Role of retinal slip in the prediction of target motion during smooth and saccadic pursuit

Visual tracking of moving targets requires the combination of smooth pursui t eye movements with catch-up saccades. In primates, catch-up saccades usua lly take place only during pursuit initiation because pursuit gain is close to unity. This contrasts with the lower and more variable gain of smooth p ursuit in cats, where smooth eye movements are intermingled with catch-up s accades during steady-state pursuit. In this paper, we studied in detail th e role of retinal slip in the prediction of target motion during smooth and saccadic pursuit in the cat. We found that the typical pattern of pursuit in the cat was a combination of smooth eye movements with saccades. During smooth pursuit initiation, there was a correlation between peak eye acceler ation and target velocity. During pursuit maintenance, eye velocity oscilla ted at similar to3 Hz around a steady-state value. The average gain of smoo th pursuit was similar to0.5. Trained cats were able to continue pursuing i n the absence of a visible target, suggesting a role of the prediction of f uture target motion in this species. The analysis of catch-up saccades show ed that the smooth-pursuit motor command is added to the saccadic command d uring catch-up saccades and that both position error and retinal slip are t aken into account in their programming. The influence of retinal slip on ca tch-up saccades showed that prediction about future target motion is used i n the programming of catch-up saccades. Altogether, these results suggest t hat pursuit systems in primates and cats are qualitatively similar, with a lower average gain in the cat and that prediction affects both saccades and smooth eye movements during pursuit.