Abstract
Retinal motion, stereo, and extra-retinal information from pursuit eye movements all provide information for catching a ball. Predicting the ball's path is also important when hitting a cricket ball (Land & McLeod; 2000) and when catching a bouncing ball thrown by another (Hayhoe et al., 2004). In both tasks, gaze was directed to the bounce point just prior to the arrival of the ball, suggesting knowledge of the dynamic properties of the ball allows prediction of the ball's path. When subjects have to catch a ball they throw themselves, is there a greater reliance on prediction, since they have added information from the throw?
We asked subjects to throw a ball against a wall, ensuring that it bounced on the floor prior to contact with the wall, and to catch the returning ball. There were therefore 2 bounce points between each throw/catch. The ball bounced approximately 175ms and 339ms after release. Subjects infrequently looked at the first bounce point (20%), opting instead to direct gaze to the second bounce point (80%), after which subjects pursued the returning ball. Secondly, fixations were significantly closer to the bounce point on the wall than on the floor (7deg v 13deg). In addition, arrival of gaze on the wall preceded the ball by 324ms. This was much earlier than subjects who had to catch the same ball when thrown by another person (53ms ahead of the bounce).
While evidence exists for the role of internal models of the body's dynamics in the control of movement, the need for internal models of the environment is less established. Fixating the wall when throwing the ball off the floor suggests we can rely on internal models of the properties of the ball and the environment in tasks such as throwing and catching a ball. Knowledge of the throw also allows a greater use of prediction.
References. LandMF.McLeodP. (2000) Nature Neuroscience, 3(12), p1340–1345. HayhoeM. M.MennieN.GorgosK.SemrauJ.SullivanB. (2004). Journal of Vision, 4(8), 156a.
Supported by NIH grants EY05729 and RR09283