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At 150 millilamberts the comparable decrement was only 0.25 minutes of arc. This suggests again that the instrument panel in forthcoming space vehicles should be of sufficient brightness so as to compensate for such effects . White (1960), working at positive-G levels below those which produced peripheral dimming, found that foveal sensitivity was re- duced during runs lasting 1.25 minutes. His results demonstrated that foveal thresholds were doubled and tripled at levels of 3 and 4 G, respectively. A further study by Brown and Burke (1958) demonstrated that visual reaction time also is affected by acceleration. Although the causative relationships are not clear, it is suggested that this break- down may be related to a decrement in form discrimination. The Visual Effects of Weightlessness and Simulated Gravity Since the end of World War II, numerous articles have been written concerning the subject of weightlessness. Inasmuch as true weightlessness can be experienced only when the force of gravity is absent or ia counterbalanced by an opposite force, much of what is contained in the literature is based on speculation. So far as is known at the present time, the absence of gravity has no direct physiological or anatomical effect on the visual mecha- nism per ae (Gerathewohl, 1952). 14
There have been a few studies, however, concerning certain specific visual phenomena. Pigg and Kama (1961) studied the effect of short exposures (14 seconds) to weightlessness on visual acuity. It was found that the acuity loss in flight at zero G was only 6 per cent and, when compared with laboratory tests, the loss was still only 10 per cent. These small decrements are considered to be negligible. Shock (1959) determined that the ability of a subject to adjust a bar to a hor- izontal and vertical position in the absence of visual cues is signifi- cantly impaired when he is totally immersed in water. "On the surface of the earth, man has constantly avail- able his perceptions" [based upon] the force of gravity and [in addition has many] indications of the direction of "up." The presence of this reference [system], along with the fact that he usually performs gross movements in only two dimensions greatly simplifies the problem of navigating his body within the confines of his immediate physical en- vironment. In a spacecraft this reference may be lacking and in addition, at some future time when large space stations have been established, man may have the added complication of being required to maneuver his body in three dimen- sions almost continuously. "Some of the possible complications of this situation may be discussed. As long periods of time will be spent by occupants of spacecraft under conditions of zero gravity, the layout of the interior of such a craft may be quite dif- ferent from the interiors of terrestrial vehicles of com- parable size. It. . .[may] not be necessary to have any floors or ceilings as such. The day may come when indi- viduals will be able to move in any direction within the vehicle without reference to a specific ground plane. The stations of different crew members within a single com- partment may be oriented such that these individuals are at unique angles with respect to one another. It is to be expected that man's training and experience in earthly surroundings may cause him some difficulty in his initial experiences in such a novel situation. Man is accustomed to a rectangular organization of the artificial enclosures in which he lives on the surface of the earth. His rooms 15
are constructed with four sides and the streets of his cities cross each other at approximately right angles. Orienting within a building or a large city is strongly dependent on what we may consider a right angle unit for the perception of direction. There are innumerable examples of problems arising from a nonrectangular spatial organization. Two prominent examples of such problems are the city of Wash- ington and the Pentagon. Strangers to each of these places find it extremely difficult to orient themselves. It is neces- sary for them to unlearn orientation schemes based on right angle layouts and to learn other appropriate techniques before they can get about without difficulty. . . . "The most efficient layout of a spacecraft designed for multiple occupancy probably will not be rectangular. It will be necessary for occupants to learn to move contin- uously in three-dimensional space and not on a single refer- ence or ground plane in any given compartment. Under con- ditions of zero G, restriction on body positions which apply at 1 G will not apply and the most efficient design of work spaces and the integration of the locations of the several occupants of a given compartment will require a careful consideration of the additional flexibility afforded by the weightless state. . . . Such problems must be considered in planning training programs for the future occupants of such an environment. [Further discussion of problems associated with work space layout will be found in the next Section.]" In view of the fact that weightlessness does markedly affect other sensory systems of the body, the role of vision in maintaining effective performance in space flight is extremely important. The results of short exposures to weightlessness have demonstrated that individuals probably can perform effectively in space if they are selected care- fully. Whether these same individuals can perform as well for ex- tended periods has yet to be determined. If a space traveler in a completely weightless state were to maintain a rigid postural position, he probably would be able to orient himself properly solely with his visual mechanism. It is absurd to 16
