Left-hand turns require ‘huge’ brain power, study shows
Subjects were easily distracted during hands-free simulation
Canadian researchers have shown for the first time that making left-hand turns at busy intersections, where the worst real-world crashes occur, requires far more brain power than right turns or other manoeuvres.
Throw in talking on a hands-free cellphone, and the brain becomes so distracted, it shuts down key areas needed for visual attention and alertness.
For their study, Toronto researchers slid volunteers into functional MRI machines, or fMRIs — scanners that capture the brain at work in real time by measuring changes in blood flow. The machines show how certain areas of the brain are activated, or “light up” under different levels of mental demand.
The team, in a feat of engineering that took more than a year and a half, fitted an fMRI with a virtual-reality driving simulator complete with a fully functional steering wheel, brake and accelerator pedals. The study was inspired by neurological patients — people who have suffered brain injury from strokes, brain tumours, trauma and other conditions that make them vulnerable to being declared “medically unfit” to drive.
“If you take their licence away, it’s probably one of the most devastating things to happen to them, next to the medical condition itself and, in some cases, even more so,” said principal researcher Dr. Tom Schweizer, a neuroscientist and director of the neuroscience research program at St. Michael’s Hospital in Toronto.
Yet little is known about the fundamental, underlying brain networks behind human driving behaviour, namely, what areas of the brain are responsible for driving?
Insurance and crash statistics show that left-hand turns at busy intersections are where the most serious crashes occur.
“They must be appreciably different in some way than just driving straight in the country,” Schweizer said.
Intuitively, it makes sense. “But, we still don’t understand, would it be completely different brain areas? Would it be a different collection of brain areas that are recruited when doing this? We had no idea.”
The study, which included collaborators from Sunnybrook Health Sciences Centre and Baycrest in Toronto, involved 16 healthy volunteers; men and women aged 20 to 30, with seven years of driving experience on average. The team looked at the brain areas activated when driving straight, versus making simple right turns, or left turns with or without oncoming traffic. They found that making a left-hand turn in traffic lights up a “huge” network in the brain “that was well over and above anything we saw with straight driving or even turning right,” Schweizer said. Specifically, they saw dramatically increased activity in brain regions involved in visual processing, spatial navigation and motor co-ordination.
“Think about it,” Schweizer says. “You’re in a busy intersection. You have to look at your own traffic light, to make sure you don’t turn on a red, and you have to look at the oncoming traffic to time your manoeuvre so you don’t get T-boned.”
Drivers also have to watch for pedestrians crossing in front of them on the walkway, from the left and the right.
A right-hand turn is not nearly so demanding.
“You have that oncoming traffic on the left, but you don’t have to co-ordinate as much,” Schweizer said.
The study was restricted to right-handers. “We wanted to keep it as homogeneous as possible.”
Next, the researchers tested the effects of distraction. Volunteers simulated making a left-hand turn at a busy intersection while answering a series of true or false questions, such as, does a triangle have four sides? The idea was to mimic what happens when someone is talking on a hands-free device, Schweizer said. Answering true-false questions “forces you to listen, to process and then come up with a response, like you would in a conversation.”
The finding, Schweizer said, “was quite striking. Clearly, there’s a finite amount of brain resources that can go around, and something had to give.”
What gave was the visual posterior cortex, the area of the brain that serves visual processing. It shut down by about 50 per cent. Blood moved from the part of the brain that controls sight, to the part of the brain, the prefrontal cortex, that controls decision-making.
Essentially, the brain sacrificed one area to recruit enough resources for the other. “It’s potentially quite dangerous,” Schweizer said.
“Hands-free does not mean brains-free,” he added.
The work is published in the journal Frontiers in Human Neuroscience.