Vector inversion diminishes the online control of antisaccades.

Antisaccades require the suppression of a stimulus-driven response (i.e., response suppression) and the computation of a movement plan mirror-symmetrical to the location of a target (i.e., vector inversion). The goal of the present study was to determine whether response suppression, vector inversion or both contribute to previously reported differences in the online control of pro- and antisaccades (Heath in Exp Brain Res 203:743-752, 2010a). Pro- and antisaccades were completed in separate blocks (i.e., blocked schedule) and a block wherein the spatial relation between stimulus and response was provided at response cuing (i.e., random schedule). Notably, the random schedule provides a relative means for equating response suppression across pro- and antisaccades. To examine online trajectory amendments, we computed the proportion of variance (R² values) explained by the spatial location of the eye at early, middle and late stages of saccade trajectories relative to the saccade's ultimate endpoint. The basis for this analysis is that between-task differences in R² magnitudes reflect differences in the use of feedback for online trajectory amendments: small R² values represent a trajectory supported via online control whereas larger R² values reflect a reduction in online control. Results show that antisaccades yielded larger R² values than prosaccades from early to late stages of saccade trajectories, and this finding was observed regardless of whether or not tasks were equated for response suppression. Thus, we propose that the intentional nature of vector inversion disrupts the normally online control of saccades and renders a mode of control that is not optimized to support error-reducing trajectory amendments.