|"The 'hidden nine-tenths' of your mental strength lies buried... discover, release and use it to gain new success, personal happiness—a fuller, richer life."|
| ||- advertisement for The Magic Power of Your Mind, W.B. Germain, 1956|
|"They say you only use 10% of it."|
| ||- advertisement for database software, 1999|
|"You only use 11% of its potential."|
| ||- advertisement for digital TV, 1999|
|"It's been said that we use a mere 10% of our brain capacity."|
| ||- advertisement for an airline, 1999|
Where Did the 10% Statement Begin?
The origin of the belief that we use only a small part of our brain is unclear. Perhaps the belief is derived from debates during the early 1800s between those who believed that brain function could be localized to particular regions of the brain and those who believed that the brain acted as a whole. These debates centered around Franz Joseph Gall (1757-1828) and Johann Spurzheim (1776-1832) who developed the field of phernology : the idea that specific human behaviors and characteristics could be deduced by the pattern and size of bumps on the skull. Not everyone agreed with Gall and Spurzheim. Marie-Jean-Pierre Flourens (1794-1867), an outspoken critic of phrenology, believed that although the cerebral cortex, cerebellum and brainstem had separate functions, each of these areas functioned globally as a whole ("equipotential"). Flourens supported his theories with experiments in which he removed areas of the brain (mostly in pigeons) and showed that behavioral deficits increased with size of the ablation. Although the work of Gustav Fritsch (1838-1927), Eduard Hitzig (1838-1907), Paul Broca (1824-1888) and Karl Wernicke (1848-1904) in the late 1800s provided strong data to counter the theory of equipotentiality, some scientists in the early 1900s appeared to once again favor the notion that the brain acted as a whole.
One prominent researcher who promoted the theories of equipotentiality and "mass action" was Karl Spencer Lashley (1890-1958). Lashley believed that memory was not dependent on any specific portion of the cerebral cortex and that the loss of memory was proportional to the amount of cerebral cortex that was removed. His experiments showed that the ability of rats to solve simple tasks, such as mazes and visual discrimination tests, were unaffected by large cerebral cortical lesions. As long as a certain amount of cortex remained, the rats appeared normal on the tests he administered. For example, in 1939 Lashley reported that rats could perform visual discriminations with only 2% of the visual thalamocorical pathway intact. He even estimated that this behavior required only 700 neurons. In another experiment in 1935, Lashley found that removal of up to 58% of the cerebral cortex did not affect certain types of learning. It is possible that overinterpretation and exaggeration of these data led to the belief that only a small portion of the brain is used. For example, although Lashley's rats may have been able to perform the simple tasks, they were not tested on other more complicated paradigms. In other words, the brain tissue that was removed may have been used for tasks that Lashley did not test. Moreover, Lashley was interested primarily in the cerebral cortex, not in other areas of the brain. Therefore, these data should not be extrapolated to other parts of the brain.
Several public figures have made reference to the 10% brain use statement. American psychologist William James wrote in 1908: "We are making use of only a small part of our possible mental and physical resources". Some famous people without training in neuroscience, such as physicist Albert Einstein and anthropologist Margaret Mead, are also attributed with statements regarding human use of only a small portion of the brain.
Regardless of its origin, the statement that we use only 10% of our brains has been promoted by the popular media for many years. Indeed, many advertisers have jumped on the statement to sell their products. According to these advertisements, if we buy their products, devices, or programs, we will be able to tap into the brain's unused powers and enrich our lives.
What does it mean to "use only 10% of your brain?" Does this statement imply that only 10% of the brain's neurons is active at any one time? If so, how could this be measured? Does the statement assume that only 10% of the brain is firing action potentials at one time? Even if this was true, the discharge of action potentials is not the only function of neurons. Neurons receive a constant barrage of signals from other neurons that result in postsynaptic potentials. Postsynaptic potentials do not always result in the generation of action potentials. Nevertheless, these neurons, even in the absence of generating action potentials, are active.
