Space neuroscience is the scientific study of the central nervous system (CNS) functions during spaceflight. Living systems can integrate the inputs from the senses to navigate in their environment and to coordinate posture, locomotion, and eye movements. Gravity has a fundamental role in controlling these functions. In weightlessness during spaceflight, integrating the sensory inputs and coordinating motor responses is harder to do because gravity is no longer sensed during free-fall. For example, the otolith organs of the vestibular system no longer signal head tilt relative to gravity when standing. However, they can still sense head translation during body motion. Ambiguities and changes in how the gravitational input is processed can lead to potential errors in perception, which affects spatial orientation and mental representation. Dysfunctions of the vestibular system are common during and immediately after spaceflight, such as space motion sickness in orbit and balance disorders after return to Earth.[1]
Adaptation to weightlessness involves not just the Sensory-motor coupling functions, but some autonomic nervous system functions as well. Sleep disorders and orthostatic intolerance are also common during and after spaceflight. There is no hydrostatic pressure in a weightless environment. As a result, the redistribution of body fluids toward the upper body causes a decrease in leg volume, which may affect muscleviscosity and compliance. An increase in intracranial pressure may also be responsible for a decrease in near visual acuity.[2] In addition, muscle mass and strength both decrease as a result of the reduced loading in weightlessness. Moreover, approximately 70% of astronauts experience space motion sickness to some degree during the first days.[3] The drugs commonly used to combat motion sickness, such as scopolamine and promethazine, have soporific effects. These factors can lead to chronic fatigue. The challenge of integrative space medicine and physiology is to investigate the adaptation of the human body to spaceflight as a whole, and not just as the sum of body parts because all body functions are connected and interact with each other.
To date, only three countries, the United States, Russia, and China, have the capability to launch humans into orbit. However, 520 astronauts from more than thirty different countries have flown in space and many of them have participated in space neuroscience research. The launch of the first living animal in orbit on Sputnik on November 3, 1957 marked the beginning of a rich history of unique scientific and technological achievements in space life sciences that have spanned more than fifty years to date.[4]
The first documented space neuroscience experiments were performed during the third human mission on board the Russian Vostok spacecraft. These experiments began after the crew from previous missions complained from nausea and spatial disorientation in weightlessness. Space neuroscience experiments typically addressed these operational issues until the Skylab and Salyut space stations were made available for more fundamental research on the effect of gravity on CNS functions. Approximately 400 space neuroscience experiments have been performed from Vostok-3 in August 1962 to the Expedition-15 on board the International Space Station in October 2007.[5]
Operational aspects
Sensory and sensorimotor disturbances when arriving in low Earth orbit are well documented, the most known of these being space motion sickness (SMS). Individual differences, spacecraft size, and body movements cause SMS symptoms. Typically lasting the first three or four days of weightlessness, symptoms range from headaches and fatigue to nausea and vomiting. The consequences vary from simple discomfort to possible incapacitation, creating potential problems during extra-vehicular activity, re-entry, and emergency egress from the spacecraft. The body receives a variety of conflicting signals from the visual, somato-sensory, and vestibular organs in weightlessness. These conflicting inputs are thought to be the primary cause of SMS, but the precise mechanisms of the conflict are not well understood. Medications currently used to alleviate the symptoms produce undesirable side effects.[6]
Astronauts must remain alert and vigilant while operating complicated equipment. Therefore, getting enough sleep is a crucial factor of mission success. Weightlessness, a confined and isolated environment, and busy schedules coupled with the absence of a regular 24-hour day make sleep difficult in space. Astronauts typically average only about six hours of sleep each night. Cumulative sleep loss and sleep disruption could lead to performance errors and accidents that pose significant risk to mission success. Sleep and circadian cycles also temporally modulate a broad range of physiological, hormonal, behavioral, and cognitive functions.
Methods to prevent sleep loss, reduce human error, and optimize mental and physical performance during long-duration spaceflight are being investigated. Particular concerns include the effect of the space environment on higher-order cognitive processes like decision-making and the impact of changing gravity on mental functions, which will be important if artificial gravity is considered as a countermeasure for future interplanetary space missions.[7] It is also necessary to develop human-response measurement technologies to assess the crew's ability to perform flight-management tasks effectively. Simple and reliable behavioral and psycho-physiological response measurement systems are needed to assess mental loading, stress, task engagement, and situation awareness during spaceflight.
