The human brain is a complex organ that remains a mystery unto itself.

And while a common myth that we only use 10 percent of our brain still prevails, valuable insight on the matter proclaims we actually use much more than that.

"We use virtually every part of the brain, and [most of] the brain is active almost all the time," explains neurologist Barry Gordon at Johns Hopkins School of Medicine in Baltimore.

 

"Let's put it this way: the brain represents three percent of the body's weight and uses 20 percent of the body's energy."

That fact makes the idea of someone living with only 10 percent of their brain intact truly mind-boggling. 

But a French man who lives a relatively normal, healthy life is doing just that.

Despite missing 90 percent of his brain, the man isn't mentally disabled. The case has been puzzling scientists for nearly a decade. And though researchers found he had a low IQ of 75, he was working as a civil servant, and was also married with two children.

The man is challenging the way we understand consciousness, which begs the question:

How can someone without the majority of their neurons still be aware of themselves and their surroundings?

Being aware of one's existence remains a confusing subject, with research still fairly thin, but since we do know it's somehow based in the brain, this man's abilities are truly confounding.

At 44 years old, the French man went to the doctor complaining of muscle weakness in his left leg.

Brain scans showed his skull was mostly filled with fluid, however. There was a mere thin outer layer of actual brain tissue, while the internal part was almost completed eroded away. Doctors concluded that hydrocephalus - a condition he had been diagnosed with it as an infant - had slowly destroyed the brain over the course of 30 years.

As a child it was treated with a stent, but because it was removed when he was 14 years old, fluid built up in the brain and resulted in the evident erosion.

His apparent well-being led scientists to question what it actually takes to survive and be aware of the life you're living, as this man certainly was despite his condition.

Researchers have linked consciousness to regions of the brain like the claustrum, a thin sheet of neurons running between major brain regions, or the visual cortex.

But these hypotheses don't correlate with the French man's awareness.

"Any theory of consciousness has to be able to explain why a person like that, who's missing 90 percent of his neurons, still exhibits normal behavior," explains Axel Cleeremans, a cognitive psychologist from the Université Libre de Bruxelles in Belgium.

Axel Cleeremans hypothesizes that consciousness is not something we are born with, but is rather learned, and that such a phenomenon can occur in different brain regions, as its location can be flexible.

"Consciousness is the brain's non-conceptual theory about itself, gained through experience - that is learning, interacting with itself, the world, and with other people," he says. 

His hypothesis, called the 'radical plasticity thesis,' is connected with recent research that has found the adult brain is more adaptable than we originally thought - even capable of taking on new roles when injury occurs.

According to Cleeremans, even though the French man's remaining brain was so small, the leftover neurons were still capable of generating a theory about themselves, which explains how the man remained conscious of his actions.

This situation is a reminder that our brains can learn more than we might think, even under grave circumstances.

It also gives us hope that one day we may be able to reverse some of the illnesses caused by neurodegeneration.