes were found to be amplitude- and direction-specific but changes were generalized across a broad range of orbital positions. Two conditions of adaptation: increasing and decreasing amplitude, produced quantitatively similar results, indicating that similar mechanisms underlie both processes. Thus, these data support the view that changes during rapid saccadic adaptation are organized principally in a retina-referenced (oculocentric) map, but only broadly, if at all, in a head-referenced (orbitocentric) map. The changes are consistent with a mechanism represented in a spatial mapping of either retinal or motor error.