Sensory functions in space
All living organisms on Earth have the ability to sense and respond to changes in their internal and external environment. Organisms, including humans, must accurately sense before they can react, thus ensuring survival. The body senses the environment by specialized sensory organs. The CNS utilizes these sensations in order to coordinate and organize muscle activities, shift from uncomfortable positions, and adjust balance properly. In common speech, five different senses are usually recognized: vision, hearing, smell, taste, and touch. All these senses are somewhat affected by weightlessness.
In fact, the human body has seven sensory systems – not five. The sixth and seventh systems are the senses of motion located in the inner ear. The former signals the beginning and end of rotation and the latter signals body tilt relative to gravity as well as body translation. The seventh system no longer provides tilt information in weightlessness; however, it does continue to signal translation, so the afferent signals to the CNS are confusing. The experience of living and working in space alters the way the CNS interprets the otolith organ signals during linear acceleration. Although the perception is fairly accurate when subjects are exposed to angular acceleration in yaw in-flight, there are disturbances during angular rotation in pitch and roll, and during linear acceleration along the body transversal and longitudinal axes. Perception of body motion is also altered during the same motion immediately after landing. There is an adaptation to weightlessness in orbit that carries over to post-flight reactions to linear acceleration.[8]
Posture, movement, and locomotions
Exposure to weightlessness causes changes to the signals from the receptors to touch, pressure, and gravity, i.e., all information necessary for postural stability. Adaptive modifications in the central processing of sensory information take place to produce motor responses that are appropriate for the new gravitational environment. As a result, terrestrial motor strategies are progressively abandoned in weightlessness, as astronauts adapt to the weightless environment. This is particularly true for the major postural muscles found in the lower legs. The modifications in posture, movement, and locomotion acquired in reduced gravity are then inappropriate for Earth's gravity upon return. After landing, postural instability approaching clinical ataxia is manifested as a result of this in-flight neural reorganization.[9]
Difficulties with standing, walking, turning corners, climbing stairs, and a slowing of gait are experienced as astronauts re-adapt to Earth's gravity, until terrestrial motor strategies are fully re-acquired. Adaptation to spaceflight also induces a significant increase in the time required to traverse an obstacle course on landing day, and recovery of functional mobility takes an average of two weeks.[10] These difficulties can have adverse consequences for an astronauts’ ability to stand up or escape from the vehicle during emergencies and to function effectively immediately after leaving the spacecraft after flight. Thus it is important to understand the cause of these profound impairments of posture and locomotion stability, and develop countermeasures.
The most significant sensorimotor problems astronauts will face during a stay on the Moon and Mars are likely to occur when walking around in their space suits. The suits are big and bulky and change the body's center of gravity. This along with the uneven terrain and limited field of view makes locomotion challenging.
Compensatory eye movements
The function of the vestibular system during spaceflight is by far the most carefully studied of all. This is especially true of the gravity-sensing otolith organs and their relationship to eye movements. The vestibular semicircular canal function seems unchanged in weightlessness because the horizontal eye movements that compensated for head yaw rotation are not affected by spaceflight. The absence of gravity stimulation of the otoliths reduces the torsional vestibulo-ocular reflex during head roll rotation in microgravity. This deficit is absent when astronauts are exposed to centrifugal forces, suggesting that the adaptive CNS changes are taking place centrally rather than peripherally.[11]
During the first days in orbit, the asymmetry of vertical eye movements in response to moving visual scenes is inverted. A return to symmetry of the vestibulo-ocular and optokinetic reflexes is then observed. Some studies have shown increased latencies and decreased peak velocities of saccades, while others have found just the opposite. It is possible that these conflicting results depend on when the measures were obtained during the mission. There is also a serious disruption of smooth pursuit eye movements, especially in the vertical plane.[12]
Human missions to Mars will include several transitions between different gravitational environments. These changes will eventually affect the reflex eye movements. A key question is whether astronauts can have different sets of reflexes among which they can rapidly switch based on the gravitational environment. Determination of the dual-adaptive capabilities of reflex eye movements in such circumstances is vitally important so that it can be determined to what extent the Sensory-motor coupling skills acquired in one-g environment will transfer to others.
