Reaching fails where saccades succeed: nonrigid motion reveals a dissociation between saccadic and limb pathways in predicting future target position

Intercepting a moving target requires prediction: knowing the current position is not enough due to sensorimotor delays. A moving "vortex" stimulus, which lacks a conventional velocity signal for smooth pursuit yet still elicits accurate saccade targeting, was used here to assess whether it also supports accurate reaching movements. In our experiments, participants reached to the vortex or to a rigid moving disk. Three manipulations probed prediction and online control: constant-speed interception, speed perturbations at reach onset, and target disappearance at reach onset. Reaction times when reaching to the vortex did not depend on target speed in the way they do for rigid targets. Moreover, when reaching to the vortex participants made larger undershoots that increased with target speed, and they compensated less for speed changes. Thus, the vortex provides motion-related information that is sufficient for accurate saccadic localization, but that is not effectively exploited for predictive manual interception. This dissociation suggests that motion information may not be used in the same manner across motor effectors.