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The Star Larvae Hypothesis
Nature's Plan for Humankind
Part 3. Space Brains

Neuroplasticity and the Enrichments of Weightlessness

Bones and muscles—adaptations to gravity—atrophy in space, but brains are poised to bulk up.

Given phenotypic plasticity and the impoverishing effects of weightlessness on bone and muscle tissue, one should be able to predict the morphology, or body shape, of native extraterrestrials. Similarly, given neuroplasticity and the enriching effects of weightlessness on brain tissue, one should be able to predict the psychology of native extraterrestrials.

The assertion that weightlessness enriches brain tissue derives from the established scientific understanding of brain development. In What's Going on in There? : How the Brain and Mind Develop in the First Five Years of Life, neurobiologist Lise Eliot describes the foundational importance of the vestibular sense in the overall somatosensory and cognitive development of infants. The vestibular sense has to do with awareness of bodily orientation (e.g., upright, inverted, prone) and motion in space (e.g., rocking, falling, spinning). The vestibular sense is mediated by the vestibular organs of the inner ear in conjunction with proprioceptive inputs from the limbs and torso.

On Earth, the tug of gravity conditions the vestibular sense, from birth onward. But in weightless space, this sense will be conditioned by different influences, namely by a body's own movements. Minus gravity, newborns won't find themselves to be confined to lie supine, like beached sea creatures. Floating, gravityless, they will be able on their own to generate endless vestibular experiences.

Eliot describes experiments in which infants from 3 to 13 months old were held by researchers in swivel chairs and spun in one direction then the other with their heads tilted in various ways. They received this treatment four times a week for four weeks. A control group received no treatment, and a second control group sat on the researchers' laps but did not spin. Eliot notes,

"The results were striking. Compared with both control groups, the babies who were spun showed more advanced development of both their reflexes and their motor skills. [. . . . ] Vestibular stimulation appears to be equally beneficial to very young infants. Newborns cry less when they are rocked, carried, jiggled or suddenly changed in position, all actions that activate the vestibular system. [. . . .] Indeed, infants who are comforted through vestibular stimulation show greater visual alertness than babies comforted in other ways. It's during these periods of quiet alertness that babies do their best learning, when they can most effectively absorb information about the world around them. [. . . . ] As one of a baby's most mature senses, the vestibular system provides a fast track into her developing brain. It doesn't take long for most parents to discover the power of this hidden sense, but isn't it nice to know that all that rocking, jiggling and carrying is not only soothing to your baby, it is actually quite good for her emerging mind?"

In weightless space, infants won't need to depend on spinning chairs, accommodating researchers or attendant parents to enjoy the varieties of vestibular stimulation. Through their own efforts, babies, even weightless newborns, will be able to self-generate all the vestibular stimulation that they will be able to tolerate. Given this prospect, living in weightless space will entail neuro-evolutionary developments.

Evolutionary change does not necessarily involve genetic change, although human genes are sensitive to gravity (see quote from Dr. James Pawelczyk, below). The changes that weightlessness will produce in brains will be due to the peculiar way in which brains develop. Brains manage their cells in a unique way. The cells of most tissues reproduce, replenishing the tissues. But brain cells, or neurons, don’t reproduce (though brain tissue can be partially replenished, as described at the end of this page). Like other cells, however, neurons do die. And yet, with its neuronal population shrinking as cells die, a human brain grows dramatically in weight and volume during its early years. Brain researchers account for this paradox by pointing out that, although a brain has fewer cells as time passes, the cells that remain continue to gain mass by sprouting rich networks of connections to their neighbors.

A brain cell has three main parts:

• The axon is a branching stem that reaches out to and establishes contact with other neurons.

• Dendrites are fibers that receive incoming signals from other neurons.

• Synapses are the junctions at which dendrites and axons meet and through which neurons exchange their chemical signals.

The general course of brain development is well understood. A baby's brain grows quickly in utero as new brain cells develop, and after birth the brain continues to grow. but not by creating more cells. Once its bearer is born, a brain grows by creating more connections among its cells. The density of synapses in brain tissue peaks in humans between the ages of three and six, then tapers off, by about 50 percent, to adult levels by late adolescence. A typical human brain reaches 95 percent of its adult volume by the age of five.

