Corrective saccades: effect of shifting the saccade goal.

A double step paradigm was used to investigate the saccade control, in particular the role of retinal feedback on correction saccades. The first target step eliciting the primary saccade had an amplitude of 10-15 deg and was followed by a second target step of 3 deg which occurred 0-300 msec after the onset of the primary saccade. The characteristics of the saccadic reactions were analyzed as a function of the time between the end of the primary saccade and the second target step. The data show that correction saccades can be modified when the second target step occurs earlier than 60 msec after the end of the primary saccade. Secondary saccades then are corrective, and their parameters show no differentiation between endogenous and exogenous refixation errors. In further experiments, additional blanking periods were inserted separating the visual system from reafferent information. Under this condition, the system is silent and starts the programming of the correction saccade only when the target is illuminated again. The data demonstrate that retinal feedback is a fundamental part in refixation.