Numerous hypotheses attempt to explain the physiologic causes of SMS. The majority of these hypotheses focus on the vestibular system and how it responds to microgravity. The vestibular system consists of two identical vestibular organs located on either side of the head in the inner ear. It functions to maintain the body's balance by detecting the position and motion of the head. Each organ is made up of three semicircular canals that detect angular accelerations in three different planes in space and two otolithic organs which detect linear accelerations. The otolithic organs depend on gravitational forces to function properly; therefore, they are especially sensitive to microgravity conditions. The semicircular canals and the otolithic organs work in conjunction with visual inputs to send signals to the brain, which then activates the appropriate muscles to keep the body balanced. Exposure to microgravity affects the relationship between the vestibular signals and visual inputs and thus causes a sensory conflict in the brain. The sensory conflict occurs because the visual inputs from the eyes do not agree with the signals from the vestibular system. A good example of this occurs when a crewmember is floating weightless and upright in reference to the Skylab's floor, but in reality he may be completely upside down due to microgravity. Accordingly, visual inputs in this situation would indicate the correct "up" and "down" orientation but the vestibular organs would not sense the correct orientation because they rely on gravity to function properly. Researchers believe that this sensory conflict is the underlying mechanism responsible for SMS, illusions of motion and spatial-orientation disturbances.
Skylab scientists noted that the incidence of SMS increased on the Apollo missions as compared to the Mercury and Gemini missions. They attributed this to the larger size of the capsule which allowed for greater mobility of the crew. In addition, it was known that SMS is caused by a combination of microgravity and head or whole-body movements that produce changes in one's spatial orientation. These findings prompted researchers to investigate human vestibular function on Skylab missions since these missions would involve a great deal of activity in a large workshop for extended periods of time. Thus, the objectives of Experiment M131 were to (1) establish the validity of specific behavioral/physiological responses influenced by vestibular activity under 1-g and microgravity conditions, (2) determine man's adaptability to unusual vestibular conditions and to predict the habitability of future spacecraft environments, and (3) investigate man's ability to accurately judge spatial coordinates without the aid of visual inputs and while under the influence of microgravity.
The MS test was designed to determine the crewmembers' susceptibility to motion sickness. The first phase of this test involved gathering data in order to formulate a numerical value for the Coriolis Sickness Susceptibility Index (CSSI). The CSSI requires data on the crewmembers' history of motion sickness and vestibular function. The second phase of the MS test focused on producing abnormal stimuli in the vestibular organs in the form of cross-coupled angular accelerations in order to induce motion sickness symptoms. Each crewmember was blindfolded and then instructed to perform standardized head movements while rotating at a preselected velocity in the Rotating Litter Chair (RLC). The blindfold was necessary to block out visual signal inputs that would interfere with vestibular signals. The head movements and the alternating clockwise and counterclockwise rotations in the RLC provided the angular and linear accelerations necessary to stimulate the vestibular organs. Head movements were performed in sets of five; each set consisted of four basic movements: forward, back, left, and right (the forward movement was executed twice). The test was stopped when the subject had completed 150 head movements or displayed specific motion sickness symptoms. Primary data consisted of verbal reports by both the subject and the observer of the subject's clinical symptoms, angular velocity of the RLC, and the number and characteristics of the head movements. These data were used to formulate a numerical value for each subject who indicated the severity of motion sickness experienced during the test. The point system ranged from 1 to 16 with higher values indicating more severe symptoms.
The OGI test was designed to investigate the crewmembers' ability to correctly report the direction of apparent lateral motion of a fixed target as induced by angular accelerations. The data collected from the OGI test allowed researchers to analyze the sensitivity of the semicircular canals and the associated vestibulo-ocular reflex (VOR). The test was carried out while the subject was seated in the RLC and wearing the Vestibular Test Goggle (VTG). The target viewed by the crewmember's right eye was adjusted so that it appeared vertical and straight ahead. The chair was programmed to rotate for a period of 20 seconds at a constant acceleration rate and then for 25 seconds at the achieved constant velocity. The same time profile was used for the deceleration phase, after which the chair would remain stationary for 25 seconds before progressing to the next higher acceleration rate. During each acceleration/deceleration period, the crewmember was signaled to open his eyes after 2 seconds, and after 5 seconds he was signaled to judge whether the target appeared to move left, right, or remain stationary. If no response was given by the end of the 20 second period, it was assumed that no movement was perceived.
