Manifestations and Theory
Another system adversely affected by the conditions of spaceflight is the vestibular complex. Two categories of vestibular side effects result from weightlessness. One category includes a variety of vestibular reflex phenomena such as postural and movement illusions, vertigo, and dizziness; the second category is space motion sickness. These two categories of response are believed to be closely tied; motion sickness often follows vertigo and postural illusions, and there is evidence to suggest that as vestibular reflex phenomena disappear with adaptation, the risk of motion sickness subsides (Reason and Brand,1975).
Vertigo and spatial disorientation-The first reported instance of spatial disorientation in spaceflight was experienced in 1961 by the Soviet pilot aboard Vostok 2 (Graybiel, Miller, and Homick, 1974). For a brief period immediately after exposure to 0 g, the pilot felt that he was flying upside down. Also, he experienced vertigo during his fourth orbit. During the Voskhod I flight of 1964, the scientist cosmonaut and the on board physician both reported the inversion illusion (one spaceman imagined he was half crouched and facing downward, and the other felt he was hanging upside down). These illusions persisted regardless of whether the cosmonauts' eyes were open or shut. Yakovleva, Kornilova, Tarasov, and Alekseyev (1982) report that 21 of 24 cosmonauts who made flights in Soyuz-type spacecraft and served in the Soyuz/Salyut orbital complex reported postural illusory reactions. During the Apollo and Skylab programs, several American astronauts reported similar sensations. There were also reports of movement illusions: feelings of tumbling and sensations of spinning (Berry and Homick, 1973; Graybiel, Miller, and Homick, 1977).
Symptoms of vertigo and spatial distortion in space have so far posed no serious hazards to crews. They have been relatively shortlived and at worst an annoyance. As reported by the Skylab 2 pilot, although he was aware of illusory phenomena, their intensities made  little impression (Kerwin, 1974). Nonetheless, it is important that we' understand the nature and control of these phenomena because of their i' potential impact on crews under emergency conditions If unplanned tumbling, rolling, or spinning of the spacecraft should occur (for instance,. during docking)), the consequent vestibular reflex responses could seriously hinder regaining control of the vessel,
Space sickness- The second category of vestibular side effects reported in space includes symptoms resembling Earth motion sickness. These symptoms range from stomach awareness and nausea repeated vomiting. Symptoms also include pallor and sweating.
Space sickness2 has been a recurring problem in the history of the space program. While this syndrome appears to decline within 3 to 5 days, in some cases the degree of illness has hindered work. capacity and disrupted the scheduling of important mission activities. Nine of the 25 Apollo astronauts suffered some degree of sickness while five of nine Skylab crewmen experienced symptoms (Berry, 1970, Graybiel et al., 1977). Soviet cosmonauts have reported similar experiences (Yuganov, Gorshkov, Kasian, Brianov, Kolosov, Kopanev, Lebedev, Papov, and Solodovnik, 1966; Volynkin and Vasil'yev, 1969). Four of the nine crewmembers of the Vostok and Voskhod missions suffered some degree of illness. There were also suggestions that illness suffered during the Soyuz 10 flight may have been a reason for the premature termination of this mission
It is interesting to note, however, that despite the presence of intense space sickness symptoms, task performance during flight has been only moderately affected. This observation meshes well with our understanding of the effects of motion sickness upon performance under Earth based simulation conditions. In a series of studies conducted at Wesleyan University (Alexander, Cotzin, Hill, Ricciuti and Wendt, 1945a, 1945b, 1945c, 1945d, 1955), subjects exposed to wave like motion showed only slight deficits in performance on eight kinds of tasks, despite suffering nausea and vomiting. Only performance on a tracking device was significantly reduced among experimental (e.g., sick) versus control (e.g., nonsick) subjects. No statistically significant decrements were found in running through sand  and around various obstacles, a 660 yard dash, dart throwing, speed and accuracy of rifle shooting, code substitution testing, or mirror drawing (tracing the outline of a star when pencil and paper are seen only as mirror images). Clark and Graybiel (1961) found similar results among subjects exposed to 2 day periods of constant slow rotation. Tasks measured included strength of grip, standing on one or two feet, walking a straight line, ball tossing, dart throwing, a steadiness test, card sorting, opening combination locks, dial setting, and arithmetic computations. Although the absolute levels of performance, on the average, matched performance under normal conditions, the pattern of performance differed. There were frequent occasions when severe sickness was associated with poor performance or failure to perform. There were many cases also in which high performance occurred despite symptoms of nausea, dizziness, general malaise, and vomiting. These results support the notion that motion sickness primarily affects motivation rather than performance potential per se. Birren (1949) incorporated this idea in an early study of seasickness and crew performance. He found that "peak efficiency" is likely to be unaffected by all except the most severe forms of motion sickness. High levels of motivation engendered by a crisis can completely overshadow the physical discomforts of motion sickness. However, "maintenance efficiency" may suffer severe decrement as a result of sickness because of loss of motivation.
