Stanford University researchers recruited 19 undergrads who were heavy-duty multi-taskers — they were at the top of their class in their ability to simultaneously read, watch TV, listen to music, send and receive text messages, check their e-mail and surf the Web — and 22 others who rarely did two or three of those things at once. Volunteers in both groups submitted to a battery of tests.

It turns out the single-taskers do a better job of filtering out irrelevant stimuli compared with the multi-taskers.

To measure this, scientists asked the volunteers to gauge whether a red rectangle had changed its orientation on a computer screen without getting distracted by a bunch of blue rectangles. The more blue rectangles there were, the worse the multi-taskers did on the test. But the distracting rectangles had no effect on the single-taskers’ performance, the study found.

As further evidence that multi-taskers are more prone to distraction, a second test found that changing the color of letters that flashed on a computer screen caused them to take 77 milliseconds longer than single-taskers to decide whether they were looking at the letter “X.” (The multi-taskers were just as accurate, however.)

But you would think that someone with a lot of multi-tasking experience would have an edge when it came to toggling between two tasks. Not so.

Volunteers were shown a letter and a number together on a computer screen. They were asked to decide whether the letter was a consonant or a vowel or whether the number was even or odd. The researchers found that it took 167 milliseconds longer for the multi-taskers to switch between the letter and the number tasks than it did for the single-taskers.

Taken together, the results certainly imply that multi-taskers “approach fundamental information-processing activities differently than” single-taskers, the researchers conclude.

But why? Does a long history of multi-tasking make it difficult for people to focus? Or do they become multi-taskers because they are naturally attracted to a wide range of stimuli? That question remains unanswered.

The answer is important, especially for single-taskers. Though they performed better on the battery of tests, it’s clear these modern times favor those who can manage multiple forms of media at one time. If it’s hard for single-taskers to adapt, the researchers said, they may “be increasingly unable to cope with the changing media environment.”