The SL test was designed to investigate the crewmembers' ability to identify the direction of internal and external space without the aid of visual cues. The first SL test consisted of separate procedures for identifying internal and external spatial coordinates and was carried out using the VTG and the RLC in the tilt position. In the first procedure, the crewmember closed his eyes for 60 seconds, opened them, and then aligned the internal target to internal references in the goggle device using the target adjuster knobs. The observer would record the setting and then offset the target for the next test. This procedure was repeated for a total of five settings. The second procedure focused on external spatial localization and called for the crewmember to move his head backward from the goggle for a period of 10 seconds in order to observe his position relative to the Skylab. The crewmember then resumed the test position with the goggles against his face and set the internal target in relation to the external reference which was the longitudinal axis of the Skylab. This procedure was also repeated for 5 settings.
The second SL test utilized the RLC in all three positions with the crewmember blindfolded and holding the Rod/Sphere device. The crewmember held the sphere in his left hand and the pointer in his right hand and attempted to align the pointer relative to internal and external references. For the internal reference judgments, the pointer was placed parallel to the apparent long axis of the body with the free end pointing toward the crewmember's head. The external judgment procedure used the perceived direction of the longitudinal axis of the Skylab as a reference point. At the completion of each exercise, the observer would record the alignment and then offset the rod to a different position. Five internal and external settings were recorded for each of the three RLC positions.
Susceptibility to Motion Sickness: The CSSI scores for the Commander (Cdr), Scientist Pilot (Spt), and Pilot (Plt) on Skylab 2 were 10.2, 8.2, and 19.8, respectively. The scores for Skylab 3 were Cdr, 23.1; Spt, 26.4; Plt, 19.2. The scores for Skylab 4 were Cdr, 7.5; Spt, 8.9; Plt, 52.8. Higher values translated into higher susceptibility.
MS Test, Operational Conditions: The Skylab 2 crewmembers did not experience any motion sickness symptoms throughout the entire mission. The Skylab 3 crew began to feel symptoms during activation of the workshop, which was approximately 11 hours after insertion into orbit. The Cdr, Spt, and Plt experienced slight, moderate, and severe symptoms, respectively. For three days thereafter, the crew experienced symptoms of motion sickness that were intensified by movement and alleviated by taking medication or restricting their movements. In light of the results from Skylab 2 and 3, the Skylab 4 crew was scheduled to take motion sickness medication through mission day 3. As a result, only the Plt experienced severe symptoms during the first three days of the mission. The Cdr and the Spt were relatively symptom free for the entire mission.
MS Test, Experimental Conditions: The data collected during the RLC/head movement exercises indicate that the Skylab 2 Spt and Plt were less susceptible to motion sickness when they executed the exercises inflight as compared to pre- and postflight. The Skylab 3 crew was virtually immune to experimental motion sickness during the inflight period and their susceptibility was lower during the initial postflight period. The results for the Skylab 4 crew closely resembled the Skylab 3 results. Skylab 4 preflight testing showed high susceptibility, inflight testing indicated a virtual immunity to motion sickness, and postflight results showed a lower susceptibility during the first two weeks of testing.
Oculogyral Illusion: The results show that, during the inflight period, none of the subjects consistently improved their ability to perceive the oculogyral illusion. Four subjects revealed a slight decrement in their ability to perform the test correctly while the remaining four showed no consistent change in their abilities. The data on the Skylab 2 Spt and Plt showed that their threshold for perceiving the illusion was higher during the inflight period as compared to pre- and postflight periods. The Skylab 3 Cdr and Plt revealed average responses that were similar to their pre- and postflight levels while the Spt performed slightly below his preflight level. The Skylab 4 Spt and Plt showed a tendency to perceive the illusion less frequently as the mission progressed, whereas the Cdr revealed no consistent change during or after the mission.