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Even hands-free calls had a negative impact on drivers’ brain activity.

Making a left-hand turn at a busy intersection is one of the most hazardous driving manoeuvres. That's because there is so much to observe including the signal light, on-coming traffic and, of course, unpredictable pedestrians and cyclists. What's more, you have to decide when it's safe to go before your time runs out.

Some neuroscientists have warned that talking on a cellphone during one of these challenging turns could be doubly dangerous. And now they have the evidence to prove it.

A new study, using MRI brain scans, shows that attention shifts away from what's happening in the intersection to focus on the cell-phone conversation. And this mental distraction, which increases the risk of having a collision, also occurs with hands-free cellular devices.

"Hands-free is not brain-free," said the lead researcher, Tom Schweizer, director of neuroscience at the Li Ka Shing Knowledge Institute at St. Michael's Hospital in Toronto.

Prior to the current study, the scientists spent more than a year developing a compact driving simulator that could fit inside the MRI machine at Sunnybrook Health Sciences Centre, where the research was conducted.

Test subjects could grip a steering wheel, manipulate a break and gas pedal and watch a projection of a street scene while their brain was being scanned. A functional MRI charts mental activity by following, in real time, the flow of oxygenated blood inside the brain.

The findings are based on an assessment of 16 young adults – average age 26 – who were put through various driving scenarios that included a left-hand turn with no distractions and making the same manoeuvre while engaged in a conversation. To mimic a phone call on a hands-free device, they had to answer true or false to a series of questions, such as: Does a triangle have four sides?

Even without any distractions, the MRI revealed that a left-hand turn at a busy intersection takes a considerable amount of brainpower. The areas governing vision and motor control "are firing at the same time," said Schweizer, "so it is very demanding."

Add a conversation into the mix and "something has to give" said Schweizer. Blood moved away from the visual cortex to the prefrontal cortex – essentially the thinking part of the brain. In that situation, on average, brain activation in the visual cortex of each participant dipped to 36 per cent from 59 per cent, while in the prefrontal cortex it rose to 39 per cent from about 11 per cent, according to the findings published in journal Frontiers in Human Neuroscience.

"Quite possibly, this is most dangerous move you could make on the road," he said. "So the visual cortex is one area of the brain you don't want to shut down."

And yet that is precisely what happens while talking on a cell phone, he noted.

For the next phase of the research project, Schweizer wants to test older drivers in the same scenarios. He expects they will do worse than the younger volunteers because the frontal cortex tends to atrophy, or shrink, with age. So older brains may try to pull even more blood away from the visual cortex to compensate for that shrinkage.