Using a cellphone — even with a hands-free device — may distract drivers because the brain cannot handle both tasks, U.S. researchers said.

Imaging tests show the brain directs its resources to either visual input or auditory input, but cannot fully activate both at the same time, the team at Johns Hopkins University in Baltimore found.

“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,” he added in a statement.

“When attention is deployed to one modality — say, in this case, talking on a cell phone — it necessarily extracts a cost on another modality — in this case, the visual task of driving.”

Writing in the Journal of Neuroscience, Yantis and colleagues said they tested people 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.

“It’s as if the participants were changing the volume on visual input and auditory input depending on where they were supposed to be directing attention,” Yantis said.

This is like driving and trying to talk on a cellphone. “You are sharing attention between vision and audition and doing the best you can,” Yantis said.

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