Clearly, motivation plays a very large part in determining the level of performance in a motion sick individual. The superior motivation of spacecrews has no doubt been a reason for the continued high level of performance achieved in flight despite the presence of space sickness symptoms. As we anticipate future, more routine flights, crew motivation may not be maintained at this high level and decrements in performance could result.
Sensory conflict- As a means of summarizing and integrating, information on vestibular effects, it is important to draw upon a theoretical base. While both spatial disorientation and space sickness pose potential threats to the conduct of a mission, space sickness has been the more obvious and disturbing factor in spaceflight to date. As a result, theoretical considerations, as well as methods of averting, or controlling problems associated with vestibular changes, have tended to focus on the issue of space sickness. However, spatial disorientation effects and space sickness are believed to be interrelated phenomena. Understanding, the mechanisms under lying space sickness is an important step in understanding the general nature and effects of vestibular alterations.
 A number of biomedically oriented theories have been proposed for use in understanding space sickness (see Reason and Brand, 1975), but let us here focus on a fairly well acknowledged psychophysiological model the sensory conflict theory. Stated briefly, this theory postulates that motion sickness occurs when patterns of sensory input to the brain from the vestibular system, other proprioreceptors, and/or the visual system are markedly rearranged, at variance with each other, or differ substantially from expectations of stimulus relationships in a given environment. In space, sensory conflict can occur in several ways. First, there can be conflicting information transmitted by the otoliths and the semicircular canals of the ear. It is postulated that the otoliths are functionally deafferented due to the effects of weightlessness, whereas the semicircular canals are not radically altered by 0 g and continue to send impulses regarding angular accelerations. When the astronaut moves about in space, the semicircular canals signal that movement to the brain, while the otoliths remain inactive, resulting in conflicting sensory input to the brain. Sensory conflict may also exist between the visual and vestibular systems during motion in space. The eyes transmit information to the brain indicating body movement, but no corroborating impulses are received from the otoliths.
A third type of conflict may exist in space because of differences in perceptual habits and expectations. On Earth, we develop a neural store of information regarding the appearance of the environment and certain expectations about functional relationships (e.g., the concepts of "up" and "down"). In space, these perceptual expectations are at variance with the new functional relationships associated with weightlessness. This new environmental orientation is succinctly expressed in the report of Astronaut Gibson (1974) of the Skylab 4 crew:
It is important to note that no single source of sensory conflict appears to account entire for the symptoms of space sickness.
 Rather, it is the combination of these conflicts which somehow produces sickness, although the exact physiological mechanisms remain unknown.
The sensory conflict hypothesis has proved very useful as a structure in which to organize the available empirical data. It clarifies why a discrepancy in symptomatology between astronauts and cosmonauts existed early in the space program. Initially, Soviet crewmen reported considerable difficulty related to space sickness, although no reports of these symptoms occurred for American crewmen prior to the Apollo missions. The American Mercury and Gemini capsules were considerably smaller than the Soviet Vostok and Voskhod vehicles, thus restricting movement to a greater degree. With less movement capability, there was less potential for sensory conflict between visual and/or canal impulses and otolithic input. In the Apollo vehicles (which were comparable in size to the earlier Soviet vehicles) considerably more opportunity existed for movement generated sensory conflicts, and space sickness did occur.
