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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 phrenology: 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 thalamocortical 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.