assume, however, that such a condition would exist except for brief intervals. If the head is moved in sub-gravity conditions there is evidence indicating that visual perception will be affected by the action of the semicircular canals and otolith organs of the inner ear. These effects usually are in the form of illusory movement of surrounding objects (oculoagravic illusion), vertigo, and general disorientation (Graybiel, 1952; Gerathewohl, 1953; Gerathewohl 8t Stallinge, 1958). These phenomena have been treated adequately elsewhere (Gerathewohl, 1959; Lofters k Hammer, 1961; Schock, 1958a, 1958b) so a detailed discussion is not attempted here. One fact which should be empha- sized, however, is that an individual in a weightless state may not be able to recognize certain of these visual illusions, as .such, until after a considerable lag. Hence, any potential astronaut should undergo extensive training and indoctrination to prevent any incapacitation due to these effects. There appear to be three basic methods of dealing with the problem of weightlessness. The first and most obvious in simply to do nothing about it, in which case the onus is on the astronaut to learn to live with it. A second method is to create an artificial "down" which may be accomplished with the use of magnetic shoes in conjunction with an appropriately designed cabin interior. Work along these lines has been done by Simons (1959). When such a technique was used there was a strong sensation of foot-down orientation. This sensation was so strong according to Simons that, "When one subject walked on the 17
ceiling and one on the floor, the foot-down orientation again overrode any sensation of disorientation and each man appeared upside-down to the other." In this regard it might be of interest to study the effect of sustained pressure on various parts of the body, e.g. , the feet on the over-all perception of the visual world. Such studies would be germaine both to the psycho-physiological process involved and to the design of display systems. A third method that has received wide attention is that of creating an artificial gravity by means of slowly rotating an entire vehicle or platform. Although from a technical standpoint such a system is en- tirely feasible, the stresses which it presents to the occupants may be severe. An excellent study performed recently in Pensacola per- tains directly to this problem (Clark b Graybiel, 1960; Graybiel, Clark, b Zarriello, 1960). In these experiments a nearly circular, windowless room, 15 feet in diameter, was constructed around the center post of a human centrifuge. The room contained complete living facilities for four persons. Subjects were rotated in this room for periods up to three days at angular velocities ranging from 1.71 to 10 rpm. These speeds were chosen primarily to sample the range that might be used in manned space platforms. The results indicated that strong oculogyral, oculogravic, and coriolis illusions were present in all normal subjects, but that they often disappeared at the end of the first or second day of rotation. In addition, the subjects experienced other symptoms such as malaise, apathy, and nausea. In some instances, the symptoms were 18
so severe that the subjects were unable to carry on. Adaptation, how- ever, ". . .occurred over periods of hours to days and the symptoms either disappeared or were reduced in severity." An obvious conclusion from this experimental procedure using the rotation room is that the subjects were exposed to substantial amounts of stress, particularly at the higher rotational velocities. Furthermore, it would appear that the intermittent stimulation of the semicircular canals is the dominant factor in the situation. This conclusion is supported by the fact that the significant stimulus was angular acceleration, and the additional fact that a subject who had lost the function of the semicircular canals, along with other sensory organs of the inner ears, did not experience these symptoms. Although this study was concerned with many different param- eters, the visual illusions experienced by the subjects are capable of inducing severe disorientation for the unwary subject. If, in the future, a space vehicle or platform is utilized which incorporates constant angular rotation, extreme care must be given to the design of the interior so as to minimize, if possible, some of the effects just described. It would appear, on the basis of available experimental evidence, that man will be able to function adequately in the absence of gravity. Further support for this comes from the manned shots of the United States and the USSR. Future orbital shots should determine whether there are unanticipated effects produced by long-term exposure. 19