The experiences of Skylab missions are of interest here. While aloft, Skylab crewmen moved between the Command Module and the larger Workshop. In this situation, two separate adaptations may have been required. Of the nine Skylab crewmen, two astronauts first experienced space sickness inside the Command Module. After adapting to this situation, one of these individuals again became sick upon entering the Workshop. Two other crewmen, who had evidenced no symptoms inside the Command Module, developed their first symptoms upon entering the Workshop. These findings highlight the importance of a large area in providing more opportunity for movement and for sensory conflict and therefore a higher incidence of sickness. These results can also be interpreted to support the research finding that adaptation to one situation does not necessarily provide protection in another. Adapting to the sickness provoking conditions of the Command Module was not a satisfactory prophylactic for preventing sickness inside the larger Workshop.
Given the dramatic adaptation which must occur in space, we may ask whether there are specific training and design considerations which would help reduce the feelings of unfamiliarity with the environment, and also perhaps reduce the problems of motion sickness and disorientation. Although there are design considerations favoring a 360° orientation, the sensory conflict theory suggests there might be advantages associated with an environment more Earth like in orientation. Even a simple feature, such as strips of  material attached to the walls to create the impression of "up" and "down" was found helpful by Soviet cosmonauts (Leonov and Lebedev, 1975). However, it is possible that improving orientation within the vehicle could exacerbate the problem if the crew member then had to reorient to a conflicting scene outside the spacecraft. In addition to design aids, we might consider providing crews with preflight experiences in their craft simulators at orientations other than horizontal and Earth based "up" and "down." This could be accomplished through water immersed simulators, where the environment could be rotated away from the horizontal (Abbott and Duddy, 1965; Adams and Bulk, 1967).
It seems logical that our most appropriate strategy for understanding motion sickness in space would be to analyze similar conditions on Earth. Following sensory conflict theory, several models can be suggested. Visual vestibular conflict in space occurs when motion is seen in the absence of vestibular corroboration. On Earth, a similar conflict can be approximated through the use of fixed based simulators equipped with appropriately moving visual displays (Barrett and Thornton, 1968; Reason and Diaz, 1971). This type of equipment has been shown to produce motion sickness in a significant number of subjects, with the incidence of illness increasing with the experience of operators in the real world environment. For example, Havron and Butler (1957) found that the greater the subjects' previous experience flying helicopters, the more susceptible they were to sickness when exposed to visually simulated helicopter flight. These results suggest that a profitable ground based training procedure for astronauts would be to examine exposure and adaptation to visually moving environments simulating, those to be encountered in space, with the astronaut in training remaining in a motionless position.
A second type of sensory conflict experienced in space, canalotolith, also can be simulated on Earth. By arranging conditions such that the canals suggest motion in the absence of otolith input, we can replicate the essential conditions hypothesized to exist in 0-g. As one example caloric stimulation of the outer ear fits these requirements. When the outer ear is irrigated with water which is either hotter or colder than blood temperature, convection currents are set up in the semicircular canals causing activation. When the subject is stationary there is no otolith signal corresponding to the canal signals and a situation results which in some ways is analogous to that accompanying head movement on O-g . In the study of motion sickness, considerably less research has been devoted to these types  of provocative stimuli than to those associated with rotation, but caloric stimulation studies are planned for future Space Shuttle flights. Based on the explanatory power of sensory conflict theory, it appears that more research is now warranted.
The sensory conflict theory suggests that just as a period of adaptation is required to adjust upon entry into 0-g, a process of readaptation on Earth may be required upon return to 1-g. This is a factor of concern for space crews returning to Earth. As examples, Apollo 16 crewmen indicated some decrement in postural equilibrium 3 days after their return to Earth (Homick and Miller, 1975). The greatest effects were observed when they were deprived of a visual reference. Following Skylab, postural instability measured by requiring astronauts to walk beams of various widths, was substantial even up to 10 days following return to Earth (Homick, Reschke, and Miller, 1977). All crewmen reported that rapid head movement produced a sensation of vertigo. One crewmember was unable to accurately sense small displacements of his head and body, and another crewman fell down in his house when the lights were extinguished unexpectedly. Similar readaptation problems have been reported by the Russians (Gazenko, 1979).