This pattern occurs because the infant brain overproduces synapses, then selectively prunes the excess ones. A New York Times review of brain research (6/24/86) tapped an artistic metaphor to describe the process: "Nature is like a sculptor using two methods. The sculptor first builds a framework and progressively adds plaster to it, producing a rough shape that approximates what he wants. Then he chips away at it until the definitive form appears." Ongoing research will clarify the details of the process, but the general pattern of an overproduction of synapses early in childhood followed by a pruning of underused ones is well documented.

The synapses that survive the "chipping" to compose the "definitive form" of the adult brain represent neurological pathways selected and maintained by the environment. How environments select the synapses that survive has been studied extensively through a simple methodology. Since the 1970s researchers have been comparing brains from subjects—rats—that are raised in experimental environments with the brains of rats raised in control environments. In the classic experiment of this type, some rats lead a privileged life, growing up in a spacious cage filled with toys and littermates. Researchers typically call this the enriched environment condition. The control subjects endure lives of privation, growing up in solitary confinement in barren, cramped cages. This is the impoverished environment condition. When the brains of adult rats from the two environments are compared, those that develop in the enriched condition weigh significantly more than those that develop in the impoverished condition. The weight difference is due to a difference in synaptic density. Researcher William T. Greenough, generalizes from these findings: "[The] results suggest the number of synapses per neuron in a variety of brain regions is determined to a significant extent by the circumstances under which the organism develops. We speculate that these changes are involved in storing information arising from experience." (quoted in Richard Restak's The infant Mind.)

Greenough's speculation suggests factors that influence synaptic retention, but it is general enough to beg a few questions: Does a particular type of experience disproportionately influence synaptic retention? Or, is intensity the determining factor? That is, what characteristics of environments specifically account for their ability to enrich or impoverish?

Move It Or Lose It—Neuroplasticity And Environmental Enrichment Effects

A typical response to this line of questioning comes from neurologist Marian Cleeves Diamond. She is straightforward about operationalizing enrichment: "In essence, an enriched environment is one which introduces more stimulation to the body’s surface receptors than does an impoverished one, whether it be for rats or human beings." This explanation, from Diamond’s review of developmental neurology, Enriching Heredity, typifies the conclusions of scientists working in this field, because it gives primacy to sensory input. But sensory input is only half the story. Motor activity—muscle output—plays as great a role in neurological enrichment as does sensory input.

This point is underscored by a variant of the standard enriched/impoverished experiment. The variant demonstrates that a developing brain has to move a body if it is to preserve an enriched neural infrastructure. Passively receiving sensory input is not enough. In the variant experiment, "observer" condition rats are raised singly in small cages that are fixed in place inside a large enriched cage. In terms of brain weight, rats free to roam in the enriched cage outperform the confined observers significantly. Researchers who have published the results of such experiments report that, "Although the observer condition rats shared the sights, sounds, and smells of their enriched condition littermates and had some contact with them through their wire-mesh cage walls, the observer condition brain weight measures differed significantly from those of the enriched condition but not from those of impoverished condition rats."

In other words, sharing the sights, sounds, and smells of their free-ranging cohorts does observers no more good than had they been subjected to the solitude, isolation, and confinement of impoverished cages. From their results the researchers conclude, "It appears that the necessary and sufficient condition for the production of enrichment effects is active interaction with varied inanimate stimulus objects." (Both quotes are from Ferchmin, P. A.; Bennett, Edward L.; and Rosenzweig, Mark R., “Direct Contact with Enriched Environment is Required to Alter Cerebral Weights in Rats,” Journal of Comparative and Physiological Psychology, Vol. 88, No. 1, pp. 360-367.) Richness of sensory input alone does not ensure generalized neurological enrichment, it turns out. Interaction with the environment—movement, that is, which produces sensory feedback—is required.

More evidence: Using a different experimental approach, psychologist Richard Held in the 1960s ran a series of experiments in which he upset the normal correlation between sensory inputs and motor outputs, with telling results. In one case, human subjects practiced strolling a winding path while wearing goggles that distorted their vision. Subjects in a second group wore the goggles while being conveyed down the path in a wheelchair. Those who walked—those who engaged the environment actively and received sensory feedback from their self-initiated movements—subsequently scored higher on tests of visually guided tasks than did those who were conveyed passively. More manipulative experiments with animals produced similar findings. Accounts of Held's canonical work can be found in any general psychology textbook.

