NASA was established in 1958, shortly after the Russian launching of the
second Earth satellite Sputnik II, the first vehicle to carry life into
orbit around the Earth. This accomplishment was preceded by the
pioneering work of Henry et al. ([ref.77]), in which animals were
exposed briefly to low-gravity states in Aerobee rockets. A
motion-picture camera photographed the behavior of two white mice in
rotating drums during this series of flights, which marked the first
time that simple psychological tests were made on animals in the
weightless condition. While this behavioral experiment was relatively
simple, it provided the basic concepts for recent studies which involved
rotation of animals during the weightless state. Subsequent flights such
as Project MIA (Mouse-in-Able) reflected a preoccupation with
physiologic measures (refs. [ref.78] and [ref.79]), although the flights
of Baker and Able included preflight and postflight performance studies
([ref.80]). Able’s behavior was recorded in detail on in-flight film,
but none of the behavior was programed or under experimental control.
The first flights in which behavior or performance was explicitly
programed were those of Sam and Miss Sam in flights of the Little Joe
rocket with the Mercury capsule, launched from Wallops Island in 1959
and 1960 ([ref.81]). The first major space achievement in the behavioral
sciences was the successful in-flight measurement of the behavior of the
chimpanzee Ham in early 1961, in which the pretrained animal performed
throughout the flight. The second achievement along these lines was in
1962 when the chimpanzee Enos made several orbits around Earth and
performed continuously on a complex behavioral task. The tasks which the
animals performed during these flights have been described in detail by
Belleville et al. ([ref.82]), and the results of the in-flight
performance have been presented by Henry and Mosely ([ref.83]). These
early flights provided much of the technological framework on which
current biological experiments on organisms during flights of extended
duration are based. Due largely to the efforts of Grunzke (refs.
[ref.84] and [ref.85]), the apparatus needed to sustain animals during
space flight, such as zero-g watering and feeding devices, are now
commonplace ([ref.86]). Advanced systems of programing stimulus
presentations and recording responses, developed for Project Mercury,
may now be seen in many basic research laboratories throughout the
country.
Several other noteworthy advances have been made as an outgrowth of the
Mercury animal flights. Immediately before the orbital flight MA-5, in
which the chimpanzee Enos was employed, it was unexpectedly found that
this 5-year-old animal was hypertensive. Subsequent centrifuge studies
showed that its vascular responses exceeded those of a control group.
Consideration of the animal’s preflight experience led to speculation
concerning the origin of this hypertension. An explanation of the
high-blood-pressure responses detected in Enos has been pursued by
Meehan et al. ([ref.87]). Persistent hypertension has been produced in
other laboratory chimpanzees restrained in the same manner as those
participating in space flight and exposed to demanding performance
tasks, a demonstration which has important implications for prolonged
manned space flight and for cardiovascular medicine in general.
Studies more directly concerned with behavior and performance have been
extended from those of Project Mercury. These extensions have been in
the following directions: (1) the establishment and maintenance of
complex behavioral repertoires under conditions of full environmental
control, (2) the refinement of behavioral techniques for assessing
sensory and motor processes, and (3) the maintenance of sustained
performance under conditions of long-term isolation and confinement and
preliminary extension of such experimental analysis to man.
Numerous studies with primate subjects, including several at Ames
Research Center, have been devoted to developing methods for maintaining
optimum performance in environments with limited sources of stimulation.
Monkeys, baboons, and chimpanzees, for example, have been isolated for
periods of longer than 2 years with no decrement in performance on
complicated behavioral tasks ([ref.88]). The behavioral techniques used
in these studies are closely related to those employed on human subjects
under NASA sponsorship at the University of Maryland ([ref.89]). The
essence of these techniques is in the proper programing of environmental
stimuli ([ref.90]). It is not sufficient to provide the subject with his
physiological requirements for survival, but he must be given the
psychological motivation for using these provisions. This statement, of
course, is an oversimplification of the problem, but it serves to
illustrate the essence of these experimental programs.
Gravity has long been known as one of the major factors influencing
various life processes and the orientation of both plants and animals.
One of the most challenging problems of space research has been to
define this influence more precisely. Related to the effect of gravity
on living processes is the problem of the effects of weightlessness. Of
particular interest to psychologists are the possible modifications an
altered gravitational environment might produce in behavioral patterns
basic to the animal’s maintenance and survival, such as eating, sensory
and discriminative processes, development and maturation, and learning
capacity ([ref.91]).
One prominent method of studying gravitational effects is to simulate an
increase in gravity by centrifugation. Smith et al. ([ref.92]) and
Winget et al. ([ref.93]) have investigated the effects of long-term
acceleration on birds, primarily chickens, while Wunder (refs. [ref.94]
and [ref.95]) and his coworkers (refs. [ref.96]-[ref.99]) have used
fruit flies, mice, rats, hamsters, and turtles. The general findings are
that, when animals are subjected to a prolonged period of acceleration
of moderate intensity, they exhibit decreased growth, delayed
maturation, and an increase in the size of certain muscles and organs,
dependent on the species. With regard to the decreased growth effect,
the data of these investigators show some exceptions. When the
gravitational increase is kept below a certain limit, growth was greater
than that of controls in the fruit fly, turtle, mouse, and chicken. The
limit below which enhancement of growth was observed varied with the
species studied.
