We use only 10 percent of our brains. The notion that our brains are not running at full speed simply doesn't hold up. 'Numerous types of brain imaging studies show that no area of the brain is completely silent or inactive. Detailed probing of the brain has failed to identify the 'nonfunctioning' 90 percent,' Carroll and Rachel Vreeman, an assistant professor of pediatrics at the Indiana University School of Medicine, write in the British Medical Journal study. Carroll says the notion may go as far back as the snake-oil salesmen of the early 20th century, who used the myth to sell a tonic that would increase brainpower.