Spatial orientation
An astronaut on board the International Space Station is wearing a head-mounted display for performing a space neuroscience experiment aimed at evaluating changes in perceived depth and distance.
In weightlessness, astronauts must rely much more on vision to maintain their spatial orientation, because the otolith organs can no longer signal the “down” direction. During prolonged exposure, however, reliance seems to shift toward an intrinsic, body vertical reference. The erroneous illusions of self-motion during head movements performed during and after return to Earth gravity are presumably due to a re-interpretation of vestibular inputs. Ground-based studies suggest that the CNS resolves the “tilt-translation” ambiguity based on the frequency content of the linear acceleration detected by the otolith organs, with low frequency indicating “tilt” and high frequency indicating “translation”. A crossover exists at about 0.3Hz where the otolith signals are then ambiguous. Exposure to weightlessness presumably results in a shift of this crossover frequency, which could then contribute to spatial disorientation and SMS.[13]
Although investigations of higher cognitive processes, such as navigation and mental rotation are limited,[14] the astronauts frequently report that the spacecraft interiors look longer and higher than they actually are, and a reduction in the perceived height of three-dimensional objects is observed in-flight compared with pre-flight, suggesting an alteration in the mental representation of three-dimensional cues in weightlessness. Perception is a model of the brain, a hypothesis about the world that presupposes the Newton's laws of motion. These laws change in weightlessness and, therefore, one could expect changes in the mental representation of objects’ shape and distance during spaceflight.[15] The rare investigations carried out in space so far have not demonstrated drastic changes, probably because the CNS continues to use an internal model of gravity, at least for a short while.[16] It can be speculated that the way of processing three dimensions will be more developed after a long absence of a gravitational reference.
Further investigations carried out in space will perhaps reveal that other higher cortical functions are impaired in weightless conditions. The combination of virtual reality with the measurement of evoked potentials and brain mapping on board the International Space Station should provide exciting results on the adaptive mechanisms of cerebral functions in weightlessness.
Neuroscience and space exploration
From Voskhod to the International Space Station, spacecraft have improved in size and comfort and have allowed more and more people traveling into orbit. However, even with all of the human spaceflight experience gained over the past fifty years, no single completely effective countermeasure, or combination of countermeasures, exists against the negative effects of long-duration exposure to weightlessness. If a crew of astronauts were to embark on a six-month journey to Mars today, the countermeasures currently employed would presumably leave them less operational after landing.[1]
Many believe that physiological adaptation to Mars gravity (0.38 G) and re-adaptation to Earth gravity (1 G) would be enhanced by frequent exposure to artificial gravity on board the spacecraft en route to and from Mars. This would require an on-board human-rated centrifuge or spacecraft rotation to produce a centrifugal force similar to gravity. This solution, while potentially effective, raises a number of operational, engineering, and physiological issues that will need to be addressed. The human physiological responses to long-duration exposure to anything other than zero-gravity or Earth's gravity are unknown. Research is needed to identify the minimum level, duration, and frequency of gravity level required to maintain normal CNS functions, as well as the importance of a gravity gradient across the body.[17]
The complex functioning of the CNS, even in the 1-G environment of Earth, has not revealed all its secrets. The most basic space neuroscience questions must be answered to minimize risks and optimize crew performance during transit and planetary operations. The results of this research will certainly find other applications in medicine and biotechnology. Our ability to understand how Earth's gravitational environment has shaped the evolution of sensory and motor systems can give us a clearer understanding of the fundamental mechanisms of CNS functions. Knowledge of the effects of gravity on CNS functions in humans, as well as elucidation of the basic mechanisms by which these effects occur, will be of direct benefit to understanding the impact of, and providing countermeasures for, long-term exposure of humans to the weightlessness of spaceflight and the partial gravity of Moon and Mars bases.
Human spaceflight is spaceflight with a crew or passengers aboard a spacecraft, the spacecraft being operated directly by the onboard human crew. Spacecraft can also be remotely operated from ground stations on Earth, or autonomously, without any direct human involvement. People trained for spaceflight are called astronauts, cosmonauts, or taikonauts; and non-professionals are referred to as spaceflight participants.
Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea, vomiting, cold sweat, headache, sleepiness, yawning, loss of appetite, and increased salivation. Complications may rarely include dehydration, electrolyte problems, or a lower esophageal tear.
The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent humans and nonhuman animals from falling over when standing or moving. Equilibrioception is the result of a number of sensory systems working together: the eyes, the inner ears, and the body's sense of where it is in space (proprioception) ideally need to be intact.
STS-58 was a mission flown by Space Shuttle Columbia launched from Kennedy Space Center, Florida, on 18 October 1993. The missions was primarily devoted to experiments concerning the physiological effects of spaceflight. This was the first in-flight use of the "Portable In-flight Landing Operations Trainer" simulation software. It was also the last time Columbia would land at Edwards Air Force Base.
The term micro-g environment is more or less synonymous with the terms weightlessness and zero-g, but with an emphasis on the fact that g-forces are never exactly zero—just very small. The symbol for microgravity, μg, was used on the insignias of Space Shuttle flights STS-87 and STS-107, because these flights were devoted to microgravity research in low Earth orbit.
The vestibular system, in vertebrates, is a sensory system that provides the leading contribution to the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear in most mammals.
Space adaptation syndrome (SAS) or space sickness is a condition experienced by as many as half of all space travelers during their adaptation to weightlessness once in orbit. It is the opposite of terrestrial motion sickness since it occurs when the environment and the person appear visually to be in motion relative to one another even though there is no corresponding sensation of bodily movement originating from the vestibular system.
Spatial disorientation of an aviator is the inability to determine angle, altitude or speed. It is most critical at night or in poor weather, when there is no visible horizon, since vision is the dominant sense for orientation. The auditory system, vestibular system, and proprioceptive system collectively work to co-ordinate movement with balance, and can also create illusory nonvisual sensations, resulting in spatial disorientation in the absence of strong visual cues.
Artificial gravity is the creation of an inertial force that mimics the effects of a gravitational force, usually by rotation. Artificial gravity, or rotational gravity, is thus the appearance of a centrifugal force in a rotating frame of reference, as opposed to the force experienced in linear acceleration, which by the equivalence principle is indistinguishable from gravity. In a more general sense, "artificial gravity" may also refer to the effect of linear acceleration, e.g. by means of a rocket engine.
Venturing into the environment of space can have negative effects on the human body. Significant adverse effects of long-term weightlessness include muscle atrophy and deterioration of the skeleton. Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells, balance disorders, eyesight disorders and changes in the immune system. Additional symptoms include fluid redistribution, loss of body mass, nasal congestion, sleep disturbance, and excess flatulence.
A motion simulator or motion platform is a mechanism that creates the feelings of being in a real motion environment. In a simulator, the movement is synchronised with a visual display of the outside world (OTW) scene. Motion platforms can provide movement in all of the six degrees of freedom (DOF) that can be experienced by an object that is free to move, such as an aircraft or spacecraft:. These are the three rotational degrees of freedom and three translational or linear degrees of freedom.
The vestibular nerve is one of the two branches of the vestibulocochlear nerve. In humans the vestibular nerve transmits sensory information transmitted by vestibular hair cells located in the two otolith organs and the three semicircular canals via the vestibular ganglion of Scarpa. Information from the otolith organs reflects gravity and linear accelerations of the head. Information from the semicircular canals reflects rotational movement of the head. Both are necessary for the sensation of body position and gaze stability in relation to a moving environment.
The vestibulospinal tract is a neural tract in the central nervous system. Specifically, it is a component of the extrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information from nuclei to motor neurons. The vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract. The function of these motor commands is to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head.
The otolithic membrane is a fibrous structure located in the vestibular system of the inner ear. It plays a critical role in the brain's interpretation of equilibrium. The membrane serves to determine if the body or the head is tilted, in addition to the linear acceleration of the body. The linear acceleration could be in the horizontal direction as in a moving car or vertical acceleration such as that felt when an elevator moves up or down.
