It is often stated that humans only use 10% of their brain capacity. This is nonsense. If the brain evolved in response to need, why would it then turn off and idle? Surely the world of today is more demanding of the brain than past eras, so if anything all parts of the brain should be engaged, and they are.
Humans map the brain as best they can, documenting reactions noticed during brain surgery and capacities lost as a result of brain damage. They thus know some of the jobs the brain performs, but are puzzled by all that gray matter that seems to have no function. Then there are the cases of remarkable performance where the human brain seems to be virtually absent, as in the cases where an encephalitic infant develops into an adult with apparently normal capacity. Part of the human brain maintains the body functions, as is evident when brain stem injury occurs above the nerves to the heart and other organs. Life stops. But these parts of the brain are older, at the base of the brain, and do not involve the larger and newer sections of the brain to any degree. Humans are also aware that they can live quite well with only half a brain, right or left side, as long as a complete half remains. As with many vital organs, evolution favored the specimen who could survive the loss of one. Thus dual eyes, dual ears, dual kidneys, dual lungs, dual limbs, dual testes and ovaries, and dual brains. While whole, they both function, and thus both halves of the brain are hard at work as cooperative team members, communicating via the connective mass between them.
Humans are puzzled at the amount of gray matter that seemingly has no function. If a human can survive losing half a brain, and an encephalitic can function with a minimal brain, then is not there excess brain capacity? This assumption is based on the apparent normalcy of humans functioning with diminished brain size. The humans walk, talk, laugh at jokes, remember to brush their teeth - apparently normal. However, as most consider it to be amazing that these individuals are not in a coma, they seldom move past astonishment to check for full capacity. A woman with only one ovary still ovulates every month, as each ovary has more than enough eggs, menopause occurring due to timing out rather than exhaustion. A single lung or kidney maintains the body under normal circumstances, but under duress a lack of capacity manifests. Just so, where diminished brain size allows the individual to learn and learn well those routines called upon regularly, when asked to stretch the lack of capacity shows up.
Each complex concept is built from many mental building blocks, and each of these building blocks likewise is composed of many parts. Children piece these building blocks together, bit by bit, piece by piece, and eventually get to the point where they can structure abstract concepts. The concept of gravity is grasped in school because the babe dropped food bits from its high chair in fascination as they plopped and bounced on the floor, and the toddler watched birds fighting gravity with their beating wings but found that flailing arms did not create a lift, and the child built towers and arches with building blocks and found the arch must support the weight above or else. All these concepts built into an abstract concept of gravity.
Diminished brain capacity allows the afflicted person to laugh when others laugh as laughter is contagious, especially when one wishes to belong. Do not small children laugh along while not understanding the joke? Diminished capacity does not allow one to create a joke beyond the slap- stick, nor does it allow one to build an abstract concept where one has not already been constructed. Thus, the brain damaged can continue old functions where the connections and structures have not been lost, but time stops for them where new abstractions must be constructed. Too many washed out bridges, so the destination is never reached. Abstract concepts, intuition, long range planning, adaptability - this is what all that uncharted gray matter is supporting, and it is not idle.