Our sense of spatial orientation results from a complex set of neural processes of sensory integration; these processes utilize

INTRODUCTION Our sense of spatial orientation results from a complex set of neural processes of sensory integration; these processes utilize information from many different physiological systems. Evidence suggests that extended exposure to micro-gravity yields adaptive changes in these neural processes. For the most part, these adaptive changes appear functionally relevant for spaceflight but yield significant transient problems when astronauts return to gravitational environments (Earth, Mars, Moon, etc.). In-flight countermeasures that reinforce the normal sensory interactions encountered in a gravitational environment (e.g., rotations about any axis not aligned with gravity are accompanied by changes in the relative orientation of gravity) will almost certainly help astronauts function adequately when they transition to a gravitational environment. This will be important to pilot astronauts landing spacecraft as well as to crewmembers trying to rapidly exit a spacecraft. Furthermore, artificial gravity has been proposed as a countermeasure for numerous problems associated with spaceflight (including cardiovascular deconditioning, bone decalcification, etc.). One undesired side effect of artificial gravity will be additional alterations in these processes of sensory integration. For all of these reasons, it is essential that we understand the predominant sensory interactions that are altered by exposure to micro-gravity.