Changes in control of saccades during gain adaptation

In a typical short-term saccadic adaptation protocol, the target moves intrasaccadically either toward (gain-down) or away (gain-up) from initial fixation, causing the saccade to complete with an endpoint error. A central question is how the motor system adapts in response to this error: are the motor commands changed to bring the eyes to a different goal, akin to a remapping of the target, or is adaptation focused on the processes that monitor the ongoing motor commands and correct them midflight, akin to changes that act via internal feedback? Here, we found that, in the gain-down paradigm, the brain learned to produce a smaller amplitude saccade by altering the trajectory of the saccade. The adapted saccades had reduced peak velocities, reduced accelerations, shallower decelerations, and increased durations compared with a control saccade of equal amplitude. These changes were consistent with a change in an internal feedback that acted as a forward model. However, in the gain-up paradigm, the brain learned to produce a larger amplitude saccade with trajectories that were identical with those of control saccades of equal amplitude. Therefore, whereas the gain-down paradigm appeared to induce adaptation via an internal feedback that controlled saccades midflight, the gain-up paradigm induced adaptation primarily via target remapping. Our simulations explained that, for each condition, the specific adaptation produced a saccade that brought the eyes to the target with the smallest motor costs.