Asynchrony between position and motion signals in the saccadic system.

The influence of position and motion signals on saccades was studied in two dimensions (2D) using a double step-ramp paradigm. We showed the presence of a predictive component in 2D catch-up saccade programming that is based on motion signals and influences both saccade amplitude and orientation. Interestingly, a significant proportion of catch-up saccades was characterized by a large curvature or a sudden change of direction in midflight for large values of retinal slip. For these saccades, a quantitative analysis showed that their trajectory could be explained by an asynchrony between position and motion signals in saccade programming. When the saccade trajectory was not straight, position error was always available first and influenced the initial orientation of the saccade, whereas retinal slip determined the final orientation. This new paradigm could be used in electrophysiological experiments, where it should prove to be very useful to study position and motion pathways separately in catch-up saccades