The data on food intake do not present a consistent picture. Wunder
([ref.94]) found that food intake in accelerated mice was markedly
reduced from that of nonaccelerated control animals. Smith, however,
found that in chickens, food intake increased up to 36 percent over
controls and has derived an exponential relation between food intake and
acceleration. After six generations of selective breeding, Smith has
produced a strain of chickens better adapted to prolonged exposure to
high g.
A very relevant finding of their research with birds was that exposure
to chronic acceleration in some way appears to interfere with
habituation to rotatory stimulation. Chickens who were being subjected
to chronic acceleration were given repeated rotatory stimulation tests
to estimate their labyrinthine sensitivity. This study revealed that
centrifuged animals showed a marked reduction in labyrinthine
sensitivity. This result appeared to persist after the acceleration was
terminated. In animals who developed gait or postural difficulties as a
result of acceleration, there was no evidence of a postnystagmus in
response to the rotatory stimulation test, which the investigators point
out may be evidence of a lesion in the labyrinth or its neural pathways.
Smith has implicated social factors as interfering with acceleration
effects. His subjects were typically accelerated four or six to a cage.
When groups were mixed midway through the experiment, they exhibited a
higher mortality rate and incidence of acceleration symptoms than did
groups whose constituency remained unchanged.
At the U.S. Naval School of Aerospace Medicine, numerous studies have
been conducted on the effects of slow rotation on the behavior and
physiology of humans and animals ([ref.100]). Rotation initially
produces decrements in performance, but adaptation to a rotating
environment ensues quite rapidly (refs. [ref.101]-[ref.103]). Perceptual
distortion, nystagmus, nausea, and other signs of discomfort are common
responses to slow rotation. These symptoms are generally reduced with
continued exposure (adaptation). Interestingly, however, adaptation is
delayed when the subjects are exposed to a fixed reference outside their
rotating environment.
At NASA-Ames, rodents have been used in experiments by Weissman and
Seldeen to delimit the stimulus effects of rotation. In these
experiments the subjects must discriminate between different speeds of
rotation in order to obtain food reinforcement. The results thus far
provide evidence that these animals are capable of discriminating
between the different speeds at which they are being rotated. The range
of speeds studied was 0-25 rpm, with tests of discrimination being made
at intervals of less than 5 rpm. Experiments such as these will lead to
the development of techniques for measuring rotational sensitivity in
many species, including man.
The optimum configuration of manned spacecraft will depend, in part,
upon biomedical considerations. A voluminous literature now exists on
the possible hazards to man of prolonged exposure to zero-g conditions.
Should prolonged weightlessness prove to be a serious detriment to
health, consideration must be given to design concepts which provide
artificial gravity.
No data exist on the minimum gravity requirements necessary to sustain
basic biological functions for extended periods. A limit of 0.2 g has
been given as the lower level at which man can walk unaided ([ref.104]).
It has also been recommended that angular velocity be maintained
at the lowest possible level in order to minimize the occurrence of
vestibular disturbances. These recommendations are based on human-factor
requirements, rather than upon biological considerations, which may
significantly modify these values. In recent studies, a technique has
been devised which promises to provide reliable criteria for biological
acceptability, since it is based on fundamental biological and
behavioral principles.
As animals progress up the evolutionary stale, their survival depends
less and less upon stereotyped physiological reactions which occur in
reflex fashion, in response to environmental stimulation. In higher
organisms, survival depends more upon the capacity of organisms to
modify their behavior. At the highest levels of functional efficiency,
the ultimate form of adaptation is seen—the manipulation of the
environment by the organism. Developments in behavioral science now
permit us to utilize the adaptive behavior of animals to investigate
many problems of biological interest. Recent studies on the
self-selection of gravity levels represent a further attempt to exploit
the adaptive capacities of animals, in order to provide information
relevant to problems of space exploration.
One such project allows animals to select their own gravity environment
in an apparatus designed to create g-forces through centrifugal action
by rotation at 60 rpm ([ref.105]). The surface of this centrifuge is
parabolic, so that the resultant of the centrifugal g and the Earth’s
gravity is always normal to the surface. When the animal moves away from
the center, increasing the radius of rotation, it is exposed to
increasing gravity. Motion toward the center reduces the gravity level.
By this means, an animal is free to select its own gravity environment.
When the animal moves toward or away from the center, he is moving from
one tangential velocity to another. He is therefore acted upon by a
third force—due to Coriolis acceleration. The effects of Coriolis forces
are a major problem difficult to eliminate in studies such as these, but
they must be taken into account in the design of spacecraft which
produce artificial gravity by rotation. Motion of the head in any
direction not parallel to the centrifugal force vector would result in
bizarre stimulation of the semicircular canals and consequent motion
sickness. This effect is likely to become even more pronounced if the
sensitivity of these organs is increased by prolonged exposure to
reduced gravity. Methods such as these are currently being developed for
conducting a refined psychophysical analysis of gravity, including
studies by Lange and Broderson on the perception of angular, linear, and
Coriolis acceleration.
The results of animal studies such as these will be of great value in
arriving at a decisive judgment concerning the need for artificial
gravity in a manned orbiting space station, or other vehicles designed
for long-term occupancy.
To aid in the interpretation of in-flight data, other studies are
underway to determine the functions of the vestibular system, as a
principal brain center related to orientation in space and to the
physiology of posture and movement, as well as with the influences of
acceleration, rotation, and weightlessness. Experiments are presently
being conducted on monkeys and cats in order to trace these complex
neurological connections and to determine their functional organization.