The use of visual feedback and on-line target information in catching and grasping.

Although visual feedback (i.e. seeing our hand while we move it) improves the accuracy and efficiency of grasping movements, these positive effects of visual feedback are not consistently found for catching. It was the aim of our study to compare the efficiency of the use of visual feedback in grasping and catching and to explore possible reasons why visual feedback effects have been found less consistently in catching than in grasping. The first reason might be technical. Less sensitive measurement methods have been used in catching; this might explain why some catching studies did not find visual feedback effects. This problem was avoided in our study by using the same methods for both the catching and the grasping task. The effects of visual feedback were examined under standard conditions and under conditions where subjects wore prismatic glasses. Nevertheless, visual feedback effects were obtained for grasping but not for catching movements. This confirms that the difference in the use of visual feedback is real and not due to technical differences between grasping and catching studies. The second reason relates to the different temporal demands of grasping and catching. During a catching task, subjects have less time to respond, and therefore might not have sufficient time to use visual feedback. We tested this explanation with a task that required subjects to reach for a stationary object (i.e. grasping) as quickly as they had for the moving object in the catching task. However, even in this time-constrained grasping task, significant visual feedback effects were found, suggesting that time constraints do not explain the lack of visual feedback effects in catching. In our last explanation, we suggest that possibly the mode of motor control is different for catching and grasping, more particularly while grasping allows for on-line corrections, such corrections might not be possible for catching movements. We tested this explanation with a catching task that contained perturbation trials. During those perturbation trials, the target trajectory was shifted, after the subject had already started to move. We found that subjects responded to the target shifts with an appropriate shift of their catching response. This shows that on-line corrections are possible in the case of catching movements. We conclude that neither differences in the temporal demands nor in the capacity to make on-line modifications explain why visual feedback is used less effectively in the catching than in the grasping task.