The "10% (of our) Brain" Myth?

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OK by now most, if not all of you have heard of the "we only use 10% of our brain" saying. Some people think it means that we only use a small part of our brain. That is falsified by fMRIs which show we do use our entire brain.

OTOH I have always heard it in the fashion that it is true that we use every region of our brain we only use it to 10% of its capability. Clearly you can also see the issue with that-> how do we know? That was the first question I asked, followed by who did the study and what journal is it in.

I never understood the first argument. and the second argument is only as good as its evidence, which is missing.

The point? The next time you hear someone say that we only use 10% of our brain ask them what they mean by that and how was it determined. You will either learn something or be entertained.