As perceptual habits are altered to adjust to the new orientations of space, these new expectations will be in conflict with Earth based orientations upon return. An important issue for the future is to determine whether, as some researchers suggest (Kornilova, Syrykh, Tarasov, and Yakovleva, 1979), these return to Earth symptoms increase in severity as the length of stay in space increases. Changes in the gravity level, whether from 0-g to 1-g or the reverse, could also produce altered perceptual relationships, sensory conflict, and therefore motion sickness. It should be remembered that one need not travel from Earth to space or from space to Earth to trigger perceptual alterations. If various parts of the spacecraft are subject to varying degrees of gravity, transitions across those parts may require separate adaptations at each sector.
Resistance to Vestibular Effects
One approach to the vestibular problems in space might be to develop instruments effective in distinguishing susceptible individuals from nonsusceptible individuals. By selecting only those individuals with high tolerance to vestibular changes on Earth, the chance of problems in space might decrease.
 Although Earth based research assumes that susceptibility to motion sickness in 1 g and space sickness in O g are related, there are significant differences. Parabolic flight research has shown that an individual may have normal vestibular responses on the ground and show markedly greater or lesser susceptibility to vestibular stimulation in weightlessness (Miller, Graybiel, Kellogg, and O'Donnel, 1969). Similarly, a number of astronauts who had considerable jet pilot flying experience and had no history of motion sickness were the most seriously affected in space, whereas some astronauts with definite motion sickness histories challenged weightlessness with no difficulty (Homick and Miller, 1975). Nevertheless, at present it seems reasonable to assume that the conditions of space produce sickness symptoms via the same general mechanisms that influence motion sickness of Earth, albeit via different stimulus conditions. If so, understanding motion sickness and its correlates could enhance our understanding of space sickness. Several characteristics have been found to be predictive of motion sickness, as discussed in the following sections.
Age- There is evidence that motion sickness susceptibility fluctuates with age (Tyler and Bard, 1949; Chinn and Smith, 1953; Money, 1970). Susceptibility appears to be at its highest Ievel between the ages of 2 and 12. There is a highly significant decline in vulnerability between the ages of 12 and 21, with the decline continuing throughout the life cycle, and it is relatively unusual to find motion sickness among individuals over the age of 50 (Reason, 1968). The suggested decrease in susceptibility to motion sickness associated with increasing age could be attributable to a simple avoidance of motion intense situations. However, researchers have demonstrated that differences in susceptibility to motion sickness vary with age even when subjects cannot avoid provocative stimuli Chinn (1956) found that among 5,000 individuals on transatlantic troopships, persons between 17 and 19 years of age showed a high incidence of sickness, whereas persons between the ages of 30 and 39 yr became ill at a low rate. A similar age related decline in motion sickness susceptibility has been observed among civil aircraft passengers ( Lederer and Kidera, 1954).
A possible explanation of these findings is that the sensitivity of the vestibular system is reduced with age. This suggestion gains support from the work of Prober (1958), Kennedy and Graybiel (1962) and Lidvall (1962), who found vestibular sensitivity to be greater in motion sic knees prone individuals compared with motion sickness resistant individuals. However, overall, it does not appear that  vestibular sensitivity can reliably predict susceptibility to motion sickness (Reason and Brand, 1975; Lent/, 1976), and it seems unlikely that a decline in susceptibility correlated with age is in any way related to a decline in the reactivity of the vestibular system. An alternate explanation holds that the increased resistance to motion sickness associated with advancing years is somehow linked with experience with the environment, and perhaps a diminution in the cortical or cognitive factors involved. This hypothesis, if supported, could help explain the difference found between Earth based susceptibility and space susceptibility. Experience with the environment may render individuals almost immune to symptomatology on Earth, yet in the new environment of space, they must learn to adapt again. Little research has been brought to bear on the environmental hypothesis and thus no conclusions can be drawn.
As we anticipate crews, and particularly passengers, with wider age ranges than previously employed, the possibility that some age groups may be more susceptible to space sickness than others should be considered. In designing crew work schedules and assessing who will conduct vital tasks during the first weeks of the mission, information regarding age related susceptibility may prove important. Further research is warranted defining the relationship between age and motion sickness and the mechanisms involved.
