The part of the brain needed to make safe left-hand turns largely shuts off during a cellphone conversation — even if the phone is a hands-free device, a group of Toronto researchers have found.

The results of a brain-imaging study led by Tom A. Schweizer, director of the neuroscience research program at St. Michael's Hospital, may explain statistics that show drivers using hands-free phones are just as likely to be involved in collisions as those using hand-held phones.

"Hands-free isn't brains-free," Schweizer said in an interview Thursday. "You're still distracting the individual. You're still processing information, which could take [brain] resources away from the primary task of driving."

The study was published Thursday online in the journal Frontiers in Human Neuroscience.

Using hand-held cellphones while driving is banned under provincial and territorial laws across Canada, but using hands-free devices is still allowed.

Schweizer said he and his colleagues undertook their study after noticing statistics showing that a large proportion of collisions tend to involve left-hand turns.

That got them interested in determining what areas of the brain were activated during different driving maneouvers.

A good technique for measuring brain activity during different tasks is functional magnetic resonance imaging (fMRI), which measures the blood flow to different areas of the brain in real-time — pointing to the areas that are activated and working the hardest.

Because small, portable fMRI scanners don't yet exist to scan drivers inside their vehicles, Schweizer and his colleagues spent 1 ½ years setting up a high-quality driving simulator inside an fMRI machine at Toronto's Sunnybrook Health Sciences Centre.

They recruited seven women and nine men between the ages of 20 and 30 and scanned their brains while they were inside the driving simulator. The volunteers had to drive straight ahead as well as make right and left turns, with and without oncoming traffic. In some cases, they had to answer true or false questions such as "Does a triangle have four sides?" while driving, to mimic a hands-free cellphone conversation.

Left-hand turns in traffic hard on brain

The scans showed that the amount of brain activity and the number of brain regions required for different types of driving varied greatly.

"Nothing compared to the amount of brain that was needed to pull off the left-hand turn at the busy intersection," Schweizer said. "It was very, very striking."

In particular, difficult left hand turns were very demanding on the part of the brain used to process vision and maintain alertness.

However, those brain areas largely turned off when the drivers were holding a simulated cellphone conversation. Instead, the blood was redirected to the parts of the brain used to process conversations.

"It kind of speaks to the fact that there's only so many brain resources one has," Schweizer said. "They're almost at capacity when making a left turn at a busy intersection. When you throw in a distracting task like talking, something's got to give. And it just so happens that what gives is the visual system, it looks like."

The researchers don't yet know for sure how that affects the driver's vision.

Also, while it appears that hands-free conversations may be as distracting as conversations with hand-held devices, the researchers don't yet know that for sure. Nor are they entirely certain that leads to an increased chance of being in a collision.

They hope to increase the difficulty of the driving task in future experiments to find out. They would also like to repeat the experiment with a group of older adults to see if there are differences linked to aging or greater driving experience.

In the meantime, the paper suggests that those who are concerned about distracted driving should consider how certain activities distract the brain from its focus on driving, rather than putting too much emphasis on whether a driver is physically holding a device.

The study added that vehicle manufacturers "have a responsibility to improve safety" by not installing communication devices in vehicles or by installing systems that deactivate communications if the driver tries to use them while the vehicle is moving. The researchers also suggest that tests of whether someone with a brain impairment or dementia is fit to drive should possibly include specific conditions such as heavy traffic or driving during a conversation with the examiner.

In the future, Schweizer said, the researchers hope their findings will lead to tests that target the areas of the brain involved in different driving scenarios to more objectively gauge whether someone is fit to drive.