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  • An enduring myth says we use

  • only 10% of our brain.

  • The other 90% standing idly by for spare capacity.

  • Hucksters promised to unlock that hidden potential

  • with methods "based on neuroscience,"

  • but all they really unlock is your wallet.

  • Two thirds of the public

  • and nearly half of science teachers

  • mistakenly believe the 10% myth.

  • In the 1890s, William James,

  • the father of American psychology,

  • said, "Most of us do not meet

  • our mental potential."

  • James meant this as a challenge,

  • not an indictment of scant brain usage.

  • But the misunderstanding stuck.

  • Also, scientists couldn't figure out

  • for a long time

  • the purpose of our massive frontal lobes

  • or broad areas of the parietal lobe.

  • Damage didn't cause motor or sensory deficits,

  • so authorities concluded they didn't do anything.

  • For decades, these parts

  • were called silent areas,

  • their function elusive.

  • We've since learned that they underscore

  • executive and integrative ability,

  • without which, we would hardly be human.

  • They are crucial to abstract reasoning,

  • planning,

  • weighing decisions

  • and flexibly adapting to circumstances.

  • The idea that 9/10 of your brain

  • sits idly by in your skull

  • looks silly when we calculate how the brain uses energy.

  • Rodent and canine brains consume

  • 5% of total body energy.

  • Monkey brains use 10%.

  • An adult human brain,

  • which accounts for only 2% of the body's mass,

  • consumes 20% of daily glucose burned.

  • In children, that figure is 50%,

  • and in infants, 60%.

  • This is far more than expected

  • for their relative brain sizes,

  • which scale in proportion to body size.

  • Human ones weigh 1.5 kilograms,

  • elephant brains 5 kg,

  • and whale brains 9 kg,

  • yet on a per weight basis,

  • humans pack in more neurons

  • than any other species.

  • This dense packing is what makes us so smart.

  • There is a trade-off between body size

  • and the number of neurons of primate,

  • including us, can sustain.

  • A 25 kg ape has to eat 8 hours a day

  • to uphold a brain with 53 billion neurons.

  • The invention of cooking,

  • one and half million years ago,

  • gave us 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 provides more energy

  • than if we ate food stuffs raw

  • and so we can sustain brains

  • with 86 billion densely packed neurons.

  • 40% more than the ape.

  • Here's how it works:

  • Half the calories a brain burns

  • go towards simply keeping the structure intact

  • by pumping sodium and potassium ions

  • across membranes to maintain an electrical charge.

  • To do this, the brain has to be an energy hog.

  • It consumes an astounding 3.4 x 10^21 ATP molecules per minute,

  • ATP being the coal of the body's furnace.

  • The high cost of maintaining resting potentials

  • in all 86 billion neurons

  • means that little energy is left

  • to propel signals down axons and across synapses,

  • the nerve discharges that actually get things done.

  • Even if only a tiny percentage of neurons

  • fired in a given region at any one time,

  • the energy burden of generating spikes

  • over the entire brain

  • would be unsustainable.

  • Here's where energy efficiency comes in.

  • Letting just a small proportion of cells

  • signal at any one time,

  • known as sparse coding,

  • uses the least energy,

  • but carries the most information.

  • Because the small number of signals

  • have thousands of possible paths

  • by which to distribute themselves.

  • A drawback of sparse coding

  • within a huge number of neurons

  • is its cost.

  • Worse, if a big proportion of cells never fire,

  • then they are superfluous

  • and evolution should have jettisoned them long ago.

  • The solution is to find

  • the optimum proportion of cells

  • that the brain can have active at once.

  • For maximum efficiency,

  • between 1% and 16% of cells

  • should be active at any given moment.

  • This is the energy limit

  • we have to live with

  • in order to be conscious at all.

  • The need to conserve resources

  • is the reason most of the brain's operations

  • must happen outside of conciousness.

  • It's why multitasking is a fool's errand.

  • We simply lack the energy to do two things at once

  • let alone three or five.

  • When we try, we do each task less well

  • than if we had given it our full attention.

  • The numbers are against us.

  • Your brain is already smart and powerful.

  • So powerful, that it needs a lot of power

  • to stay powerful.

  • And so smart

  • that it has built in an energy efficiency plan.

  • So don't let a fradulent myth

  • make you guilty about your

  • supposedly lazy brain.

  • Guilt would be a waste of energy.

  • After all this,

  • don't you realize it's dumb to waste

  • mental energy?

  • You have billions of

  • power-hungry neurons to maintain.

  • So hop to it!

An enduring myth says we use

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B2 TED-Ed brain energy proportion body myth

【TED-Ed】What percentage of your brain do you use? - Richard E. Cytowic

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    VoiceTube posted on 2014/03/26
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