Gender- Research to date suggests that women are more susceptible to motion sickness than men. Reason (1968) found that women students reported a significantly greater incidence of motion sickness than men of comparable age and travel experience, both before and after the age of 12. These results have been confirmed by Lentz and Collins (1977). An unresolved question is whether this gender linked susceptibility. is based on physiological differences, psychological differences, or are reflective of a socialization process in which it is more acceptable for women than men to report illness. Bakwin (1971) and Abe, Amatomi, and Kajiyama (1970) have suggested that susceptibility to motion sickness may be genetically determined They report that at age 3 significantly more girls suffer from motion sickness than boys. Also, in partial support of the genetic argument, Lentz and Collins (1977) found that, compared with nonsusceptibles, susceptible individuals more frequently indicate that their parents or siblings are also susceptible. However, such reports could reflect a learning process as well as a genetic inclination.
 Given the growing involvement of women in the space program, the extent and degree that space sickness will affect female personnel needs to be determined. Also, understanding the factors which underlie gender related differences in motion sickness might help explain the general process of motion sickness.
Personality- A review and study by Collins and Lentz (1977) suggests that certain personality factors distinguish individuals who are susceptible to motion sickness from those who are resistant to motion sickness. Using a self report test battery (composed predominantly of paper and pencil tests), Collins and Lentz confirmed several significant differences between susceptible and nonsusceptible individuals. Motion sickness susceptible individuals showed a high degree of anxiety and scored higher on tests assessing general medical and psychiatric problems and on tests measuring potentially serious neuropsychiatric and psychosomatic disturbances than did nonsusceptibles. These authors also confirmed previous work with the Eysenck Personality Inventory which showed that both the introversion and the neuroticism scales are correlated with motion sickness history (Guedry and Ambler, 1972; Reason and Graybiel, 1972; Wilding and Meddis, 1972; and Kottenhoff and Lindahl, 1960). Collins and Lentz also found that nonsusceptible individuals had higher scores on the masculinity scale of the Guilford Zimmerman Temperament Survey than did susceptibles, regardless of their gender.
The results of the 16 Personality Factors Test employed by Collins and Lentz provide a summary of those traits descriptive of susceptibles and nonsusceptibles. Nonsusceptibles can be characterized as emotionally stable, venturesome, self assured, relaxed, adjusted, thinking oriented, not neurotically disposed, and good leaders. Susceptibles are defined as emotionally oriented, tenderminded, and subjective, and as possessing traits generally the opposite of those defining nonsusceptibles.
Our understanding of the behavioral correlates of motionsickness susceptibility has been further expanded by research conducted at Ames Research Center by Cowings and her associates (sowings, 1977a, 1977b; Cowings, Billingham, and Toscano, 1976, 1977; Cowings and Toscano, 1977, 1982; Stewart, Clark, Cowings, and Toscano, 1978; Toscano and Cowings, 1978). These authors confirmed that susceptible individuals demonstrate higher levels of anxiety than nonsusceptibles. However, the most intriguing findings reported in the Ames studies come from autonomic functioning  profiles. Here, subjects were divided into two groups: "sympathetic dominant" and "parasympathetic dominant," based on autonomic balance measures developed by Wenger (1941). Individuals with sympathetic dominant autonomic profiles showed less anxiety, were less able to perceive their own autonomic functioning, and, under conditions of unusual vestibular functioning, showed fewer symptoms of motion sickness than did parasympathetic dominant individuals. These same sympathetic dominant individuals were shown to be much more susceptible to hypnosis than were their parasympathetic dominant counterparts. Parker and Wilsoncroft (1978) also examined the relationship between autonomic dominance and susceptibility to motion sickness. Like Cowings, these authors found sympathetic dominant subjects to be more resistant to motion sickness than parasympathetic dominant subjects. However, these authors conclude that resistant subjects are more (rather than less) anxious than susceptible subjects, and that anxiety acts to protect against symptomatology.
In comparing the results of these studies, several questions emerge. Although the personality patterns of those found to be motion sickness resistant and those found to be motion sickness susceptible are generally consistent between the Cowings and the Collins and Lentz studies, it is not obvious that the "extraverted, venturesome, emotionally stable, good leader" (Collins and Lentz) would be highly susceptible to hypnosis (sowings). The more significant question, however, involves the tie in between autonomic dominance and motion sickness susceptibility. The Collins and Lentz description of personality traits associated with resistance to motion sickness suggests an individual usually thought of as parasympathetic dominant. Yet Cowings has identified individuals with traits overlapping those described by Collins and Lentz to be sympathetic dominant. The relationship between autonomic dominance, personality traits, and motion sickness susceptibility needs to be more fully understood if these variables are to contribute to selection strategies.
