The influence of the consistency of postsaccadic visual errors on saccadic adaptation.

The saccadic system is a prime example of motor control without continuous visual feedback. These systems suffer from a strong vulnerability against disturbances. The mechanism of saccadic adaptation allows adjustment of saccades to alterations arising not only from anatomical changes but also from external changes. The weighting of errors according to their reliability provides a strong benefit for an optimized control system. Thus the consistency of visual error should influence the characteristics of adaptation. In the typical adaptation paradigm a visual error is introduced by stepping the target during the saccade by a given amount. In this paradigm, the retinal error varies with the accuracy of the saccade and the step size. To study the influence of error consistency we use a variant of the adaptation paradigm which allows to specify a constant error size. Intrasaccadic target step sizes were calculated with respect to the predicted landing position of each individual saccade. The consistency of the visual error was varied by introducing different levels of noise to the intrasaccadic target step. Different mean intrasaccadic target step sizes were examined: positive target step, negative target step, and a condition in which the mean of the error distribution was clamped to the fovea. In all three conditions saccadic adaptation was strongest when the error was consistent and became weaker as the error became more variable. These results show that saccadic adaptation takes not only the average error but also the consistency of the error into account.