Held proposed that the exercise of "sensorimotor feedback loops" in the brains of the active subjects helped produce their higher test scores. An enriching environment’s "richness" seems to be a measure of the complexity and abundance of the sensorimotor feedback loops that the environment exercises. The more complex the input-output feedback relationships that a brain has to manage—the more synapses regularly exercised, it would seem—the more enriched and long-lived will be that brain's synaptic network. The point is that physical output and sensory input both must be present to produce generalized enrichment effects.

Sensorimotor feedback
drives the complexity
of neural circuitry, and
weightlessness expands
the potential complexity
of sensorimotor feedback.

But of the two activities, physical output should be considered the primary influence. Our bodily movements continually alter what we see, hear, and touch, as well as our vestibular and proprioceptive experiences. For this reason, motor activity, not sensory input, should be considered the sine qua non of neurological enrichment. A brain that develops while it receives input passively, or with only a small capacity to respond with movement, will forego most of its potential for enrichment, as "observer"-condition subjects show.

In The Descent of the Child, Elaine Morgan quotes John E. Eisenberg's The Mammalian Radiations, in which Eisenberg compares modes of locomotion among mammals along with their corresponding brain sizes. He summarizes,

"One will note that complex locomotor patterns involving movement in three different directions, such as arboreality or aquatic adaptations, are strongly associated with high encephalisation quotients, whereas movement in essentially two dimensions generally is associated with a lower encephalisation quotient."

Morgan comments,

"This would explain why all primates, being originally arboreal, have slightly bigger brains than most non-primate species. This correlation would lead us to expect that an arboreal primate descending to the two-dimensional world of the savannah would have lesser needs in respect of brain size. But locomotion in water—swimming and diving—is three-dimensional and, other things being equal, it would tend to lead to higher encephalisation."

If such reasoning is sound, then one would expect a correlation between the gymnastics evoked by weightlessness and unprecedentedly high levels of encephalisation. Native extraterrestrials should bear brains wildly hypertrophied by terrestrial standards, because they will spend their entire lives moving in three dimensions. (These observations similarly throw light on the anomalously enriched brains of cetacea—whales and dolphins—which also navigate in a three-dimensional world.)

(WARNING! Digression: The binge-purge strategy characterizes not only the development of brains, but also the evolution of life generally. Episodes of mass speciation have alternated with episodes of mass extinction. During the past several hundred thousand years, cycles of glaciation have intensified evolutionary competition, and the cycles have driven evolution—while preserving Gaia's climatological support systems—toward a pinnacle of planetary adaptedness in the form of Homo sapiens. Humankind spans all climates; we are the most geographically dispersed species on Earth, having adapted to all climates by engineering locally hospitable mini-climates. This is the process of urbanization, and its extensions now include the weightless International Space Station. Glaciation cycles function as genetic filters—stress tests—that drive the Earth's gene pool toward climatological adaptability—an adaptability that ultimately takes the form of humankind’s ability to engineer industrial technologies that function as an exo-body for the species—and allow it to occupy climates as hostile as that of outer space. A readable account that places this genomic plasticity in the larger context of Earth's ontogeny is Peter Ward and Donald Brownlee, The Life and Death of Planet Earth. See also Gould and Eldredge's theory of Punctuated Equilibrium.)

Neuroplasticity And The Enrichments Of Intrauterine Weightlessness

Evidence from prenatal research corroborates the emphasis on movement as the primary factor in producing enrichment effects. Fetuses cavort in their amniotic capsules like astronauts, and their gyrations apparently pay off neurologically. In The infant Mind, Richard Restak notes that ultrasound imaging reveals two predictable patterns of intrauterine movement:

"In the first movement pattern, the head is flexed backward and turned to one side. This is accompanied by rotation of the trunk and the rest of the body to the same side. In the second movement pattern, leg movements occur, almost as if the fetus were pedaling a stationary bicycle, resulting in a somersault as the legs contact the uterine wall. . . . These movements serve the purpose of stimulating brain development, especially those structures having to do with balance, coordination, and coping with the forces of gravity."

How spinning in three-dimensions in an environment of simulated weightlessness would prepare an organism for the relative flatness of a gravity-bound world is unclear. Newborn brains might be better adapted to a weightless environment, given their prenatal experience. In any case, the womb bears the hallmarks of an enriching environment. It exercises (kinesthetic and tactile, vestibular and proprioceptive) sensorimotor feedback loops.

