A new study suggests drivers making a left-hand turn while talking on a hands-free cellphone is more dangerous than previously thought.

In a small study, researchers put 16 adults between the ages of 20 and 30-years-old to the test.

Using a driving simulator, complete with working foot pedals and a steering wheel, the test subjects manoeuvred through a series of driving tasks that became progressively more difficult over the course of an hour.

All the while, researchers at Toronto's Sunnybrook Hospital mapped their brain activity with an MRI.

What researchers saw in the MRI scans was startling. They found that turning left requires a high level of brain activation. In fact, they say it uses more areas of the brain than turning right or even driving straight.

"When it came to a left-hand turn at a busy intersection, virtually the entire brain lights up, giving us objective evidence it is a very demanding task," said Dr. Tom Schweizer, a researcher at St. Michael's Hospital, who led the study.

That's key, because according to the National Highway Traffic Safety Administration in the U.S., left turn crashes happen more often. More than 22 per cent of the 2.3 million collisions that occurred in the U.S. in 2008 happened when drivers made left turns at intersections compared to 1.2 per cent during right turns. Transport Canada says about 30 per cent of fatalities and 40 per cent of serious injuries in crashes in Canada occur at intersections and left turn crashes are among the most dangerous.

The drivers in the Canadian study were put through the paces -- asked to go straight, and turn left and turn right six times without facing any oncoming traffic as they laid down in the MRI driving simulator. Then they were told to turn left six times as traffic approached. They then repeated the left turns facing oncoming traffic as a voice peppered them with true or false questions such as "does a triangle have four sides?" to simulate a hands-free cellphone conversation. To answer the questions, they pushed buttons on the steering wheel similar to ones drivers use to adjust volume control.

When the mock motorists made a right turn, the MRI showed little to no change in brain activity. Turning left with traffic coming at them activated more areas of the brain.

However when they turned left with oncoming traffic and a voice distracting them, it provided a much different picture. The MRIs showed a dramatic shift that's illustrated in bright red on the images -- blood moved away from the visual cortex part of the brain, which controls sight, and surged to the prefrontal cortex, which controls decision-making and monitoring a conversation.

And that's disturbing because managing traffic flow is a task that is visually demanding, said Schweizer.

"You have to pay attention to the traffic light, to oncoming cars and pedestrians. So in that moment in time, you're processing a huge amount of information in that snapshot," he told CTV News.

While participants also saw other motorists behind them in the simulated traffic scenario, Schweizer admits they were more cautious and spent more time at the intersection, deciding whether it was safe to turn, than they would have in the real world.

Researchers also couldn't induce the pressure and anxiety drivers feel in real traffic when another motorist is giving them the finger or honking their horn because they're in a hurry, he said. And study participants knew they weren't going to die if they crashed in the simulation, although none did crash, he said.

While driving with a cellphone in hand is banned across the country except in Nunavut, use of hands-free cellphones is allowed in most provinces although some provinces prohibit hands-free cellphone use for novice drivers.

In light of the study results, should the law be revisited to include hands-free cellphone use by motorists? "Yes," said Schweizer.

The study suggests it's the distraction -- not the cellphone device itself -- that's the problem for drivers.

But it's not just cellphones drivers should be concerned about -- a great radio talk show could also be too distracting, he suggested.

The nine men and seven women in the study were all healthy and had a mean driving experience of 7.4 years. Results for older, more experienced drivers could differ so a larger study involving people of various ages would be needed to confirm the findings, he said.

Schweizer and his team are currently seeking grant money to do a similar study with brain damaged patients and elderly patients in the next year.

The bottom line? Hands-free isn't brain free -- having a cellphone conversation could be a lethal combination if drivers are turning left at a busy intersection, said Schweizer.

The study, funded by the Natural Sciences and Engineering Research Council of Canada and the Ontario Ministry of Economic Development and Innovation, is published online Thursday in the journal Frontiers in Human Neuroscience.