In this chapter we have discussed some of the consequences and selection implications of the biomedical alterations that occur in space. Although the physiological changes associated with weightlessness induced deconditioning have been extensively researched, considerably less attention has been given to the perceptual and behavioral correlates of these changes. As an example, bedrest studies have been the predominant model for the study of  space related biomedical changes. However, very few of these experiments have included performance measures, and those that have measures have generally experienced procedural difficulties or resulted in inconsistent findings. There is both an opportunity and a need to extend hypokinesia studies to include psychomotor and cognitive performance measures.
Aside from the issue of behavioral changes associated with biomedical alterations in space, there are psychophysiological research questions surrounding various deconditioning countermeasures. In the past we have relied upon the selection of athletic men for the astronaut program and have employed rigorous physical training programs to improve or at least maintain their high levels of conditioning. Research now indicates that extensive pre mission physical conditioning may be inappropriate or even detrimental to space adjustment. Similarly, the type and amount of in flight exercise which may be a useful countermeasure for particular physiological systems has received renewed attention. The question of the physical characteristics of selected crews has become recent research suggesting that older and/or less physically fit individuals may have certain advantages in adjusting cal stressors of space. Studies simulating the capacity of women to tolerate space biological stressors also must be considered. On most measures men and women respond similarly to the stresses of weightlessness; on a small number of responses men appear to have an advantage. However, women consume less food and oxygen, weigh Iess, and appear to be more radiation resistant. Moreover, there is evidence that, overall, women tolerate the combined pressures of space better than men. Information on possible gender differences needs to be gathered through in flight experience.
The use of medication as a countermeasure to the biological stressors of space has important implications for understanding performance. Research conducted in extreme environments indicates that alterations in drug action can occur when normal biomedical functioning is modified. We need to assess whether any changes in drug action occur in space and what the behavioral ramifications of these changes may be. Related questions involve assessing how weightlessness influences rhythmicity and how the time of drug administration may influence drug action and possible behavioral correlates. The new fields of chronobiology and chronopharmacology explore these prospects and need to be integrated with the general question of how drugs operate in space.
 Artificial gravity, either in the form of vehicle rotation or on board centrifuge devices, has frequently been suggested as a means of compensating for adverse spaceflight alterations. Vehicle rotation would require a rather massive structure to simulate completely Earth's gravity. Even then the movement dynamics aboard such a vessel would be considerably different from those on Earth and would require a period of adaptation. The benefits and limitations of such a simulation need to be understood, as does the way gravity simulations of less than 1 g might mitigate the adverse effects of weightlessness. The use of on board centrifuge devices is probably a more practical near term possibility for use in space. Current experiments with animals suggest that this technique may be prophylactic for some uses. However, this approach does not appear to prevent bone decalcification and may produce some unpleasant side effects related to the abnormal vestibular stimulation produced by centrifugation. The overall usefulness of this technique for future missions needs to be assessed.
Another question concerns the behavioral correlates of weightlessness effects on the vestibular complex. Most investigators conclude that two distinct problems result from the unusual circumstances of the vestibular system in space. One involves spatial disorientation, illusions, and vertigo, and a second class of responses mimics what on Earth is called motion sickness. Most attention has been directed toward the latter category because of the frequency of its occurrence and the potentially debilitating effects that illness could have upon the flight crew. There is a need to continue development of theoretical models that can link together available data and provide direction for future efforts. The sensory conflict theory appears to be a promising choice.
Several countermeasures now in use for the treatment of motion sickness are discussed in this chapter. Future possibilities for minimizing space sickness include the use of biofeedback and the identification of more effective selection procedures. The relationship of demographic variables such as age and gender to motion sickness needs to be examined more fully. Also, studies correlating certain personality characteristics and propensities toward motion sickness appear promising.
The third major system discussed is the visual complex. Although it now appears unlikely that weightlessness significantly alters visual acuity, routine studies are needed to document how such functions as dark adaptation, brightness and color sensitivity,  accommodation, etc., are affected during spaceflight. Radiation may have an effect on vision as noted in previous missions, leading to perceptions of "light flashes." An important question is whether there is any cumulative effect associated with this irradiation which might jeopardize crews of long duration missions. Another aspect of space travel that may adversely affect vision is vehicle vibration. With manual landings a routine requirement of Shuttle like operations, it will be important to continue investigations on how vibration alters vision and what steps can be taken to minimize or compensate for vibration induced losses.