Is it really true humans only use 10% of their brains?

Myth. There's two ways you can interpret this myth.

First, that it's a statement of how active our brain is. In this case, we use all of our brain, just not all at once. A brain is a lot like an integrated circuit. If everything were active all at once, there wouldn't be a coherent signal. Signals must follow a processing flow which means that parts of the brain will be quietly waiting input (you don't want to activate all your muscle routines while you're trying to write something, only your writing muscle routines).

The second interpretation is that we can only consciously access 10% of our brain. Since we don't know the extent of "where" our consciousness is in the brain (or what kind of activity it consists of), and therefore can't quantify "how much" of our brain we have conscious access to, such a statement is nonsensical.

 

There's no part of the brain that is quietly waiting input. All areas of the brain are active all the time. The brain has organized itself so that it keeps the global attractor that drives the alpha-theta rhythms intrahemispherically in a state whereby the large variance of individual neurons are kept just barely subthreshold. Typically, it's just a few neurons here and there that are pushed over the edge. Most of the time, this does nothing, but cross a certain threshold, and the entire cortex converges into a burst state.

The brain is not anything like an integrated circuit, Pythagorean, you should know this. In the cortex, everything is active all at once, this is what creates the basal chaotic states that allow sensory input to drive the cortex into learned attractor states.

It's just myth/folklore. The entirety of your brain is active all the time. If it weren't your neurons would die. If you're talking psychologically about how much of one's brain power they utilize, etc., that a different story. But the brain uses just as much energy to make you a dumb ass as it does to make you a genius. And it does it 24/7.

 

It's a rather simple point I'm making; it doesn't need to be complicated with chaotic attractors. There are quiet periods for different circuits components (we use the word "quiescent" more often in the literature). If all of your brain was synchronized in activity, you'd essentially be having a brain-wide epileptic seizure. Muscles that you didn't want to move would move. Neuropeptides (hormones) that you didn't want being released into your bloodstream would be released into your bloodstream.

If we only had active states, we wouldn't be able to have bistability. Why would we treat neurons on binary if they didn't have two interesting states (one of them being quiescent). You can see many examples of systems with quiescent circuitry in nature.

http://www.sciencedirect.com/science/article/pii/S0006349590825911

http://link.springer.com/article/10.1007/BF00198776

http://www.sciencedirect.com/science/article/pii/0014488665900774