We humans like to think we're good at using our nimble brains to deal with two challenges at once. But it gets complicated. We can talk and watch television at the same time, but we can't carry on two conversations simultaneously.
That's because, according to researchers, multitasking is easier if it involves two different stimuli, like watching and listening, than if both stimuli are of the same type, especially visual.
Communications professor Zheng Wang of Ohio State University found that when subjects were hit with two visual challenges -- concentrating on images on a computer screen while text messaging -- their performance plummeted. Their visual capacity was, to some degree, overwhelmed.
Their performance also suffered if they concentrated on the images while talking on the phone, thus using different sources of stimuli, audio and visual. But it was not nearly as difficult as when both challenges were of the same type.
Here's the troubling part: Participants who faced two visual challenges thought they had performed much better than they had. Their ability to concentrate on the images dropped by 50 percent, yet they thought they had done just fine. Participants who had to watch and talk at the same time also saw their performance drop, by 30 percent, and they were slightly more aware of that.
"People's perception about how well they're doing doesn't match up with how they actually perform," Wang said in releasing the study, published in the journal Computers in Human Behavior.
Wang said she found her own research alarming because the experiments were designed around a major source of concern in the world of multitasking: text messaging while concentrating on a visual challenge, such as driving a car.
In her experiments, 32 student volunteers who watched a computer while text messaging performed much worse than participants who watched the computer while talking on the phone.
Her findings support an earlier study at Cornell University that demonstrated that there's not much danger in listening to the radio while driving a car, but it's really hard to understand two people who are talking at the same time. Two talkers can overwhelm the auditory system.
Wang's study suggests that if a driver simply has to communicate with someone who isn't in the car, it's better to phone instead of text because that involves two different sources of stimuli. But it's still dangerous. Researchers at Johns Hopkins University found that even a hands-free phone isn't safe.
"Directing attention to listening effectively 'turns down the volume' on input to the visual parts of the brain," psychologist Steven Yantis said when that study was released in 2005. "When attention is deployed to one modality -- say, in this case, talking on a cell phone -- it necessarily extracts a cost on another modality, in this case, the visual task of driving."
Scientists have paid a lot of attention to multitasking in recent years, partly because the National Safety Council has blamed 25 percent of all car crashes on drivers using cellphones. Here are a few other findings:
Multitasking is hardest in the early morning and late at night, apparently because the time of day affects our reaction time.
The older you get, the harder it gets, according to scientists at the University of California, San Francisco. They found that the "working memory," the part of the memory system used to retain information long enough to use it, is adversely affected in elderly persons by multitasking. And according to a study last year by Wang, even though research shows it's counterproductive, we love multitasking because it makes us feel good. It's nice to think you can do two things at once.
Multitasking, of course, didn't begin with the cellphone. It's been around for a very long time.
Anthropologist Monica Smith of the University of California, Los Angeles, laid out the history of multitasking in her book, "A Prehistory of Ordinary People."
It began, she said, millions of years ago with our bipedal ancestors. When they started walking on two feet, they were free to use their hands for other tasks. That allowed them to pick up fresh fruit and take it home to the kids, or throw a spear while chasing a rabbit, or make weird noises while blowing through the horn of a bison.
Smith goes so far as to say multitasking "is what makes us human."