The Fallible Mind

Emotion, perception, and other tricks of the brain

Using Just 10% of Your Brain?

Why multitasking is a losing proposition, and our brain energy is limited.

An earlier post debunked the myth that we use only 10% of our brain. According to this tale, 90% of our brain stands idly by, waiting to be recruited for spare capacity. The fantastic logic behind this idea should be apparent: the reverse argument implies that we are poised to tap into extraordinary latent abilities. Neither of these premises is remotely true. Yet hucksters boasting methods “based on neuroscience” promise to unleash hidden potential. What they actually unlock is your wallet.

Nonetheless, two–thirds of the public and nearly half of science teachers believe the 10% myth, which shows how hard it is for facts to crowd out mistaken attitudes. More than 75% of science teachers believe that enriching a child’s environment—with Baby Einstein videos, for instance—strengthens the brain. It is true that growing up locked in closet or otherwise deprived of human contact demonstrably impairs a growing brain; it does not follow that enriching a child’s environment beyond what is typical will boost cognitive development.

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The idea that we don’t utilize all of our brain capacity emerged perhaps in the 19th Century when neurology first burgeoned as a science. In the 1890s William James, the father American psychology, said, “Most of us do not meet our mental potential.” James meant this as a challenge rather than an indictment of insufficient brain usage. Nonetheless, the charge stuck. For a long time also, scientists could assign no function to the great expanse of the frontal lobes or the equally large association areas of parietal cortex. Circular reasoning concluded that because damage to these areas didn’t cause motor or sensory deficits, they therefore didn’t “do anything.” For decades these brain parts were known as “silent areas” because their function remained unknown. Today we appreciate that they underscore executive and integrative functions without which we would hardly be human.

These cortical expanses are crucial to reasoning, planning, deciding, and adapting to circumstances. They lie behind our most abstract reasoning. We usually talk about the brain in terms of computing power, but look what happens when we regard it in terms of energy cost.

Rodent and canine brains consume 5% of total body energy. Monkey brains use 10%. An adult human brain accounts for only 2% of the body’s mass, yet consumes 44% of daily glucose burned, far more than expected for its relative size. Brain size scales with body size. Human brains weigh 1.5 Kg, elephant brains 5 Kg, and whale brains 9 Kg. Yet on a per weight basis, human brains are packed with more neurons than other species. This dense packing is what makes us so smart.

There is a tradeoff between body size and the number of neurons that a primate, including us, can sustain. A 25 Kg ape has to eat 8 hours a day to sustain a brain of 53 billion neurons. The invention of cooking one–and–a half–million years ago gave humans a huge advantage. Cooked food is rendered soft and predigested outside of the body. Our guts more easily absorb its energy. Cooking frees up time and gives us more energy than if we ate raw foodstuffs. As a result we sustain a brain of 86 billion densely packed neurons.

It still turns out that neuronal activity is costly in terms of energy use. Most of the daily calories that the brain burns go toward housekeeping: pumping sodium and potassium ions across membranes to maintain an electrical charge and keep the structure intact.

The brain is an energy hog. It consumes an astounding 3.4 x 1021 ATP molecules per minute (ATP is the coal of the body’s furnace). The mere cost of maintaining resting potentials in all of our neurons means that little energy is left for neuronal spikes—the nerve discharges that actually get things done.

Even given a few percent of firing neurons in a local region, the energy burden of generating spikes over the entire cortex is unsustainable. Far less than 10% is the energy limit we must live with. It is for this reason that selective attention exists—our focus on one thing at a time. It is why multitasking is a fool’s errand: We simply lack the energy to do two things at once, let alone thre or five. When we try to multitask we do each task less well than if we’d given it our full attention.

 Any practical solution to the energy limit demands that we give ourselves more brainpower. But how? Fans of Dr. Who might turn to the Ood, aliens who hold a glowing orb of a supplemental brain in their hand. This might sound silly, but real life has caught up with science fiction. Every day you see hundreds of people wielding supplementary brain that they hold in their hands. We just call them smartphones.

Richard E. Cytowic, M.D., is a neurologist best known for bringing synesthesia back to mainstream science. His latest book is Wednesday Is Indigo Blue.

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