One might suspect that motion sickness susceptible (compared with resistant) individuals would be more aware not only of those functions associated with their motion sickness, but also of their internal functions generally. Cowings (1977b) used an autonomic perception questionnaire (Mandler, Mandler, and Uviller, 1958) to assess the ability of subjects to perceive the processes of their own viscera. It appears that susceptibles arc much more aware of their own bodily functions than are nonsusceptibles; this is perhaps one  reason they respond to the disruptive effects of unusual vestibular stimulation upon their bodily functions.
A related finding has been made by Barrett and Thornton (1968), who examined the relationship between motion sickness susceptibility and field dependent/independent perceptions.3 During visual motion simulator training, field independent subjects experienced significantly greater discomfort and were more prone to motion sickness than field dependent counterparts. Although it is logical to assume that those who rely on their internal sensations are aware of them, it does not follow that those who operate primarily on external cues are necessarily unaware of internal cues. Studies to clarify these findings would further elucidate the area of motionsickness susceptibility. Field independence has also been associated with introverted personality styles, while extroversion is typical of field dependent subjects (Witkin, Lewis, Hertzman, Machover, Meissner, Brettnall, and Wopner, 1954). These relationships suggest that introverts may be more susceptible to motion sickness than are extraverts. This possibility deserves further attention.
Drugs-One approach to the treatment of space sickness is the use of medications to help reduce symptomatology (Wood, Kennedy, and Graybiel, 1965). Unfortunately, anti motion sickness pharmaceuticals have not been completely satisfactory. Although these pharmaceuticals have reduced the severity of symptoms for some astronauts at some times, they have been ineffective on other astronauts. The pilot on Skylab 3 took an anti motion sickness preparation, but vomited anyway. The Skylab 4 commander and pilot continued to experience space sickness periodically during their first 3 mission days, despite the administration of pharmaceuticals on a regular basis (Graybiel et al., 1977).
There are other limitations to the use of drugs as a prophylaxis against motion sickness. To produce the maximum benefit, the chemicals have had to be taken before the symptoms occur. Once illness is established, the drugs are not altogether helpful in alleviating symptoms. Furthermore, medications may have side effects or  aftereffects that can potentially influence performance (Wood, Manno, Manno, Redetzki, Wood, and Vekovius, 1984), hence astronauts have been reluctant to ingest the medications regularly. Unfortunately, changes in physiological functions resulting from medications can interact with and overshadow biomedical alterations produced by weightlessness, and interfere with the evaluation of data (Vernikos Danellis, Winget, Leach, Rosenblatt, Lyman, and Beljan, 1977). As with other drugs, we do not understand adequately what effects usage may have in space.
Head movement schedules-The use of self induced head movement on planned schedules has been suggested as a way to speed up the process of adaptation to unusual vestibular stimulation (Graybiel and Wood, 1969; Homick and Miller,1975). This procedure is based on a principle analogous to vaccination. By exposing the subject to a mild level of provocative (i.e., response producing) stimulation insufficient to result in sickness, it is hoped that the process of adaptation to greater levels of stimulation will be accelerated. Earth based studies of head movement adaptation schedules have shown encouraging support for this vaccination approach (see Graybiel, 1975, for review) For example, Graybiel and Wood (1969) showed that subjects required to execute experimenter paced head movements during rotation adapted significantly faster to the provocative condition than when head movements were not required. Unfortunately, in flight data have not been as encouraging. The astronauts of Apollo 10 were instructed to carry out a series of slight head movements paced at about 2 sec intervals during the early stages of the mission, but because of busy schedules, only the lunar module pilot actually carried out the exercises. On the first day, movement induced stomach awareness and nausea developed within 1 min and movement had to be discontinued. The same result occurred the second day and again the head movements had to be stopped.
Controlled head movements have also been used in combination with medications. During Skylab 3, all three crewmen suffered space sickness before treatment. On the second mission day, they were instructed to take an anti motion sickness preparation and to execute, after a 1 hr delay, a series of head movements. Although remaining ill, they did report feeling marginally better by evening.
