We only use 10% of our brains. Really? So the extra 90% of our brains are just sitting there, not being used? Why would we have all that extra brain for? Usually this claim is put in the introduction of fancy new self-help materials that are splattered with the prefix "neuro-" preceding every other word. Then the books or articles are offering YOU, the lucky reader, the secret to unlocking all of this incredible unused mental power that every other poor ignorant sucker just leaves dormant in their heads, for the low low price of just $29.95! Wow, amazing! This meme is just nonsensical and untrue, here's why:
First, the brain (obviously) consists of neurons. Neurons are living cells, and they fire (discharge) many times a second for their entire existence. So we are always "using" 100% of our brain, all day every day, even when we are asleep, awake, brushing our teeth, watching a movie, driving, or taking standardized tests. It's not the firing itself of individual neurons that is important, it's the differential firing rates and the overall patterns of communication that allows the brain to perform useful computations (think, feel, perceive, act, etc.).
Second, if there were any part of our brains that we weren't ever using, evolution would be quick to whittle that away because brain tissue is expensive and costly to possess (it is metabolically greedy consuming 20-25% of our oxygen and food). Plus, extra large brains need extra large heads (skulls) for protection, which gives rise to all sorts of unintended problems like dangerous childbirth, difficulty walking, running, leaping, unfused skulls until after birth, etc. Unless the benefits somehow outweighed the costs, evolution is not going to equip us with brains that go 90% unused. This just isn't a plausible idea.
In any case, it's not even clear what is meant by the phrase "we only use 10% of our brains" since it doesn't define what is meant by "use" and it doesn't specify what the 10% is referring to, or how the 10% number is calculated. It's basically just a made-up meme that apparently the self-help/neuro-craze crowd seems to have latched onto and perpetuated for the last few decades.
90% of communication is non-verbal. Again, this meme fails to define what the 90% is referring to, or how it's calculated. Plus it would imply that we could communicate almost perfectly without using any sounds or words whatsoever. Try explaining a complicated physics problem or tell an intricate story without resorting to words. Just limit yourself to the use of non-verbal communication for a day, see how well you can communicate with your family, friends, and colleagues. All this communicating should be easy since 90% of communication is non-verbal, right?
Maybe today this statistic is more true than ever, given the amount of communication we do in the virtual world via texting, Twitter, email, Facebook, etc. But usually the stat is used to refer to interpersonal, face-to-face styles of communication involving body language, facial expressions, clothing and other social ornamentation, and all the many sublte signs of communicating that humans have invented. It's true these are important and interesting, but they aren't 90%.
The highest fidelity human sense, carrying the most information to the brain, is by far the visual system. In fact nearly half of the cortex is devoted to visual processing. This allows us to discern thousands of faces that are only slightly different from each other, see and understand objects even in cluttered environments, track moving objects, move ourselves through the spatial environment, see in 3D, with high detail, and in color. So I won't deny that the visual channel, as opposed to the auditory (verbal) channel, carries much more raw information.
And yet what do we do when we can't talk directly to someone verbally, either face-to-face or over the phone, but still want to communicate with them? We use vision to verbalize our communication. We write our thoughts down, converting verbalizations into symbolic visualizations (writing) and send them to someone via letter, text, chat, tweet, books, graffiti, blogs, hieroglyphics, tattoos, or sky-writing. Then the receiver uses their high fidelity visual system to convert those visualizations into a verbal stream of sounds inside their own head. Almost like magic. So even when we are communicating "non-verbally", it's basically just a verbal-to-visual-back-to-verbal communication chain. It seems 90% or more of communication is actually verbal and not the other way around.