But once its bearer towels off, a fetal brain's prospects plummet. Birth is a crisis for a developing brain. It demotes the gymnastic fetus to the lowly status of sidelined newborn. No matter how intense its sensory experience, a newborn can’t respond with much movement. No more brain-stimulating gymnastics for the kid in the crib. Infants are essentially beached marine mammals. Outside the watery environment of the womb, they are unable to respond in any gross muscular way to their sensory inputs. They are "observers" in the clinical sense of the rat experiments.

Healthy newborns eventually overcome their immobility by sequentially mastering specialized skills. They squirm, thrash, and in a few months learn to roll over and crawl. Infants will pull themselves up by clutching onto furniture at ten months or so and take a step somewhere around their first birthday. They go on to walk, run, climb, jump, pedal bicycles, and in other ways establish working relationships with gravity.

This programmed sequence segues later in life into rote habits of adulthood. In its mature state in a workplace, a typical middle-class American brain will spend much less time moving in complex ways than it did on the playground in its formative youth. It may be subjected to long hours immobilized at a desk engaging a computer screen through keystrokes and mouse clicks, in a bucket seat engaging a drivetrain through slight arm and foot movements, and reclining in an overstuffed chair engaging TV fare through buttons on a remote control.

The paralysis of the newborn, the skills acquired in sequence during childhood, and the relative sloth of adulthood collectively must engage and maintain a relatively meager set of sensorimotor feedback loops. Synapse-rich toddlers become brain-damaged adults as they schlep into their senior years the few synapses that survive "the trimming of exuberant collaterals," as some researchers have labeled the synaptic selection process. And in this relatively impoverished state modern urbanites function normally, for the most part, being by adulthood well adapted to the vestibular and proprioceptive impoverishments of urbanity.

In contrast to the strictures just described, an enriching curriculum awaits brains that develop in weightlessness. Not having to spend their first postnatal months beached on the gravitational shore and instead enjoying the freedom to fly, is a prospect that a brain’s "exuberant collaterals" could only welcome for the sake of their own survival.

Bodily Movement Replenishes the Neuronal Population

Moreover, the neural enrichment that weightlessness promises stands to be augmented by the addition of new neurons throughout life. In contrast to the traditional view that no new neurons form after birth, research conducted in the 1990s revealed that brains develop new cells throughout their lives and that bodily movement stimulates the development of these cells. Space brains might become pumped up not only in terms of synaptic density per neuron, but also in terms of the sheer numbers of neurons that they possess.

New cells in adult brains don't arise from the same process as do the cells of other organs. That process, mitosis, involves the division of mature cells. The brain cells that arise after birth develop instead from layers of immature stem cells that are retained deep in the brain from its embryonic days. The cells mature as they migrate out of the immature layers.

The new research is summarized by neurobiologists Gerd Kempermann and Fred H. Gage in the May 1999 Scientific American. The authors conducted their own enriched/impoverished experiments using a variation of the standard methodology. Two groups of mice were raised in standard cages, one with running wheels and one without. "The mice having unlimited access to the wheels made heavy use of the opportunity and ended up with twice as many new nerve cells as their sedentary counterparts did, a figure comparable to that found in mice placed in an enriched environment," the researchers report, confirming the preeminence of bodily exercise in accounting for enrichment effects.

The cover story of the March 26, 2007, Newsweek goes further and reports on the link between exercise and brain growth in human subjects.

"After working out for three months, all the subjects appeared to sprout new neurons; those who gained the most in cardiovascular fitness also grew the most nerve cells. [. . . .] So far, though, for reasons no one really understands, the few studies that have examined stretching, toning, and weight lifting have found little to no effect on cognition."

This observation underscores the connection between complexity of sensorimotor feedback (cardiovascular exercise vs stretching, toning and lifting) and enrichment effects.

So, what’s an enriched brain supposed to do with all that extra gray matter? The Newsweek article suggests the direction in which enrichment carries a brain: "[T]he hippocampus is especially responsive to BDNF's effects, and exercise seems to restore it to a healthier, 'younger' state. 'It's not just a matter of slowing down the aging process,' says Arthur Kramer, a psychologist at the University of Illinois. 'It's a matter of reversing it.'" (BDNF is brain-derived neurotrophic factor, a brain chemical described in the article as "Miracle-Gro for the brain".)

Weightless neurology and psychology, then, would seem to be geared up for juvenilization.

NEXT > Neuroplasticity and Neurological Neoteny

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