Parabolic Flight
Parabolic flights are one possibility of simulating microgravity on earth, besides the free fall tower and others. It is the method of choice which permits the longest period of microgravity, in order to study phenomena attending astronauts during their stay in space.
This maneuver can not be made with all types of air-crafts as not all construction types and materials do resist to the physical forces attained during parabolic episodes. Besides the technical demands to the aircraft one needs also to take logistic impact on civil and military airspace into consideration, whenever parabolic flight campaigns are planned.
Our experiment is flown by the A300-Zero-g from NOVESPACE in Bordeaux (F), which collaborates with ESA, and all national space agencies adhered to it.
Microgravity
Strictly physically speaking there can’t be any loss of gravity during the parabolic maneuver, as the aircraft, its passengers and experiments are still under the influence of the earth’s gravitational forces. So there is no zero gravity in fact!
The simulation is possible by free-falling, which is one of the characteristics of a parabolic flight path. Following this specific path the aircraft and its content fall at the same speed. By changing the parameters of the parabola, also other gravity simulations are possible, such as Moon (16.7% of earth gravity) and Mars (38% of earth gravity) gravitational states.
Fortunately our vestibular system, responsible for the sensing of acceleration, is fooled as well, so that the simulation can be used as a scientific approach to all kind of neuro-biological problems concerning orientation and navigation during prolonged exposure to microgravity in space.
Vestibular System
The vestibular system of mammals is situated inside the skull’s temporal bone, which also bears the external auditory canal visible from outside. It is quite well protected because it is embedded in a zone called “pars petrosa” of the mentioned bone (petra is Greek word for stone). This anatomical nomenclature emphasizes the hard and compact architecture of this boney part of the skull.
It is the organ which provides the brain with the sensory input concerning acceleration in all three dimensions of space. Is one part of this organ damaged or irritated this is felt as disorientation and dizziness.
As gravity is an acceleration as well and acts on the vestibular system, it is understandable, that the exposure to zero gravity might interfere with the sensory information provided by this system and thus cause some problems.
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Hippocampus
The hippocampus constitutes the largest part of the archicortex, the phylo-genetically oldest part of the cerebral cortex. In humans and non-human primates it is hidden in the ventromedial temporal lobe. Its involvement in the control of a multitude of behaviors is reflected in its numerous connections with cortical and subcortical areas. The hippocampus can roughly be split up into two functional zones.
The dorsal part is primarily involved in spatial navigation, spatial orientation and working memory. The ventral part is responsible for controlling the prefrontal cortex (higher cognitive functions including planning and decision making) and the amygdala (processing of emotions). Anatomy and function of the hippocampus are well preserved among most mammal species.
Thus, rodents can be used as animal models to study hippocampal functions in health (e.g., higher cognitive functions) and disease (e.g., Alzheimer’s disease, epilepsy, multiple sclerosis, including a number of psychiatric disorders that affect higher congitive functions).rzhrzh zhzhezhhzhzh rhr zrhrzhrzhrzhrzh rrzhrzhrzhrzhrzhrz zrhrzhrzhrzhhzrhrwzhrzhrzhwrzhrzh rzhrzhrzhrzhrwzhrwzvetvthrzhrzh rzzrrz zrhrzhrzjhrz zrjrzjh
Spacial Navigation Problems in Space
Spatial navigation has been the most important strategy for exploring our world since the very beginning of mankind. During the centuries where exploration was important for gaining access to new natural resources and political territorial advantages men relied on the most important extra-contextual or allocentric cues for navigation: the sun and the stars. Nowadays scientists have developed the necessary hard – and software to even navigate beyond the frontiers of our planet exploring the adjacent lunar and planetary complexes. The means which are used to navigate into space are considerably different from the methods used by our ancestors; nevertheless human orientation in the space surrounding him is of extraordinary importance even for Astronauts (meaning US and other Astronauts, Russian Cosmonauts, Chinese Taikonauts) using the best technological resources, especially when this technology fails.
