Reference frames and internal models for visuo-manual coordination: What can we learn from microgravity experiments?

Gravity plays a role in many different levels of human motor behavior. It dictates the laws of motion of our body and limbs, as well as of the objects in the external world with which we wish to interact. The dynamic interaction of our body with the world is molded within gravity's constraints. The task of catching a ball that has been thrown toward a human subject typifies the kind of constraints that the nervous system must take into consideration during visuo-manual coordination on earth. By dissecting and examining the components of this task, one can see what kinds of problems must be solved by the central nervous system to generate coordinated motor actions in response to incoming sensory information. In this review, we use the example of a ball catching task to outline various issues in the field of human motor control and to ask the question as to how the microgravity environment of lower earth orbit can be used to probe the functioning of the human motor system.