Dual-task interference

You can see them a kilometer away. You notice the car driving a) slowly or b) erratically or c) both. And then you get closer…and can see the cellphone glued to the driver’s ear.

Everyone pays lip service to the notion that cellphoning while driving is a bad idea…and yet some people still do it. They probably think that they, unlike everyone else, can easily handle two competing tasks at once.

Science–specifically research recently published by two neuroscientists at Vanderbilt University–says otherwise. Our brains, it seems, simply aren’t up to the task.

That sounds strange, considering the human brain’s power and complexity: 100 billion interconnected neurons, able to process (in computer terms) one trillion instructions per second. Even if computers continue to advance as quickly as they have been, they won’t match the brain’s level of computational power until 2030. (What happens when they do is a topic for another column…or a science fiction novel.)

Yet, research has shown that when a person attempts to perform two tasks at once, the brain regions involved are less active than when either is focused on its task alone. In other words, brain power doesn’t increase to meet demand; in fact, it decreases. Both tasks get done, but it takes longer to do them concurrantly than it would have to do them consecutively.

Last July, psychologists from the University of California at Los Angeles reported on a related study, in which they gave participants (all in their 20s) a set of cues concerning cards that displayed various shapes, and asked them to make predictions about which cards would be displayed. They conducted the test with two different sets of cards. With one set, the participants learned without any distractions. With the second set, they had to simultaneously listen to high and low beeps through headphones and keep a mental count of the high ones.

In both cases, they were able to learn the card-predicting task, but when they were asked questions about the cards afterward, they were able to remember the cards they had studied without distraction better than those they had studied with distractions.

The difficulty of doing two things at once, mentally, is called dual-task interference. It’s long been thought the brain must have some kind of central bottleneck that causes the problem–and the Vanderbilt University researchers may have identified it.

Paul E. Dux and René Marois used functional magnetic resonance imaging to rapidly sample brain activity while study subjects performed two demanding tasks: pressing a computer key in response to one of eight possible sounds and speaking a syllable in response to one of eight possible images.

Their brain scans revealed that both the lateral frontal and prefrontal cortex and the superior frontal cortex, regions known to play a critical role in cognitive control, are simply unable to process two tasks at once. Instead, they queue neural activity: when presented with two tasks simultaneously (within 300 milliseconds of each other), the neural response to one task is delayed until the first task is completed.

The time lost to dual-task interference can be as high as a second–which, as the researchers point out, is a long time when you’re driving a car.

And that’s why, as the headline of a press release about a 2005 study put it, “Cell Phone Users Drive Like Old Folks.”

University of Utah researchers found that when young people aged 18 to 25 in a driving simulator talked on cell phones, they promptly began driving like senior citizens, reacting to brake lights from a car in front of them as slowly as people 65 to 74 who were not using a cell phone.

Young or old, drivers who talked on cell phones were 18 percent slower in hitting their brakes, had a 12 percent greater following distance (apparently to compensate for paying less attention) and took 17 percent longer to regain speed after braking. Not surprisingly, the number of simulated rear-end collisions doubled.

The Vanderbilt University research has implications beyond driving-while-cellphone-impaired, of course. Any time people are asked to operate in a complex environment, such as an airplane cockpit, dual-task interference has to be allowed for.

The human brain simply isn’t designed to carry out two complex tasks at the same time. And no matter how much you might like to think dual-task interference doesn’t apply to you, oh cellphone-using driver, it most assuredly does.

Unless, of course, you’re not human.

But, as I said earlier, that’s a topic for another column…or a science fiction novel.

Permanent link to this article: https://edwardwillett.com/2007/01/dual-task-interference/

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