It would seem from the limited in flight data available, that adaptation schedules are either not sufficiently understood or are not satisfactory in counteracting the problem of space sickness. Even if perfected, there are other problems inherent in the use of this  technique. It requires time away from duties during flight. Also, there is evidence that protection conferred under one set of stimulus conditions does not transfer to another. With some exceptions (see discussion by Reason and Brand, 1975), research indicates that adaptation is normally highly specific to the particular stimulus conditions under which adaptation is acquired. As one example, Guedry (1965) found that in a room slowly rotating in a counterclockwise direction, adaptation to provocative head movements did not transfer when the subjects were rotated in a clockwise direction.
From the data presented in this section, it seems that neither drug usage nor adaptive head movement schedules are particularly attractive countermeasures at this time. However, there are other directions of research which may prove practical and effective.
Head restraint- An approach opposite to that of head movement schedules has also been tried in space. Crewmembers onboard the Salyut 6 mission employed a special head restraint helmet (Neck Pneumatic Shock Absorber) designed to reduce spacesickness symptoms by limiting head movement (Matsnev, lakovleva, Tarasov, Alekseev, Kornilova, Mateev, and Gorgiladze, 1983). A soft cap, secured by unstretchable straps attached to the shoulders, minimizes head tilt and turn. Rubber cords, also attached to the cap, further restrict movement unless the crew member exerts considerable force with his neck muscles. Flight test results from the Salyut mission indicate that the device was beneficial in controlling the development of space sickness.
The use of such a head restraint device is a logical extension of the fact that space sickness symptoms are best ameliorated by lying flat (thus placing the head in a position similar to the erect posture maintained by the head restraint device) and avoiding all head movements (see Reason and Brand, 1975).
Although this device has proven useful in the limited tests conducted in space, many research questions need to be addressed. Does restriction of head movement pose any problems for performance of tasks requiring rapid or agile shifts in the position of the head? Is hand eye coordination altered by the fact that the hands can be easily monitored only when the head is in the erect position? Does the device permit adaptation to the stimuli associated with the development of space sickness such that eventually the restraint can be removed without fear of illness? If adaptation does occur, is the period of adjustment prolonged because of the lack of head movement? This latter question points up the paradox of treating space  sickness. I n theory, more active head movements produce faster adaptation to the conditions of space (Graybiel, 1975). However, increased movement also increases the risk of nausea which could debilitate crewmembers and jeopardize performance. Alternatively, restricting head movements (by lying still) lessens the probability of symptom development, but also prolongs the period of adaptation. It remains to be determined if in fact the Neck Pneumatic Shock Absorber produces a similar extension of the adaptation period.
Cuban boots- In addition to the head restraint cap, Soviet cosmonauts have also tested specially designed shoes or "Cuban boots," named after the Cuban cosmonaut Arnaldo Mendez who tested the prototype (Engler and Cheshire Engler, 1983). These shoes provide a pressure of up to 60 torr to the bottom of the feet, making the cosmonaut feel as if he is standing on solid ground. A Soyuz 38 cosmonaut reported that use of the Cuban boots reduced the severity of spatial illusions and motor disturbances, phenomena thought to be produced by conditions which also produce motion sickness. Although the Cuban boot is not likely to eliminate vestibular problems in space, it is certainly an intriguing compensatory mechanism to simulate 1 g conditions and deserves further attention.
Autonomic response control- Various investigators have begun to explore the use of biofeedback and acquired autonomic response control in the prevention of motion sickness. Cowings et al. (1977) have demonstrated that subjects trained to control heart rate, respiration rate, and blood pressure using biofeedback and autogenic training procedures can volitionally diminish and/or prevent their own motion sickness symptoms. Among 50 subjects trained in the past few years and tested under conditions of provocative motion sickness stimuli (rotating chair producing coriolis acceleration), 85% have improved their ability to withstand the test conditions (Rasmussen, 1980) Likewise, Levy, Jones and Carlson (1981) showed that biofeedback treatment of 20 air crewmen, disabled by chronic and severe sickness in flight, resulted in an 840/G rate of return to flight duty. In this study, subjects were trained to voluntarily control such functions as galvanic skin response, skin temperature, and muscle tonus.
