Abstract
Saccades and smooth pursuit eye movements are essential to visual perception and to guide many of our actions. The predictability of the environment allows not only to promptly respond to visual stimuli but also to drive anticipatory eye-movements towards the expected target location or motion direction. In two-directions motion tasks where the uncertainty about target motion direction was experimentally manipulated, several previous studies have documented anticipatory smooth eye movements (ASEM), starting ahead of target motion onset and directed in the most likely direction. Under these conditions, we previously showed that mean anticipatory eye-velocity is a linear function of direction probability. However, it is still an open question how robust and generalizable such tuning of anticipatory eye movements is with regard to more complex probabilistic manipulations of the environment. To address this question, we recorded (Eyelink1000) pursuit eye movements in healthy human volunteers while performing two novel motion tracking experiments. First, we show that ASEM can still be evoked in more complex environments, such as when the target can move in four different directions, with different probabilities for each direction. Our results indicate that anticipatory eye velocity towards the most probable direction increases linearly with its actual probability, similarly to the two-directions condition. In a second experiment with a two-segments target motion trajectory, we manipulated the probability of a 90° right/left motion turn, being conditional upon the direction (up/down) of the first target motion segment. Results demonstrate that a bias in the conditional-probability does also evoke and modulate ASEM. However, the relationship between conditional probability and ASEM is more variable across participants and may not be exactly linear. Overall, our results extend the previous results about probability-based anticipatory eye movements to more complex tasks and environmental contingencies.