In a sombre and authoritative academic tone, Morgan Freeman's latest movie character delivers the following line: "It is estimated most human beings only use 10 per cent of the brain's capacity. Imagine if we could access 100 per cent. Interesting things begin to happen".
As a conceit for the director Luc Besson's new sci-fi thriller Lucy, this often-quoted idea has obvious Hollywood potential. It also drove the plot of the 2011 thriller Limitless, starring Bradley Cooper. But according to leading neuroscientists, the idea that we only use a fraction of our brain's computing power is nothing more than an urban myth.
Lucy, which is released next month, stars Scarlett Johansson as a woman who is kidnapped and implanted with a drug that unleashes her untapped brainpower, allowing her to control time, execute bad guys with worrying ease, and deliver some vicious beatings.
But according to Barbara Sahakian, professor of clinical neuropsychology at the University of Cambridge, the idea that we only use a small percentage of our brain "doesn't make any sense".
"It's impossible to work out how much of our brain we are using quantitatively. However, it is definitely much more than 10 per cent," Professor Sahakian said.
The 10 per cent figure, she explains, is purely "artificial" and was first widely circulated in Dale Carnegie's 1936 best-selling book, How to Win Friends and Influence People. It is thought that Carnegie simply made up a figure, to substantiate a point in his book.
The erroneous percentage could also come from a misunderstanding about how most of our brain cells work. According to neuroscientists, 90 per cent of the cells in our brains are support cells, called glial cells, which provide nutrients to the other 10 per cent, which are the neurons. Neurons are the cells which produce thoughts. In short, they are our grey matter.
Professor Sahakian does agree that we don't always use our brains to their full potential, however: "Most of the time we are operating far below our maximum brain capacity due to various factors, including tiredness." She points to a recent study of school-age children which found "their maths and reading improved with exercise".
She added: "It is not just the brain's productivity improving; the brain is also doing better."
Sahakian also explains how so-called "smart drugs" could play an important role in the future. She said, "Currently, we can enhance our cognition with smart drugs, we can enhance our alertness, attention, memory and executive functions such as planning and problem solving. In the future, we will have drugs that can produce even greater enhancements with no, or low, side effects. It is highly likely that at that time, smart drugs will be in common use."
The "10 per cent" myth is apparently still widely held among the American public, with a survey last year by the Michael J Fox Foundation for Parkinson's Research finding that 65 per cent of Americans believe it to be true - 5 per cent more than believe in evolution.
According to Barry Gordon, a neurologist at John Hopkins University in Baltimore, people like to believe the 10 per cent myth so they can blame their shortcomings on supposed useless parts of their own brain.