Keeping the Brain Quiet
If all neurons of the brain were generating action potentials at the same time, it is highly likely to result in dysfunction. In fact, some neurotransmitters, such as GABA, act to inhibit the activity of neurons and reduce the probability that an action potential will be produced. Massive excitation of neurons in the cerebral cortex may result in seizures such as those that occur during epilepsy. Inhibition of neuronal activity is a normal and important function of the brain. In other words, some areas of the brain keep other areas quiet.
It is also important to keep in mind that neurons are not the only type of brain cell. Although there are an estimated 100 billion neurons in the human brain, there are another ten to fifty times that number of glial cells in the brain. Glial cells do not generate action potentials. Glial cells function to:
- support the brain structurally
- insulate axons
- clean up cellular debris around neurons
- regulate the chemical composition of the extracellular space
Would we behave normally without 90 billion neurons and billions of glial cells? Would we be just fine if 90% of our brains was removed? If the average human brain weighs 1,400 grams (about 3 lb) and 90% of it was removed, that would leave 140 grams (about 0.3 lb) of brain tissue. That's about the size of a sheep's brain. Clinical evidence indicates that damage to even a small area of the brain, such as that caused by a stroke, may have devastating effects. Some neurological disorders (e.g., Parkinson's disease) also affect only specific areas of the brain. Disabilities may arise after damage to far less 90% of any particular brain area. Because removal of small essential brain areas may have severe functional consequences, neurosurgeons must map the brain carefully before removing brain tissue during operations for epilepsy or brain tumors.
In addition to clinical evidence, brain imaging methods appear to refute the 10% brain use statement. For example, positron emission tomography (PET) scans show that much of the brain is active during many different tasks. Often when brain scans are published, they have been manipulated to show relative amounts of brain activity rather than absolute activity. This graphical presentation of the data shows differences in brain activity. Therefore, it may appear that some areas of the brain are inactive when, in fact, they were active, but at a lower level compared to other sites. Brain scans only show activity for the carefully designed isolated tasks being tested, such as memory or visual processing. They do not show activity related to other untested abilities. Imagine the brain is a restaurant kitchen. If you looked in on the kitchen at one time, you may see the chef preparing salad. However, you may not know that the main course is cooking in the oven. Similarly, if you image the brain during a visual task, you will not see the other patterns of activity associated with performing different (simultaneous) tasks.
Evolution and Development Weigh In
From an evolutionary perspective, it is unlikely that a brain that is 90% useless would develop. The brain is an expensive organ to maintain and utilizes a large supply of the body's energy resources. Certainly there are redundant pathways that serve similar functions. This redundancy may be a type of "safety mechanism" should one pathway for a specific function fail. Still, functional brain imaging studies show that all parts of the brain function. Even during sleep, the brain is active. The brain is still being "used"; it is just in a different active state.
From a developmental perspective, the 10% of the brain statement also fails. The adage "use it or lose it" seems to apply to the developing nervous system. During development, many new synapses in the brain are formed. After birth, many synapses are eliminated later on in development. This period of synaptic development and elimination goes on to "fine tune" the wiring of the nervous system. It appears that correct input is required to maintain a synapse. If input to a particular neural system is eliminated, then neurons in this system may not function properly. Nobel prize winners David H. Hubel and Torsten N. Wiesel demonstrated this in the visual system. They showed that complete loss of vision would occur when visual information was eliminated during early development. It seems reasonable to suggest that if 90% of the brain was not used, then many neural pathways would likely degenerate.
Brains are quite adaptable and do have the ability to recover after damage. When a brain is damaged, remaining neural tissue can sometimes take over and compensate for the loss. The ability of the brain to recover lost functions does not indicate that the damaged tissue had no function. Rather, this ability illustrates the brain's capacity to reorganize and rewire itself.
It appears that there is no hidden storehouse of untapped brain power. We use all of our brain.
Lashley, K.S. Studies of cerebral function in learning: XI. The behavior of the rat in latch-box situations, Comp. Psychol. Monogr., 11:1-42, 1935.
Lashley, K.S. The mechanism of vision: XVI. The functioning of small remnant of the visual cortex, J. Comp. Neurol., 70:45-67, 1939.