It would appear that the use of autonomic response control techniques has several advantages over procedures previously used in the treatment of motion sickness. Toscano and Cowings (1978) found that learned control of symptom suppression could be transferred from. one situation to another. . Subjects learning control of autonomic functions to suppress illness when exposed to acceleration  in one direction showed significant suppression of symptoms when rotated in the opposite direction Nor has training been found to be differentially affected by initial motion sickness susceptibility (Toscano and Cowings, 1978, 1982), or by gender differences (Rasmussen, 1980). Also, it appears that autonomic response control training can be used effectively by subjects for whom traditional countermeasures have proved inadequate (Levy et al.,1981).
Despite the promising results reported by investigators employing biofeedback in the treatment of motion sickness, several major issues remain unresolved. It is not clear at this time why and how biofeedback works. In fact, there is evidence to suggest that biofeedback should not be effective. Graybiel and Lackner (1980) measured changes in blood pressure, heart rate, and body temperature in conjunction with the onset of motion sickness. Among 12 subjects studied, no significant variations in these physiological parameters were observed during progressive stages of motion sickness. Other studies have failed to indicate any consistent relationship between the appearance and severity of motion sickness symptomatology and changes in blood pressure, heart rate, or body temperature (Money, 1970; Reason and Brand, 1975). Logically, if there is no consistent variation in these physiological parameters as a function of the onset of motion sickness, it is difficult to understand how learned control of these functions could be useful in the reduction or prevention. of motion sickness or space sickness symptoms. There is insufficient data at present to reconcile the positive effects found by Cowings and her associates and by Levy, Jones, and Carlson with the results of Graybiel and others. However, certain prospects suggest themselves. The observed decline in motion sickness susceptibility could be related, not to the particular autonomic response controlled, but rather to the cognitive aspect which accompanies the process of control. We know from previous research that concentration upon some difficult or engaging mental task tends to alleviate the symptoms of motion sickness. Guedry (1964) found that only three out of 10 subjects given mechanical comprehension and spatial relations problems while being rotated suffered motion sickness, whereas twice the number of control subjects not given cognitive tasks experienced severe symptoms: two individuals in this group failed to complete the experiment and four vomited repeatedly.
Although mental activity may lessen the symptoms of motion sickness, it seems unlikely that the effects that have been reported for biofeedback could be explained fully by simple attention to a task. Indeed, Toscano and Cowings (1982) have addressed this issue.
 They demonstrated that subjects given training on a cognitive task do not withstand the stress of coriolis acceleration well at all when compared with biofeedback trained subjects. There is a compelling need to understand why biofeedback does appear to work and whether it is, in fact, related to the learned control of specific autonomic responses.
Another issue regarding the use of autonomic response control procedures to treat motion sickness concerns the approximately 15% failure rate found by both Cowings and Levy. It is impossible to say at this time why this procedure works for some and not for others. It could prove valuable to devote investigations specifically to these failures. Perhaps an understanding of why autonomic response control procedures fail to confer protection from motion sickness in some cases would help us understand why it does seem effective in others.
An additional critical research question concerns the degree of effectiveness of biofeedback training in the specific treatment of space sickness. Although there are similarities between conditions on Earth that produce motion sickness and those prevalent in space that produce space sickness, the situations are not identical. It remains to be determined whether biofeedback training, effective in treating motion sickness on Earth, can also counteract sickness in space.
Another area of interest concerns the possible occurrence of symptoms after autonomic control is relaxed. Is response control exerted in the initial phase of flight sufficient to confer protection throughout the mission? Can the suppression of symptoms result in the delayed occurrence of illness? What is the relationship between intensity of illness and the duration of discomfort? These kinds of questions go beyond the issue of biofeedback and relate to the various methods of countering motion and space sickness.
2 Traditionally, the terms "motion sickness," "space motion sic sickness," , and space sickness" have been used interchangeably to describe the illness experienced by space travelers which mimics the symptoms of motion sickness on Earth. More recently space sickness has emerged as the preferred term, although "space adaptation syndrome" also has been used (Toufexis 1983).
3 It has been found that, when placed in a situation with conflicting cues some individuals rely on the external stimulus field (field dependence), whereas other individuals base their judgments primarily upon internal kinesthetic sensations (field independence) (Witken, 1949a, 1949b).