Limb position sense and sensorimotor performance under conditions of weightlessness.

This is a review of the current state of knowledge of the effects of weightlessness on human proprioception. Two aspects have been highlighted: the sense of limb position and performance in sensorimotor tasks. For the sense of position, an important consideration is that there probably exists more than one sense: one measured in a blindfolded, two-limb position matching task, the other, by pointing to the perceived position of a hidden limb. There is evidence that these two senses are supported by distinct central projection pathways. When assessing the effects of weightlessness this must be considered. Whether there is a role for vestibular influences on position sense during changes in gravitational forces is an issue for future experiments. A consideration that has proved helpful for the study of sensorimotor tasks under conditions of weightlessness is to examine the performance of subjects who have lost their proprioceptive senses, either congenitally, or later in life, as a result of disease. In weightlessness, normal subjects appear to have particular difficulties with feedback-controlled tasks. A major factor is the influence of vision on performance. In addition, the stress of working in a weightless environment leads to additional cognitive load, making the execution of even simple everyday tasks difficult.