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Your brain's energy bandwidth is limited, and multitasking is a fool's errand.
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One of the most enduring myths about the brain is that we use only 10 percent of it. Presumably the other 90 percent sits on idle, or perhaps just serves for spare parts. By inference the myth suggests that people can harness unused potential in order to boost their IQ or develop “psychic” skills and other extraordinary abilities.

Many false ideas about the brain persist. For instance, most of what popular culture believes about right-left differences between the brain hemispheres is flat-out wrong. But the notion that we use only a fraction of our brainpower is a doozy. According to Haverford College, two-thirds of the public believe the 10% myth, and according to a study in Frontiers in Psychology, even 47 percent of secondary school teachers take it as true! If schoolteachers haven't got their facts right then what hope is there of setting the public record straight? What accounts for the 10% myth’s persistence despite ample proof that it's wrong?

No one knows for sure, but the particular idea probably got started in the last century when neurology was just developing as a science. For a long time it was known that a motor area controls the opposite side of the body, and that a sensory area sitting on the strip of cerebral cortex just behind it mirrors it. The primary brain areas for hearing and vision were also well known. But there were also many brain parts that we now collectively know as "association areas" given that they perform high–level calculations important for perception, thought, and behavior. In other words they are the basis for a person’s “smarts.”

Take the largest of the association areas, the two frontal lobes that account for a full third of brain tissue inside the skull. Because damage to this large expanse of brain produces no obvious motor or sensory symptoms, medical scientists from decades ago concluded that it served no obvious purpose. Together with the other regions whose functions weren’t apparent they became known as the “silent areas.” (This is an example of circular logic: because we can’t figure out what they are doing, they must not be doing anything.)

Even during sleep all brain areas show activity. The corpus callosum is shown in red.

On its face the conclusion is absurd—how could such a significant percentage of brain tissue do nothing? But even scientists are not immune to idiotic thinking, and history is peppered with bloopers. For example, as late as the 1950s well-respected individuals claimed that the brain dealt with physical movement and reflexes—and nothing more. Another wrong idea we look back on in amazement is that the corpus callosum, the enormous fiber bundle that carries cross-traffic between the right and left hemispheres, served no purpose.

Simply on an anatomical basis the idea that it was inert should have been dismissed, because the number of nerve fibers in it exceeds all those coming in from the senses combined. Yet neurosurgeons working in the 1950s and 1960 who severed these huge connections in an operation meant to control seizures concluded that the corpus callosum had no function.  

To be fair, encounters with split-brain individuals are highly counterintuitive. Logic says you have done something drastic by cutting someone's brains in half. Yet judging by conversation, social interactions, and even the standard neurological exam, they don’t seem impaired at all. The answer to this riddle is that patient’s weren’t tested in the right way. Examiners weren’t looking for the right symptoms. When you actually restrict test input and the patient's response to one hemisphere at a time, then profound symptoms emerge.

How could our predecessors have made such an error? They mistakenly gave prime importance to movement and sensation at the expenses of everything else! These functions account for only a small slice of brain tissue, and so perhaps the 10% was born and has persisted ever since.

Astute readers may ask what prompted such narrow-minded thinking in the first place. In the last century around the time of Freud, a turn in thinking happened that was strongly anti-biological and resisted associating the brain with higher thought. Unbelievable, but true.

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