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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.

Just as we might predict the morphology, or general bodily shape, of native extraterrestrials, given the impoverishing effects of weightlessness on bone and muscle tissue, so might we be able to predict the general psychology of native extraterrestrials, given the enriching effects of weightlessness on brain tissue.

Evolutionary change does not necessarily involve genetic mutations, although human genes are sensitive to gravity (see quote, below). The evolutionary changes that weightlessness will produce in brains will be due to the peculiar way in which brains develop, which weightlessness will exploit and exaggerate once our descendants live in space full time. Brains manage their constituent cells in a way unlike other tissues. Most tissues replenish themselves because their cells reproduce. But the cells in a brain steadily decline in number during the brain's lifetime. Unlike other types of cells, brain cells, or neurons, don’t reproduce, but, like other cells, they do die. And yet, with its neuronal population steadily declining, 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 grow 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.

A baby's brain grows quickly in utero, and after birth it 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. The human brain reaches about 95 percent of its adult volume typically by the age of five.

The general course of brain development responsible for these results is well understood. The infant brain overproduces synapses and 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 doubtlessly 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 are veterans of a natural selection process. They represent neurological pathways selected and maintained by the environment. How environments direct synaptic survival has been studied for the most part through a simple methodology. Since the 1970s researchers have been comparing brains from experimental subjects—rats—that are raised in particular environments with the brains of rats raised in very different 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 other rats 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 grew up in the enriched condition weigh significantly more than those that grew up 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 "Infant Mind" by Richard Restak, Doubleday, 1986.)

(WARNING! Digression: This binge-purge strategy characterizes not only the development of individual brains, but also the evolution of terrestrial life. Over the span of geological time, 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 retaining in the biosphere the climatological support systems that Gaia depends on—toward a pinnacle of biological 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 inventions now include the weightless International Space Station. Glaciation cycles, in other words, function as genetic filters—stress tests—that drive the Earth's gene pool toward climatological adaptability—ultimately taking 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".)

Greenough's speculation throws some light on the factors that influence synaptic retention, but it is general enough to beg a few questions: Which experiences tend to preserve the most synapses? Is the general intensity of experience the determining factor? Or does a particular type of experience disproportionately influence synaptic retention? In other words, what characteristics of enriching environments specifically account for their ability to enrich?

A typical response to the last question 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" (Free Press, 1988), typifies the conclusions of scientists writing on this topic, because it gives primacy to sensory input. But sensory input is only half the story. Motor activity—muscle output—has as great a role to play in neurological enrichment as does sensory input.

This point is underscored by a variant of the standard enriched/impoverished experiment. The variant makes clear that a developing brain has to move a body if it is to preserve an enriched neural infrastructure. Stimulation 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 would 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. Held's canonical work can be found described 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 environment’s "richness" might be a measure of the complexity and abundance of these loops. 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 communications network. The point is that physical output and sensory input both must be present to produce enrichment effects.

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

But of the two processes, physical activity should be considered the primary influence. Our bodily movements continually alter what we see, hear, and touch. 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 demonstrate.

Evidence from prenatal research corroborates the emphasis on movement as the primary factor in brain enrichment. Fetuses cavort in their amniotic capsules like skylarking astronauts, and their gyrations apparently stimulate brain growth. In "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 early spinning in three-dimensions in an environment of simulated weightlessness would prepare an organism for the relative flatness the gravity-bound world is unclear. Newborn brains might be better adapted to a weightless environment, given their prenatal experience. In any event, the womb bears the hallmarks of an enriching environment. It exercises (kinesthetic and tactile) 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 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 a 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.

Normal healthy terrestrial newborns eventually overcome their immobility by mastering a sequence of specialized skills. They will 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. Typically, they go on to walk, run, climb, jump, pedal bicycles, and in other ways establish working relationships with gravity.

This programmed sequence of behaviors segues later in life into rote habits of adulthood. In its mature state a typical middle-class American brain will spend much less time engaging complex muscular patterns in the office than it did in its formative youth on the playground. 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 development, and the repetitive 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 urban adults function normally, for the most part, being by adulthood relatively well adapted to the regimentation of the environment.

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, let alone having the freedom to fly, is a prospect that a brain’s "exuberant collaterals" could only welcome for the sake of their own survival. 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 acquire 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.

But these new brain cells won’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 [running] 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 on 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), specifically, 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".)

The direction of weightless neurology and psychology would seem to be programmed toward juvenilization.

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