When a person is moving around, changing directions rapidly, or leaning sideways, forward, or backward, it would be impossible for that person to maintain a stable image on the retina of the eye unless some kind of an automatic control mechanism was in place to stabilize the eyes. By now, particularly if you participated in Student Investigation 2, you know that this automatic control mechanism is known as the vestibulo-ocular reflex (VOR).

In addition, the eyes would be of little use in detecting an image unless they remained "fixed" on each object long to gain a clear image. Fortunately, each time the visual field moves or your head moves, signals primarily from the semicircular canals cause the eyes to rotate opposite to the direction of the head. This eye movement pattern, called nystagmus, is characterized by a series of successive compensatory movements of the eye, and it provides evidence that the VOR is at work. Nystagmus is the eye's attempt to hold on to an image and to transition to a new image in shifted stages. This allows an image to remain on the retina slightly longer. As the head continues to rotate, the eyes continue to rapidly and involuntarily oscillate back and forth, reestablishing a stable image each step of the way.

There is another source of evidence that indicates the power of the VOR, our sensations of visually induced roll (VIR). VIR is when the body actually attempts a corrective shift in the opposite direction of the rotating visual field to compensate for the sensation of rolling. Your body then feels a false sensation of self motion due to the rotating field. This kind of sensation is used to advantage in the design of flight simulators, large- screen movies, and in amusement park exhibits or rides in which the sensation of movement is created or enhanced by visual surround motion.

Dr. Young's experiment involved preflight, inflight, postflight measurements to understand how VOR, nystagmus, and VIR functions might be affected by microgravity. He hypothesized that, in the process of adapting to weightlessness, the influence of the non-vestibular cues will be magnified. Therefore, he expected to see enhanced visual involvement in the body's process of establishing some sense of equilibrium. Of course, if particular visual processes require gravity to function "Earth-normally," then those visual processes would not be enhanced in space, and, in fact, may produce distortion and confusion. Let's see how the experiments were carried out and what Dr. Young found.

The measurement of VOR is indirect. One cannot simply go to a distinct VOR organ somewhere in the body or in the head and place an electrode on it and measure this reflex. As you know, various indicators, such as counterrolling and nystagmus, provide functional information about how the VOR is operating. A specific variable that Dr. Young measured to understand how the VOR system changed in space was the astronauts' perception of rotation and how long it lasts. This perception results in part from a set of integrated vestibular, visual, proprioceptive, and tactile spatial orientation sensory information that has been "stored" in the central nervous system. This is known as our angular velocity storage capability (Figure 18). There is no particular little box in the head, in which this information is actually stored, but the signals that relate to movement history are stored and remembered by the body.

Figure 18. Angular velocity storage capacilities. 6

These stored signals allow a continual flow of information that enables us to predict future movements based on the way we perceive or "feel" our orientation at any given moment. The amount of time that the information can be stored is characterized by a dominant time constant that is unique for every movement and that is dependent on a variety of factors, including:

• vertical and horizontal semicircular canal activation;
• otolith input;
• input from the visual system;
• activity from the proprioceptive/motor system;
• the speed of rotation;
• past conditioning from repetitive movements.

In the measurement of VOR, subjects are spun on a rotating chair and they follow a protocol similar to one that you participated in for Student Investigation 1 . Each experiment session consisted of a series of successive rotation tests of two basic types, referred to as either "head erect" or "dumping" tests (Figure 19). The chair was rotated exactly the same way for both tests. Does this sound familiar so far?

Figure 19. "Head erect" or "dumping" tests.6

The chair was accelerated to an angular velocity of 120° per second in a clockwise (CW) or a counterclockwise (CCW) direction, and then brought quickly to a stop After one minute. The direction of chair rotation was alternated in each successive test so the astronauts would not begin to adapt to the rotations and to be sure that any inherent differences between left and right didn't influence the results. The subject's head stayed centered on the rotation axis in both tests during the rotation. In the dumping test, however, the subject actively pitched the head forward upon instruction from the test operator, immediately after the chair stopped, bending mostly at the neck and shoulders. The subject maintained the head in the 90 degree, nose down position for at least 50 seconds After the chair stopped. In the head erect test, the subject maintained an upright position until the end of the test (Figure 19)

The subject's electro-oculogram (EOG) was recorded for the entire experimental period. This provided a measurement of how long after the rotation the eye continued to make its nystagmus movements, indicating the decay of the slow phase eye velocity (SPV). From this data, the VOR time constant could be calculated. Also, a video camera recorded head movement and the subjects wore a head-mounted angular accelerometer so that all of the data could be integrated to enable one to understand which movements were connected to which responses. Preflight, inflight, and postfllight data were collected on four subjects. During the nine day shuttle mission, inflight data was collected either on flight day 4 or 5 (FD-4 or FD-5). Figure 20 illustrates the combined preflight and inflight slow phase velocity (SPY) data for four subjects. The results of both the head erect test and the nystagmus dumping test are shown. In addition, the VOR time constant values calculated from the SPV data are also shown in Figure 20 and summarized in Table 2.

