As anyone with the temerity to question the existence of psychic powers will soon discover, a cherished rejoinder is the logical fallacy known as the "argument from ignorance." Lacking solid evidence of their own, defenders argue that if it cannot be proved that something is not the case, this somehow counts as evidence that it is true.

In debates with advocates of the paranormal, I frequently encounter an extension of this ploy that rests on the widely quoted, but never supported, assertion that normal people only use 10% of their brains. So, the argument continues, if we don't know what the remainder of the brain is there for, it could be the repository for awesome mental powers that only a few adepts have mastered--this enlightened minority can supposedly tap their latent cerebral potential to accomplish levitation, spoon bending, clairvoyance, precognition, telepathy, psychic healing, and other fantastica scarcely conceivable to mere mortals condemned to subsist on the drudge-like 10%. Of course, the one-tenth figure is itself debatable, but even if it were accurate, it would in no way entail the existence of psychic powers, which must stand or fall on their own demonstrable merits. The 10% myth is so prevalent that I have become curious about its origins and why it persists despite its inherent improbability.

As someone who spends much of his professional life pondering how the brain works, I am quite willing to admit the extent of our ignorance about how this kilo and a half of gray matter manages to produce thoughts, feelings, and behaviours. Nonetheless, I am quite at a loss to understand how my debating adversaries came to know with such pontifical certainty that we normally use only 10% of it. To the best of my knowledge, this alleged fact appears nowhere in the literature of neurophysiology or physiological psychology. On the contrary, it is at variance with much of what we do know about the brain. The "dormant brain" thesis seems to be another of those "factoids" that accumulate a patina of believability through repetition. It is firmly ensconced in the conventional wisdom though no one who has cited it to me has ever been able to state who first said it or what evidence there is in its favour.

Logic alone should give pause to the 10-percenters. When asked if the 10% figure is true, I often respond, "How well do you think you'd continue to function if 90% of your brain were suddenly incapacitated?" Typically dismal estimates implicitly concede the implausibility of the claim. We all know stroke victims who have lost considerably less brain tissue and are severely debilitated.

Furthermore, virtually all educated people now accept that the human brain is the product of millions of years of evolution. Given the conservatism of natural selection, it seems highly unlikely that scarce resources would be squandered to produce and maintain such an underutilized organ. The brain is costly to run, consuming approximately a quarter of the metabolic resources of the resting body. How long would you endure huge hydro bills to heat all ten rooms of your home if you never strayed beyond the kitchen?

The brain has evolved a certain amount of redundancy in its circuitry as a safety precaution but little, if any, of it lies perpetually fallow. The armamentarium of modern neuroscience decisively repudiates this notion. EEGs, CAT-, PET- and MRI- scans, magnetoencephalography, regional cerebral blood flow measures, etc. all show that, even during sleep, there are no silent areas in the brain. Such tranquility would be a sign of gross pathology. We also know from these techniques and from studying the effects of head trauma that the brain is not an undifferentiated mass — distinct functions are distributed throughout the brain. According to the 10% scenario, 90% of each functional area would have to be unused in order not to lose certain functions totally in a 90% dormant brain. We can't rule this out, but it seems implausible in light of animal research where electrodes are inserted directly into the brain to map its microcircuitry.

Henry Ford once said, "Whatever you have, you must use it or lose it." Muscles atrophy from disuse and so, apparently, do brain circuits. My own research and that of many others indicates that neural systems deprived of normal input either fail to develop or deteriorate permanently. If 90% of our brains were really idle, we would expect massive degeneration and no such signs show up in normal people on the various scanners. Of course, the 10% utilization could refer instead to storage capacity, processing speed, or some other index of brain activity (rather than simply to volume) but I know of no way to determine the theoretical limits of such processes in order to estimate the average person's efficiency. Research indicates that it is not lack of storage capacity that hinders performance most; it's the difficulty of retrieving what we've safely stored.

