As far as misguided sayings go, “You only use 10% of your brain” ranks highly. There’s no way to discern what it means exactly, and most interpretations fall victim to weird fallacies; for example, if the brain is a wholly material structure in an emergent deterministic framework (or even fundamentally deterministic, as may prove to be the underlying case of quantum mechanics), then “you” are your brain and body. Your perceived free will may deceive you into thinking that you control your body, but ultimately all actions you perform are predictable. Therefore, you cannot “use” yourself in any real sense; you merely are yourself. This “usage” of self is illusory.
But let’s set this argument aside for now. Let’s just assume that “you use ___% of your brain” is a phrase that indicates a percentile of brain activity. An upper bound is needed to give fractional activity, so if we assume that activity refers to firing rate, than the upper bound is every neuron in your brain firing at maximum speed. Not only would this result in ludicrous, possibly fatal seizing, but it would also be remarkably discordant since different neurons have different firing rate maxima. Using 100% of your brain, in this situation, would be profoundly awful. Thus, “using only 10% of your brain” is simply safe and healthy, not a sign of unreached potential.
Here’s another possibility; maybe 10% brain usage refers to the percent positive change in the brain’s glucose consumption based on an arbitrary standard, like baseline awake-state alpha rhythm-level glucose consumption. Already some issues arise here because if we rely on an arbitrary standard, 10% brain usage depends on brain state. In addition, in order for this to make sense, we still need an upper bound. There are simply biological limits to how much glucose a neuron can process within a given time frame, so we can use those as our upper bound. However, as is the case with firing rate, a brain consuming 100% of possible glucose would be utterly dysfunctional. For a small example, you would be simultaneously trying to sit and stand. For a larger example, your prefrontal cortex would literally be overwhelmed by trying to think about everything you possibly could at once. It such a model, you should be thanking goodness that you only use 10% of your brain. And furthermore, at any given moment, you would be using any number of percentiles below 100%, not a consistent 10%.
Since potential’s been brought up, maybe that’s what the saying refers to. In fact, a common variation on “You only use 10% of your brain” is “You only use 10% of your brain’s full potential“. The recent film, Limitless, attempts to wrangle with this concept, of course on the basis of total logical shenanigans. Whatever do we mean when we wish to remove the brain’s limits, as the film title suggests? Should our brain spontaneously stimulate growth of totally new neural regions to allow itself to be powered via photosynthesis? And if there is no limit to its capabilities, how can there be a fractional standard by which to measure brain usage?
Maybe what people mean is that we only use 10% of the brain’s full potential in terms of a theoretical plastic Hebbian maximum. That is, every circuit in your brain that facilitates some task does so optimally. However, given that brain plasticity is stimulus-dependent, one must perform the skills they wish to become optimal at. Given time constraints, however, it would certainly be difficult to become fully proficient at every possibly skill that exists. The show Dollhouse suggests in its “doll architecture” the artificial imposition of a Hebbian framework wherein all skills can be imposed into an already optimized system. However, this is probably impossible for a) reasons; the brain is very unlikely to have enough space within the skull to accommodate such elaborated neural clusters and b) an immediate transformation of long-term potentiation and other lengthy metabotropic processes that give rise to Hebbian learning is, quite frankly, implausible if not outright impossible (unless you’re in a simulated computer environ where variables can be changed hither-thither).
The fact of the matter is, we use just about all of our brain just about all of the time. In fact, when you don’t call upon certain populations of neurons for a enough time, they tend to die or are “reassigned” to work in a nearby network that is getting used so they aren’t wasting energy– the “use it or lose it” principle (there are notable exceptions, but that’s a separate discussion). Even while you sleep, your brain is doing scads of things: maintaining autonomic processes, keeping your limbs paralyzed, consolidating memories, and so on.
So, the next time you reassure yourself about a personal failure by thinking to yourself, “Well, I was only using 10% of my brain,” remember: it was really more like 100%.