A coping mechanism that keeps part of the brain's attention on the road and the steering wheel lets experienced drivers tolerate many mental stresses and distractions, researchers say, but texting breaks that built-in auto-pilot.

In experiments using a driving simulator, drivers distracted by complex or emotional questions constantly compensated for erroneous steering reactions. But the same adaptability did not kick in for drivers distracted by texting, the study found.

"Our working hypothesis was that pure emotional and cognitive distractions were about the same with pure physical (i.e., sensorimotor) distractions," but according to these results, they are not, said lead author Ioannis Pavlidis of the Computational Physiology Laboratory at the University of Houston in Texas.

The researchers studied 59 subjects who completed several test drives in the simulator. For the first few, participants focused on relaxing and getting familiar with the machine while sensors recorded perspiration levels on their faces as a measure of the state of their sympathetic nervous system, which governs the unconscious "fight or flight" response.

The simulator measured steering angle and lane departures to the left or right while the subjects were driving the course. The subjects drove the course four times under stress: once with cognitive stress coming from a researcher posing the driver challenging questions, and another under emotional stress, where the questions were emotionally charged. One simulated drive had the driver sending text messages, representing "sensorimotor stress," and a fourth time there were mixed stressors.

Steering became more jittery than normal in all four conditions, but lane deviations only became "unsafe" while drivers were texting.

In the mentally and emotionally challenging conditions where drivers still had their eyes forward, their driving trajectories were actually straighter than under normal conditions, hinting at a kind of coping mechanism that pays extra attention to the task of driving when the brain is busy, the study team speculates in Scientific Reports.

"The driver's sympathetic system is already loaded, as driving itself is a task that needs psychophysiological resources," Pavlidis told Reuters Health by email. "Atop of that, if you add another stressor (cognitive, emotional, or physical), it arouses the sympathetic system even further, as it antagonizes for some of the same resources needed for the driving task."

"Jittery" steering may come from latent fight-or-flight energy, he said.

These distractions are "potentially dangerous, if they are instantly corrected, which is the case with the pure emotional and cognitive stressors," Pavlidis said. "They are immediately dangerous if they are left occasionally uncorrected (as is the case with texting), because they lead to significant left or right lane deviations."

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Texting undoes the "auto-pilot" mechanism we have to deal with driving and other routine tasks, he said, because to text you must look away from the road and disrupt the eye-hand feedback loop.

"Vehicle control requires hands on the wheel so a distraction such as holding a phone could impact stabilization and make it a bit more 'jittery'," said Despina Stavrinos of the University of Alabama at Birmingham, who was not part of the new study. "When driver's eyes are off the road they may also overcorrect in steering when they shift attention to driving."

Drivers can drift into emotional or mental distraction without realizing it, Pavlidis said.

"The best thing to do is to not drive at all when angry or upset - take a few moments to settle down before returning to the road," Stavrinos told Reuters Health by email.

"In the case of texting it is hard because smartphones are addictive and this is the case not only in driving but in all aspects of life, as we sadly see all around us everyday," Pavlidis said.