Tuning to a hip-hop beat: Pursuit eye movements reveal processing of biological motion

Abstract

Smooth pursuit eye movements can anticipate predictable movement patterns, thus achieving their goal of reducing retinal motion blur. Oculomotor predictions have been thought to rely on an internal model of the target kinematics. Since biological motion is one of the most important visual stimuli in regulating human interaction, we asked whether there is a specific contribution of an internal model of biological motion in driving pursuit eye movements. Unlike previous contributions, we exploited the cyclical nature of walking to measure eye movement’s ability to track the velocity oscillations of the hip of point-light walkers. We quantified the quality of tracking by cross-correlating pursuit and hip velocity oscillations. We found a robust correlation between signals, even along the horizontal dimension, where changes in velocity during the stepping cycle are very subtle. The inversion of the walker and the presentation of the hip-dot without context incurred the same additional phase lag along the horizontal dimension, whereas a scrambled walker incurred no phase lag relative to the upright walker. Those findings support the view that local information beyond the hip-dot, but not necessarily configural information, contribute to predicting the hip kinematics that control pursuit. We also found a smaller phase lag in inverted walkers for pursuit along the vertical dimension compared to upright and scrambled walkers, indicating that inversion does not simply reduce prediction. We show that pursuit eye movements provide an implicit and robust measure of the processing of biological motion signals.