BALTIMORE — Using a cell phone — even with a hands-free device — may distract drivers because the brain cannot handle both tasks, according to researchers at Johns Hopkins University. Imaging tests show that the brain directs its resources to either visual input or auditory input, but cannot fully activate both at the same time. “Our research helps explain why talking on a cell phone can impair driving performance, even when the driver is using a hands-free device,” said Steven Yantis, a professor in the Department of Psychological and Brain Sciences who led the study. “Directing attention to listening effectively ‘turns down the volume’ on input to the visual parts of the brain.” Researchers tested individuals aged 19 to 35 by showing them a computer display while they wore headphones playing voices. At the same time, the volunteers’ brains were scanned using functional magnetic resonance imaging. They were told to look for specific numbers, for instance, on a computer screen, while hearing recorded voices saying a stream of numbers. When the volunteers paid attention to visual tasks, the auditory parts of their brain recorded decreased activity, and vice-versa.