Thus, I must conclude that alleged neural tithing is more likely a metaphor for widespread human longings than a fact of neurology. So why do so many people cling to this ubiquitous dictum of neuro-mythology? The remarkable ability of developing brains to reorganize and recover from brain damage may be one reason. Children have been known to recover a surprisingly normal level of function after loss of an entire cerebral hemisphere to injury or disease. This is far less than 90% of their brains, of course, but dead nerve cells are not replaced. These children still show some deficits, however. Unfortunately, this ability of the remaining hemisphere to assume the additional duties of its devastated mate wanes with age, as a visit to any neurological ward will quickly convince you. Popularized accounts of the these youngsters' recoveries probably fueled the misconception that they never really needed the extra brain tissue in the first place.

This misapprehension was reinforced recently in an otherwise informative TV documentary aired on PBS and the Knowledge Network. It featured the British physician John Lorber and an extraordinary group of young patients. Referred to Lorber because of fairly minor neurological complaints, they were of normal or above-normal intelligence and were coping well, educationally and socially. Astonishingly, CAT-scans revealed that their cerebral hemispheres had been compressed into a slab less than an inch thick by enlargement of the underlying fluid- filled ventricles. This had occurred over an extended period as the fluid dammed up behind constricted outflow channels. The youths' lack of retardation despite this tremendous neural shrinkage led the producers to ask the misleading question that, unfortunately, became the title of the episode: "Is the Brain Really Necessary?"

What Lorber's remarkable cases demonstrate is not, as the documentary suggests, the irrelevance of the brain to our mental lives, but rather the amazing ability of the brain to adjust to massive disruptions, providing they occur slowly enough and early enough in life. We do not know how much of this thinning of the cerebral hemispheres was due to cellular loss and how much to compacting of cells into less than their normal volume. The fact that these patients can get by with reduced brain volume does not imply that they wouldn't have put any additional tissue to good use had it been retained. I also suspect that their degree of normalcy may have been somewhat exaggerated for dramatic effect. Nonetheless, Lorber's cases are an eloquent testimonial to the resilience of the young brain and its ability to reorganize and carry on. Mature brains subjected to more rapid increases in intracranial pressure, due to growing tumours for instance, certainly show much more drastic impairments.

The origins of the Great 10% Myth remain obscure, but I have learned that it has long been a staple of self-improvement courses like those of the Dale Carnegie organization. It was canonized by no less a personage than Albert Einstein who once uttered it as a speculative reply to the constant barrage of questions about the source of his brilliance.

I believe this vision of the vegetative brain may have arisen initially from lay persons' misinterpretations of early neurological research. Early studies showed that large portions of rat cortex could be removed with apparently little disruption in behaviour (later tests found deficits that weren't detectable with earlier methods). Similarly, misinterpretation of certain terms used by comparative neurologists may have compounded the error. With evolutionary advancement, the cerebrum of mammals has enlarged greatly but a progressively smaller proportion of it is concerned with strictly sensory or motor duties. This was demonstrated in the 1930's by electrically stimulating the exposed cortical surface in a variety of species. Because the current was unable to evoke obvious responses from these increasingly large non-sensory and non-motor areas, they were referred to as "silent cortex." As we have seen, they are anything but silent — they are responsible for our most uniquely human characteristics, including language and abstract thought. Areas of maximal activity shift in the brain as we change tasks and vary attention and arousal but there are normally no dormant regions awaiting new assignments.

In the end, I think it boils down, once again, to the comforting nature of most occult and "New Age" beliefs. It would be nice if they were true--death would have no sting and there would be no shortages in life, materially or mentally. We could all be Einsteins, Rockefellers or Uri Gellers if we could just engage that ballast between our ears! This "cerebral spare tire" concept continues to nourish the clientele of "pop psychologists" and their many recycling self-improvement schemes. As a metaphor for the fact that few of us fully exploit our talents, who could deny it? As a spur to hope and a source of solace it's probably done more good than harm, but comfort afforded is not truth implied. As a refuge for occultists seeking the neural basis of the miraculous, the probability is considerably less than ten percent.

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From the Rational Enquirer, Vol 3, No. 2, Oct 89. Barry Beyerstein works at the Brain Behaviour Lab, Dept. of Psychology, SFU.

The publishing was authorized by the author of the original essay.

The original essay is available at http://eslvcr.fireplug.net/bcskeptics/ratenq/Re3.2-Brain10.html

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