The Orbiting Frog Otolith (OFO) was a NASA space program which sent two bullfrogs into orbit on 9 November 1970 for the study of weightlessness. The name, derived through common use, was a functional description of the biological experiment carried by the satellite. Otolith referred to the frog's inner ear balance mechanism.
Weightlessness is the complete or near-complete absence of the sensation of weight. This is also termed zero-G, although the more correct term is "zero G-force". It occurs in the absence of any contact forces upon objects including the human body.
The eye-tracking device (ETD) is a headmounted device, designed for measurement of 3D eye and head movements under experimental and natural conditions. The tracker permits comprehensive measurement of eye movement and optionally head movement. It represents a tool for the investigation of sensorimotor behaviour, particularly of the vestibular and oculomotor systems in both health and disease.
The righting reflex, also known as the labyrinthine righting reflex, is a reflex that corrects the orientation of the body when it is taken out of its normal upright position. It is initiated by the vestibular system, which detects that the body is not erect and causes the head to move back into position as the rest of the body follows. The perception of head movement involves the body sensing linear acceleration or the force of gravity through the otoliths, and angular acceleration through the semicircular canals. The reflex uses a combination of visual system inputs, vestibular inputs, and somatosensory inputs to make postural adjustments when the body becomes displaced from its normal vertical position. These inputs are used to create what is called an efference copy. This means that the brain makes comparisons in the cerebellum between expected posture and perceived posture, and corrects for the difference. The reflex takes 6 or 7 weeks to perfect, but can be affected by various types of balance disorders.
Astronaut training describes the complex process of preparing astronauts in regions around the world for their space missions before, during and after the flight, which includes medical tests, physical training, extra-vehicular activity (EVA) training, procedure training, rehabilitation process, as well as training on experiments they will accomplish during their stay in space.
The following page is a list of scientific research that is currently underway or has been previously studied on the International Space Station by the European Space Agency.
↑ Crampton GH (1990). Motion and Space Sickness. CRC Press: Boca Raton.
↑ Clément G, Slenzka K (2006). Fundamentals of Space Biology. Research on Cells, Plants and Animals in Space. Springer: New York.
↑ Reschke MF, Krnavek JM, Somers JT, etal. (2007). A brief history of space flight with a comprehensive compendium of vestibular and sensorimotor research conducted across the various flight programs. NASA Johnson Space Center: Houston.
↑ Clément G (2011). Fundamentals of Space Medicine - 2nd Edition. Springer: New York.
↑ Clément G, Bukley A (2007). Artificial Gravity. Springer: New York.
↑ Clément G, Gurfinkel VS, Lestienne F, Lipshits MI, Popov KE (1984). "Adaptation of postural control to weightlessness". Experimental Brain Research. 57 (1): 61–72. doi:10.1007/bf00231132. PMID6519230.
↑ Reschke MF, Bloomberg JJ, Harm DL, etal. (1998). "Posture, locomotion, spatial orientation, and motion sickness as a function of space flight". Brain Research Reviews. 28 (1–2): 102–117. doi:10.1016/s0165-0173(98)00031-9. PMID9795167.
↑ Clément G, Moore S, Raphan T, Cohen B (2001). "Perception of tilt (somatogravic illusion) in response to sustained linear acceleration during space flight". Experimental Brain Research. 138 (4): 410–418. doi:10.1007/s002210100706. PMID11465738.
↑ Clément G, Denise P, Reschke MF, Wood SJ (2007). "Human ocular counter-rotation and roll tilt perception during off-vertical axis rotation after spaceflight". Journal of Vestibular Research. 17: 209–215.
↑ Villard E, Tintó Garcia-Moreno F, Peter N, Clément G (2005). "Geometric visual illusions in microgravity during parabolic flight". NeuroReport. 16 (12): 1395–1398. doi:10.1097/01.wnr.0000174060.34274.3e. PMID16056146.
↑ McIntyre J, Zago M, Berthoz A, etal. (2001). "Does the brain model Newton's laws?". Nature Neuroscience. 4 (7): 693–695. doi:10.1038/89477. PMID11426224.
↑ Young LR (2000). "Vestibular reactions to space flight: Human factors issues". Aviation, Space, and Environmental Medicine. 71: A100–A104.
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