Table 2. A summary of VOR time constants for Subjects M, N, P, and T. Preflight and inflight data are shown for both the head erect test (Te) and the dumping test (Td).6
Subject	Preflight Te (seconds)	Preflight Td (seconds)	Inflight Te (seconds)	Inflight Td (seconds)
M	14.0	7.6	15.9	14.7
N	19.3	11.0	20.4	21.2
P	15.8	9.1	35	16.7
T	30.3	16.0	17.8	13.4
Mean ± SD	19.9 ± 7.3	16.0 ± 5.1	22.3 ± 8.7	16.5 ± 3.4

The first thing to look at in Table 2 is the difference in time constants between the values for the head erect data and the dumping data. In the gravitational environment of Earth, the time constant for the slow phase eye velocity I(PV) decay with the head tilted face down (Td) is a little more than half, or 55%, of the time constant that is obtained with the head erect test (Te). If you will compare the preflight time constant values for both tests, Dr. Oman's data also seems to show the dumping time (preflight Td) is a little more than half of the head erect time (preflight Te). However, by comparing the inflight time constants for both tests, you can see that for most subjects, the dumping time constant (inflight T_d) did not change much from the head erect time constant (inflight T_e). This is an important finding. This shows that nystagmus dumping does not occur to the same extent in space as it does here on Earth. This suggests that nystagmus dumping must be dependent on gravity to some extent since, by the removal of gravity, it is changed.

The data in Table 2 suggests another important finding. Since the inflight time constants for both the head erect test and the dumping test are similar for subjects M, N. and to some extent T, this indicates that the subject's angular velocity storage capability may not be reduced inflight. As explained previously, this indicates that their VOR is probably maintaining most of its function in space. This finding is surprising since the angular velocity storage capability is found to be greatly reduced during similar tests on the KC-135 parabolic flight airplane.

The important thing to remember here is that the inflight data was not collected until the fourth or fifth day in space. Do you think the results would have been different if data had been collected earlier in the mission, say on the first or second day in space? The earliest parts of the mission are the time when the brain and sensory organs are most confused and when the astronauts are most ill. In fact, some internal reorganization and reprogramming may have already occurred in the astronaut's sensory and balance system by the time the data was collected. Even so, it is interesting that the nystagmus dumping phenomena did not work in space the same way it does on Earth.

Figure 21. As the astronaut observes a rotating visual field, his body feels a false sense of rotational motion.

Let's now look at the technique and results for Dr. Young's investigation of visually induced roll (VIR). Recall that this occurs as you observe a rotating visual field and your body feels a false sensation of rotational motion due to that rotating field. VIR is measured by having the astronaut place his head inside a "rotating dome" whose interior is covered with dots that serve as a visual stimulus Figure 21. This moving visual field rotates at a constant velocity of 30, 45, or 60 degrees/second. The astronaut watches the visual field rotate for 20 seconds at a time with a 10 second pause between each view. A sensation of self-rotation is felt by the crew member.

Before placing one's head in the dome, a soft contact lens, marked with a "starburst" pattern to serve as eye landmarks for postflight data analysis, is inserted and wetted with distilled water so that it will temporarily stick to the lens of the cornea. As the dome rotates, the crew member moves a joystick to indicate the perception of self-motion. Rolling eye movements and body position are also recorded by split-screen, closed-circuit television. Neck movements, produced when the crew member attempted to "correct" for the perceived roll, were recorded using gauges that measure neck strain (Figure 15). These gauges were attached to a biteboard that was built onto the flight model of the rotating dome and that the astronauts would bite to keep their head stable while they either floated or while their body was secured to the floor with bungee cords. During the preflight and postflight tests, the biteboard was eliminated since, on Earth, the astronauts could stand up. Various measurements were recorded during the experiment, including the extent of swaying off center that the astronauts experienced while looking at the rotating visual field of the rotating dome. The more the astronauts sway, the greater the level of their instability. The interesting data shown in Figure 22 illustrates the postural sway results for two crewmembers, Subject M and Subject T.

Postflight Postural Instability
Figure 22. (a) Postflight postural instability, in response to the rotating dome without biteboard stabilization, was evident for the entire postflight week of testing for Subject M. (b) The day after return from space was the most unstable for Subject T, but postural stability resumed to normal levels within a day or two. (R = recovery, which refers to having landed back on Earth.)

By comparing the level of sway before flight and after flight, you can see that both astronauts exhibited great instability upon their return to Earth. Subject T seemed to return close to normal soon after returning from space, but Subject M took longer to recover, and After a week, was still experiencing significant swaying It must be remembered that the data presented are from only two subjects. Therefore, it is very difficult to draw any general conclusion about the disorientation that astronauts experience upon their return home. Also, many other variables were measured during Dr. Young's investigation of VIR and the integrated picture that is produced by considering all the data is necessary for a full explanation about what is happening. This data does suggest, however, that exposure to weightlessness causes the astronauts to rely more on their visual cues since many of their confirming vestibular signals are absent in space. Upon returning, the astronaut's balance systems are initially unstable while the brain and vestibular organs are "reprogrammed" to understand and interpret the gravitational stimulus that is present on Earth. During this reprogramming time, astronauts shift their body's reliance away from visually dominated cues back to the utilization of a fully operating VOR.

The good news is that, upon their return home, astronauts do eventually recover their "Earth-normal" sensory and balance function completely. However, a few astronauts have even experienced a very brief "readaptation sickness" when they return home, since the brain must once again recall or relearn how it and the other sensory organs used to work together before the space flight mission took place. Of course, it is not clear whether their sickness is due to the readaptation process or to the overwhelming excitement they feel for having completed a space mission and returned home to their families! We've learned in this section how the visual components of the sensory and balance system serve as indicators for a changed vestibulo-ocular reflex (VOR) in space. We have also seen the level of instability that at least two astronauts experienced because of space flight. All that we have seen seems to suggest that, in space, humans tend to shift their emphasis from vestibular cues to visual cues. In fact, the alteration of the visual-vestibular interaction caused by the loss of gravity is considered one of the possible causes of space motion sickness.

Let's now review Dr. Young's results from a set of experiments designed to understand how space flight affects our awareness of our body's position and posture.