90% of a Brain Is a Terrible Thing To Waste

January 1999
by Steven Novella, MD

The average person uses only 10-12% of their brain. Almost everyone has heard this statement of fact in one context or another, and most people, in accordance with human nature, accept this as just another amazing but true pronouncement of science without too much scrutiny. References to this “fact” are numerous in the popular culture, from ads to movies.

The appeal of this idea is clear. If we humans only use a small percentage of our brains, then all of us possess vast untapped potential, waiting for us to use. What incredible and mysterious abilities might be hiding in the supposed unused 90% of our brains? New-agers have capitalized on this false idea as a justification for belief in ESP or other supernatural mental powers.

brainThe history of the belief is more obscure. It is not clear exactly where the 10% figure came from, but it is about 100 years old. At no point did neuroscientists ever believe or even speculate that humans used such a small fraction of their brains. About the same time the 10% figure first appears, however, the brain was being mapped for the first time, with specific neurological and mental functions being localized to specific structures within the brain. At one point it was noted that about 10% of the human brain had been mapped out in this fashion, and perhaps this statement was misinterpreted to mean that the other 90% had no mundane function.

The evidence against this belief, regardless of its origin or psychological appeal, is conclusive. First, in the past hundred years the brain has been thoroughly mapped out. One classic technique for brain mapping was to carefully examine patients who had suffered strokes, then, upon there death, examine their brain to see which structure had been damaged. If the patient could not speak, for example, and on autopsy it was discovered that his left temporal lobe was injured by the stroke, then the left temporal lobe was believed to be the center for language in the brain. Other techniques, including animal studies and later electroencephalography (brain wave analysis), blood flow studies, and anatomical imaging were used. Today the most sensitive technique is functional MRI (magnetic resonance imaging). With this technique the metabolic activity of the brain can be imaged from moment to moment. When a subject is asked to perform a mathematical calculation, for example, the fMRI image will show the calculation center of the brain light up with activity.

Today the entire brain is mapped in extensive detail, and a specific function has been found for each part of the brain. Most brain functions are lateralized, meaning that they exist on only one side of the brain. The notable exception to this is the frontal lobes, which possess many redundant structures. Neuroanatomy is a highly advanced discipline, to the extent that the complex connections within the brain, between its various structures, have also been mapped out in detail. For this reason clinical neurologists can often localize a lesion within the brain with precision based solely upon a patient’s deficits and symptoms. Such localization can then be confirmed with detailed imaging, such as with MRI.

Experience with numerous patients over the past century has demonstrated that if any part of the brain becomes damaged a specific deficit will be produced. Sometimes even a tiny lesion, if it occurs in a vital structure, may produce severe deficits. Small lesions may occur in non-vital locations and not produce noticeable symptoms, but such lesions do affect the overall functioning of the brain. Detailed examination of the higher cognitive functions can demonstrate subtle deficits from these otherwise hidden lesions. Also, if many of these lesions occur, then cognition can be impaired to the point of producing a severe dementia. If 90% of the brain were damaged, any 90%, a person would be in a comatose state, unable to muster the brain power even to produce consciousness.

What if brain cells were destroyed in a diffuse manner, so that the structures of the brain were all preserved but the overall number of cells were reduced? This type of damage, which is seen is certain degenerative disease states, such as Alzheimer’s disease, also produces dramatic decreases in brain function, even when only 10-20% of brain cells are lost. Patient’s with these diseases will typically lose all higher neurological function when 50% of their brain cells are lost in this manner, and don’t survive long enough to lose 90%.

From a physiological perspective, the brain certainly acts as if all or most of it is functioning, even in everyday operation. The brain is a hungry organ, comprising about 2% of total body mass but consuming 20% of the oxygen and glucose used by the body. Modern techniques to measure the blood flow to each part of the brain, the consumption of glucose, and the electrical activity of the brain, demonstrate that the entire brain has a certain baseline metabolic rate in the quiet awake state. When specific mental tasks are undertaken certain parts of the brain will kick into high gear and increase their metabolic functioning.

From an evolutionary point of view, the concept also poses severe conceptual problems. Why, for example, would a species evolve a large, hungry organ and then only use 10% of its capacity. The large human brain also comes at a high cost, primarily increased difficulty in delivery. This problem led to shorter gestations, meaning that humans are born earlier and more helpless than would otherwise be necessary. It also led to changes in the female pelvis with a consequent decrease in the efficiency of female bipedalism. A large brain could not be selected for by evolutionary forces, unless these disadvantages were more than outweighed by specific survival benefits. Certainly, evolution would not select for only a 10% efficiency in such an expensive and vital organ.

By multiple independent lines of reasoning it is clear that humans typically use most of their brain for normal functioning. The 10% fallacy, however, seems to be deeply ingrained in the culture and is likely to persist even beyond the publication of this humble expose.