Most serious traffic accidents occur when drivers are making a left-hand turn at a busy intersection. When those drivers are also talking on a hands-free cell phone, "that could be the most dangerous thing they ever do on the road," said Dr. Tom Schweizer, a researcher at St. Michael's Hospital in Toronto.

The hospital is fully affiliated with the University of Toronto.

Researchers led by Schweizer tested healthy young drivers operating a novel driving simulator equipped with a steering wheel, brake pedal and accelerator inside a high-powered functional MRI. All previous studies on distracted driving have used just a joystick or trackball, or patients passively watching scenarios on a screen.

Immersing a driving simulator with a fully functional steering wheel and pedals in an MRI at Sunnybrook Health Sciences Centre allowed researchers to map in real time which parts of the brain were activated or deactivated as the simulator took them through increasingly difficult driving maneuvers.

The researchers were able to show for the first time that making a left-hand turn requires a huge amount of brain activation and involves far more areas of the brain than driving on a straight road or other maneuvers. 

When the drivers were also involved in a conversation, the part of the brain that controls vision significantly reduced its activity while the part that controls monitoring a conversation and attention was activated.

"Visually, a left-hand turn is quite demanding," Schweizer said. "You have to look at oncoming traffic, pedestrians and lights, and coordinate all that. Add talking on a cell phone, and your visual area shuts down significantly, which obviously is key to performing the maneuver."

The simulation had the drivers making six left turns with oncoming traffic, which required them to decide when to turn safely. It then distracted them, by making them answer a series of true-false audio questions, such as "Does a triangle have four sides?" 

The MRIs showed that blood moved from the visual cortex, which controls sight, to the prefrontal cortex, which controls decision-making.

"Brain activity shifted dramatically from the posterior, visual and spatial areas [of the brain] to the prefrontal cortex," said Schweizer, a neuroscientist and director of the Neuroscience Research Program at the hospital's Li Ka Shing Knowledge Institute.

"This study provides real-time neuroimaging evidence supporting previous behavioral observations suggesting that multitasking while driving may compromise vision and alertness. 'Hands free' not does mean 'brains free,'" Schweizer explained. 

Schweizer said his study needed to be replicated in larger groups and with various age groups, as well as with people with known brain impairments such as Alzheimer's disease.

This study was funded by the Natural Sciences and Engineering Research Council of Canada and the Ontario Ministry of Economic Development and Innovation. To access the study, click here.