We've all heard it a million times: Don't drink and drive. Now comes the latest in safe automobile practices:  Don't think and drive.

Researchers at a Complutense University in Madrid, Spain, found that when drivers think about things unrelated to the road, especially subjects that bring pictures to mind, their odds of being in an accident increase.

When drivers concentrate on a spatial or visual task their eyes fixate on certain objects longer, which impairs their perception of what's happening around them, according to the study, published in a recent edition of the Journal of Experimental Psychology: Applied.

Previous studies have found that a high percentage of car accidents are due to driver failure to pay attention to the road. They look but they don't see, the researchers noted.

To examine whether a driver's ability to pay attention is affected by internal distractions, Luis Nunes and his co-authors placed a camera on the dashboard of a car and recorded the eye movements of 12 people driving along two secondary roads in Europe.

As they performed various verbal and spatial-imagery tasks, pupil size and the amount of time they spent viewing a particular object were measured. During the visual challenges, which included imagining the letters of the alphabet, drivers fixated on certain things longer and glanced at their speedometer and rear-view mirror less frequently than they did when performing a verbal task, results showed.

A sports fan thinking about a particular sequence of a soccer match or a doctor mentally rehearsing an upcoming operation are everyday examples of how drivers may be risking their safety, Nunes said.

The drivers' peripheral vision was also affected by thinking in pictures, making it difficult to detect changes in traffic.

Of course, not all thoughts can be described as strictly verbal or visual, complex or simple, Nunes said. But they can be weighed according to their likelihood to distract. The imagery content of a thought like "Human rights are one of the most important objectives to achieve" is probably low, he explained. Conversely, the notion "My room has a small round table and a window on the left" has high imagery content.

It's better to perform a relatively simply verbal task, like listening to music, the researchers concluded. Music can actually keep you alert, unless, of course, you find yourself producing mental music videos or writing a musical score.

Knowing what kinds of thoughts are the most distracting is probably the best defense. Unfortunately, most of the participants rated the imagery tasks as being easier than the verbal tasks, although the data showed the former produced more marked effects on a driver's attention.

How to stop thinking while driving? "If you discover yourself thinking about a problem that needs a lot of concentration while driving, simply leave it for later or you could have a more serious problem because of an accident," Nunes said.

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