Week 4 Sensation & Perception
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Week 4 Sensation & Perception INSTRUCTIONS: Read the article “Weightlessness and Perception” and answer the questions that follow. WEIGHTLESSNESS & PERCEPTION In the second half of the 20th century space travel became a reality. Until the first space travelers braved the unknown, scientists and doctors could only speculate on how the human body would react to weightlessness. Some of their speculations proved true, such as space motion sickness that is commonly experienced by space travelers at the beginning of a flight. Others proved false. One thing has become quite clear—the body can adapt to weightlessness and then readjust to gravity. Researchers are attempting to use what has been learned in space to treat people on Earth with problems such as balance disorders. As most doctors can attest, it is difficult to bodies back and forth, they say it feels as predict what will happen when a brand-new though they are stationary and the challenge is presented to the human body. spacecraft is moving. The reason is Time and again, space travel has revealed its embedded in our dependence on gravity for marvelous and sometimes subtle adaptive perceptual information. ability. But only in the past few years have The continuous and pervasive nature scientists begun to understand the body’s of gravity removes it from our daily responses to weightlessness, as the consciousness. But even though we are only data—the cumulative experience of nearly reminded of gravity’s invisible hand from 700 people spending a total of 58 person time to time by, say, varicose veins or an years in space—have grown in quantity and occasional lightheadedness on standing up, quality. Pursuit of this knowledge is our bodies never forget. Whether we realize improving health care not only for those who it or not, we have evolved a large number of journey into space but also for those of us silent, automatic reactions to cope with the stuck on the ground. The unexpected constant stress of living in a downward outcome of space medicine has been an pulling world. Only when we decrease or enhanced understanding of how the human increase the effective force of gravity on our body works right here on Earth. bodies do we consciously perceive it. Otherwise our perception is indirect. Feeling Gravity’s Pull Our senses provide accurate Although many factors affect human information about the location of our center health during spaceflight, weightlessness is of mass and the relative positions of our the dominant and single most important body parts. This capability integrates signals one. The direct and indirect effects of from our eyes and ears with other weightlessness precipitate a cascade of information from the vestibular organs in interrelated responses that begin in three our inner ear, from our muscles and joints, different types of tissue: gravity receptors, and from our senses of touch and pressure. fluids and weight-bearing structures. Many of these signals are dependent on the Ultimately, the whole body, from bones to size and direction of the constant terrestrial brain, reacts. gravitational force. When space travelers grasp the wall of The vestibular apparatus in the inner their spacecraft and pull and push their ear has two distinct components: the semicircular canals (three mutually spacecraft’s forward velocity carries it perpendicular, fluid-filled tubes that contain around the curve of the planet—space hair cells connected to nerve fibers), which travelers say they do not feel as if they are are sensitive to angular acceleration of the falling. The perception of falling probably head; and the otolith organs (two sacs filled depends on visual and airflow cues along with calcium carbonate crystals embedded with information from the direct gravity in a gel), which respond to linear receptors. acceleration. Because movement of the The aggregate of signal changes crystals in the otoliths generates the signal produces, in half or more of space travelers, of acceleration to the brain and because the a motion sickness that features many of the laws of physics relate that acceleration to a symptoms of terrestrial motion sickness: net force, gravity is always implicit in the headache, impaired concentration, loss of signal. Thus, the otoliths have been referred appetite, stomach awareness, vomiting. to as gravity receptors. They are not the only Space motion sickness usually does not last ones. Mechanical receptors in the muscles, beyond the first three days or so of tendons and joints—as well as pressure weightlessness, but something similar has receptors in the skin, particularly on the been reported by cosmonauts at the end of bottom of the feet—respond to the weight of long flights. limb segments and other body parts. At one time, scientists attributed Removing gravity transforms these space motion sickness to the unusual signals. The otoliths no longer perceive a pattern of vestibular activity, which conflicts downward bias to head movements. The with the brain’s expectations. Now it is clear limbs no longer have weight, so muscles are that this explanation was simplistic. The no longer required to contract and relax in sickness results from the convergence of a the usual way to maintain posture and bring variety of factors, including the alteration of about movement. the patterns and levels of motor activity Touch and pressure receptors in the feet necessary to control the head itself. A similar and ankles no longer signal the direction of motion sickness can also be elicited by down. These and other changes contribute computer systems designed to create virtual to visual-orientation illusions and feelings of environments, through which one can self-inversion, such as the feeling that the navigate without the forces and sensory body or the spacecraft spontaneously patterns present during real motion. reorients. In 1961 cosmonaut Gherman Titov Over time, the brain adapts to the new reported vivid sensations of being upside signals, and for some space travelers, down early in a spaceflight of only one day. “down” becomes simply where the feet are. Last year shuttle payload specialist Byron K. The adaptation probably involves Lichtenberg, commenting on his earlier flight physiological changes in both receptors and experiences, said, “When the main engines nerve-cell patterns. Similar changes occur on cut off, I immediately felt as though we had the ground during our growth and flipped 180 degrees.” Such illusions can maturation and during periods of major recur even after some time in space. body-weight changes. The way we control The lack of other critical sensory cues our balance and avoid falls is an important also confuses the brain. Although orbital and poorly understood part of physiology. flight is a perpetual free fall—the only Because otherwise healthy people returning difference from skydiving is that the from space initially have difficulty maintaining their balance but recover this Most of the body’s systems return to sense rapidly, post flight studies may allow normal within a few days or weeks of doctors to help those nonspace travelers landing, with the possible exception of the who suffer a loss of balance on Earth. musculoskeletal system. So far nothing Bernard Cohen of the Mount Sinai indicates that humans cannot live and work School of Medicine and Gilles Clement of the in space for long periods and return to Earth National Center for Scientific Research in to lead normal lives. This is clearly good Paris undertook just such a study after the news for denizens of the upcoming Neurolab shuttle mission, which ended on International Space Station and for any May 3, [1998]. To connect this work with future interplanetary missions. In fact, the patients suffering from balance disorders, station, assembly of which should begin late Barry W. Peterson of this year or early next year, will provide Northwestern University and a team of researchers with a new opportunity to researchers, supported by the National investigate the effects of space travel on Aeronautics and Space Administration and humans. On its completion in five years, the the National Institutes of Health, are creating station will have 46,000 cubic feet of work the first whole-body computer model of space (nearly five times more than the Mir or human posture and balance control. Skylab stations) and will include sophisticated laboratory equipment for the Down to Earth next generation of medical studies. When space travelers return to the Recognizing the need for a comprehensive world of weight, complementary changes attack on all the potential human risks of occur. If the effects of weightlessness are long-duration space travel, NASA has completely reversible, everything should selected and funded a special research body, return to its normal condition at some time the National Space Biomedical Research after the flight. We now know that most Institute, to assist in defining and responding systems in the body do work reversibly, at to those risks. least over the intervals for which we have Many of the “normal” changes that data. We do not yet know whether this is a take place in healthy people during or just general rule. after spaceflight are outwardly similar to Space travelers certainly feel “abnormal” events occurring in ill people on gravitationally challenged during and just Earth. For example, most space travelers after their descent. As one person said after cannot stand quietly for 10 minutes just nine days in space: “It’s quite a shock. The after landing without feeling faint. This first time I pushed myself up, I felt like I was so-called orthostatic intolerance is also lifting three times my weight.” Returning experienced by patients who have stayed in space travelers report experiencing a variety bed for a long time and by some elderly of illusions—for example, during head people.