THE SPACE FLIGHT INVESTIGATION

But what is it about gymnastic movements that causes these athletes to develop such an intense relationship with gravity? As we've learned, movements of your head cause a variety of sensory confirmation steps and compensating factors to o ccur in your body. Here on Earth, the vestibular organs operate only to detect head movements relative to gravity, while the other sensory systems that we reviewed in detail earlier detect our body movements. With the gym nasts' active head accelerations, both rotational and linear, their vestibular systems are kept very busy indeed and they have developed an unusually sensitive vestibular organ. Of course, gymnasts generously use their proprioceptive sense to feel the ben ding and movements of the joints and tendons as well as the many muscle contractions that occur for each move, their tactile and hearing senses to feel and hear the pressure of the air rushing past them as they fly through the air, and their sense of sigh t to watch everything rush past them, upside down and then right side up again!

The best and most accessible indicators of vestibular function to use in order to study and understand how the balance system of the body works, are the movement and various reflex mechanisms embedded in the head, particularly the eye. Ho w do researchers measure head movements and reflexes? Although we can't enter the skull to directly observe and measure vestibular function, we do have what is considered a "window" to the inside of the head that we can use to observe some of the events t aking place in the head as we move it. That window is the eye, and its movements are an indicator of vestibular function. That is, many of our eye movements reflect what is occurring in our head and brain. You have already learned in Student Investigation 2 about the vestibulo-ocular reflex and how, when tilting your head, this reflex causes the rolling of the eyeballs in the opposite direction to the head movement, a phenomenon called ocular counterrolling. Another kind of eye movement occurs when something in your visual field is moving and your eyes shift in one direction and then quickly jump back in the other direction to stabilize the image. This is called nystagm us and was the topic of Student Investigation 1. In order to record the nystagmus response, a person may wear specially designed contact lenses that mark certain portions of the eye with a "starburst" design so that any movement of the eye can be detected. Then the eye movements are videotaped for analysis.

Eye reflex activity can also be measured by placing electrodes on the face or temples near the eye and recording the electrical signals that are produced by the rotation of the eyeball. The eye itself ha s an electrical field around it, due to the intense metabolic activity in the retina. This field moves with the eye, and you can pick it up with electrodes placed on the skin around the eye. The voltage reading that you can pick up, which is usually very small, is proportional to eye movement. The technique to measure and record this reflex is called electrooculography (EOG). (Electro = refers to electrical signals, oculo = refers to the eye, and in research, graphy = the recording of the data). Flinching and blinking are examples of these eye reflex contractions but there are many others that we don't even realize are occurring during our day-to-day balancing act.

Of course, muscle reflexes in the neck are also good indicators of various head movements since, After all, it is the neck muscles that are used to move the head. Try tilting the top half of your body over to the side as you sit there loo king straight ahead, and feel the neck muscles activate. Your neck is performing corrective reflex movements the whole time you are moving. The electrical signals produced by these reflexed can also be measured using a technique called electromyography (EMG) (myo = muscle.) In fact, EMG is also used to study the balance reflex responses of the leg muscles in relation to movement. We will be discussing both EOG and EMG in association with our examination of Dr. Young's space flight experiment in this secti on.

Dr. Young's experiment consisted of six different tests to assess sensory-motor adaptation:

Figure 15. The rotating dome experiment.

Figure 16. The awareness of position task.

Figure 17. The otolith-spinal reflex test.

1. A rotating chair was used to test movements of the eye and the vestibulo-ocular reflex (VOR). One chair protocol (procedure) required that the test subject be rotated about a vertical axis and stopped suddenly.
2. A second procedure using the rotating chair required the subject to pitch his or her head forward after the chair is stopped to investigate the phenomenon of nystagmus dumpin. We investigated our own nystagmus dumping response in Student Investigation 7.1. Both procedures using the rotating chair were designed and conducted by Dr. Charles Oman from the Massachusetts Institute of Technology.
3. In a different test, a specially designed rotating dome was used (Figure 15). When a subject sees the dot-patterned dome rotating in one direction, clockwise or counterclockwise, directly in front of her face, she senses movement in the opposite direction. The subject uses a joystick to indicate her perceived direction and velocity of rotation.
4. In the awareness of position task, crew members were asked to view various targets on a visual screen and then, with their eyes closed, point to them with a light pointer (Figure 16). This allowed investigators to examine if any changes occur in the accuracy and control of limb position due to space flight. This particular part of the entire experiment package was designed and conducted by Dr. Douglas Watt from McGill University in Montreal, Canada.
5. Also, the otolith-spinal reflex, or the reflex that causes one to catch oneself when sensing a fall, was tested in the "drop" experiment. For the study, a crew member wore a harness with bungee cords attached to the floor to serve as a substitute for gravity. The person then hung by the arms holding a suspended T-handle (Figure 17). When the T-handle automatically released and dropped the crew member, EMG electrodes measured the electrical activity in the leg muscle to determine the relationship between the vestibular system and muscle reflex responses in space. Dr. Douglas Watt was responsible for this part of Dr. Young's overall experiment package.
6. Crew members also wore an Acceleration Recording Unit to measure the acceleratory forces of their head movements, both natural and exaggerated, and recorded any space motion sickness symptoms on a pocket voice recorder. The goal of this experiment was to see if there was a connection between the vigor with which head movements were made and the intensity of space motion sickness. This particular experiment was carried out by Dr. Charles Oman, and he found that people who moved their heads more were also the ones who were sicker.

• the effects of space flight on the vestibulo-ocular reflex (VOR) and on other visual components of balance;
• how one's awareness of position and posture can change in microgravity and how the otolith-spinal reflex may contribute to those changes.