The human nervous system has developed and evolved to cope with the ever-present force of gravity. Gravity 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 interaction of our body with the world is molded within gravity's constraints and the human nervous system has certainly adapted to meet those specific conditions.

Gravity plays a role at many different levels of human sensori-motor behavior. The function of the vestibular system has been extensively studied in microgravity due to the clear link between this sensory organ and the gravitational field. The unique microgravity conditions of lower Earth orbit have been used to study, for example, the vestibular-ocular reflex, gaze and head stabilization and the control of standing posture. Gravity might also provide directional cues during development of the nervous system. By studying embryonic and post-natal development in animals one can better understand not only what motor strategies are employed by the nervous system to cope with the forces of gravity, but also how gravity influences the development of the vestibular apparatus itself. Finally, the presence of gravity can influence higher-order cognitive functions such as face recognition, perception of self-motion, navigation and strategies for eye-hand coordination. Thus, one can ask how the human nervous system accounts for the effects of gravity for a wide variety of everyday tasks.

The interest of studying these questions for the development of manned space-flight is obvious - understanding the physiology of is essential both for the ergonomic design of tools and devices to be used in the microgravity environment and for the development of effective counter-measures to assure the health of an astronaut returning from a long-duration mission. What is perhaps more important to the vast majority of people, however, is to better understand how the nervous system functions here on Earth.

In this respect, Earth orbit provides a unique opportunity to observe the influence of gravity on sensorimotor function. More specifically, microgravity can be used to better understand how the CNS combines information from different sensory modalities in a normal gravitational environment. Normally it is very difficult to separate the influence of different sensory cues on visuo-motor function. For instance, head tilt evokes changes in perceived head orientation both through otolith activity and through neck proprioception. Similarly, whole-body tilt introduces a whole new set of haptic cues in addition to the vestibular sensation of rotation. In fact, the CNS may make use of the constant reference afforded by gravity to align information from different sensory modalities. In microgravity, one is able to tease out the contribution of different modalities by independently varying sensory information in a way that is not possible on Earth. In this review of recent microgravity studies I hope to illustrate how the microgravity environment of lower earth orbit has and will be used to probe the functioning of the human sensory and motor systems.

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