The best example for this being the spectacular rescue of Apollo 13 (11.04.1970 – 17.04.1970), where technical failures, due to the explosion of an oxygen tank, had in part to be compensated by precise manual navigation in order to save the three Astronauts lives and the survival of the American Apollo program. An even more recent example for the importance of spatial navigation in this context is the rescue work for Salyut-7 (19.04.1982 – 07.02.1991) during the mid 1980ies, where several difficult extra vehicular activities (EVA’s) and especially the manually performed docking maneuvers by the crew of Commander Vladimir Dzhanibekov of the Soyuz T-13 mission (06.06.1985 – 26.12.1985) were necessary in order to re-establish the operability of this important predecessor of the Mir- station.
Other problems which have to be taken into consideration are the difficulties of the Astronauts in adapting to the lack of vestibular inputs for linear acceleration [1] in µ -gravity and the contradictory visual information which leads to motion sickness and orientation problems.
Some of these phenomena, such as visual reorientation illusions (VRI’s) or inversion illusions can arise during normal work in the cabin of the orbiting spacecraft, others such as height vertigo and 3 D navigation problems occur especially during EVA’s [2]. Some of these phenomena have been addressed by the work of J.S. Taube and colleagues by recording the firing of head direction cells posterior to the anterior dorsal thalamic nucleus (ADN) of rat brains during parabolic flights [3]. This neuro-physiological approach scrutinizes one of the major brain regions implicated in head position dependent spatial orientation. Its functional connection to the hippocampus, which is thought to be the main neuronal substrate processing spatial memory, pushed latter very important brain region into the spotlight of one of the Neurolab experiments during the STS-90 Shuttle mission [4, 5]. As described in the literature of spatial orientation and navigation in µ-gravity [2], the problems in navigation encountered by the Astronauts are most likely linked to the processing of spatial, visual and perceptive information in the central nervous system (CNS) instead of the adaptation of the vestibular organs or the vestibulo-ocular or vestibulo-spinal reflexes to microgravity.
Latter influences also being investigated in humans amongst other methods by means of the VVIS experiment on Neurolab (STS-90)[6]. During the same Neurolab Shuttle mission in 1998, four Astronauts have been subject of experiments about the susceptibility of VRI and motion illusion with the use of head mounted displays [2].
The introduction of the Morris Water maze place navigation task in 1982 [7], triggered a worldwide boost in interest for the understanding of the brain structures involved in spatial learning and spatial memory. First developed for rats, the water maze paradigm was rapidly adapted for the behavioural assessment of spatial navigation and working memory in mice, which due to the relatively easy manipulability of their genome soon replaced the rats as the most widely used animal model for human brain diseases.
During the last three decades of observation of the navigational capabilities of rodents in specific tasks the dorsal hippocampus has been identified as one of the brain structures mainly involved in spatial learning and spatial navigation. Investigators started very early to analyze the navigational behaviour of rodents with lesions to the dorsal hippocampus to search for eventual alternative strategies for spatial orientation and found severe behavioural deficits in these candidates [7, 8].
1. Clément, G., Using Your Head: Cognition And Sensimotor Functions In Microgravity. Gravitational and Space Biology, 2007. 20(2).
2. Oman, C., Spatial Orientation and Navigation in Microgravity, in Spatial Processing in Navigation, Imagery and Perception. 2007. p. 209-247.
3. Taube, J.S., et al., Rat Head Direction Cell Responses in Zero-Gravity Parabolic Flight. J Neurophysiol, 2004. 92(5): p. 2887-2997.
4. Homick, J.L., P. Delaney, and K. Rodda, Overview of the Neurolab spacelab mission. Acta Astronautica, 1998. 42(1-8): p. 69-87.
5. Knierim, J.J., B.L. McNaughton, and G.R. Poe, Three-dimensional spatial selectivity of hippocampal neurons during space flight. Nat Neurosci, 2000. 3(3): p. 209-210.
6. Bellossi, F., et al., EDEN: A payload dedicated to neurovestibular research for neurolab. Acta Astronautica, 1998. 42(1-8): p. 59-67.
7. Morris RG, G.P., Rawlins JN, O’Keefe J, Place navigation impaired in rats with hippocampal lesions. Nature, 1982. 297: p. 681-3.
8. Deacon, R.M.J., A. Croucher, and J.N.P. Rawlins, Hippocampal cytotoxic lesion effects on species-typical behaviours in mice. Behavioural Brain Research, 2002. 132(2): p. 203-213.
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