In a test created by Mr. Ophir and his colleagues, subjects at a computer were briefly shown an image of red rectangles. Then they saw a similar image and were asked whether any of the rectangles had moved. It was a simple task until the addition of a twist: blue rectangles were added, and the subjects were told to ignore them. (Play a game testing how well you filter out distractions.)
The multitaskers then did a significantly worse job than the non-multitaskers at recognizing whether red rectangles had changed position. In other words, they had trouble filtering out the blue ones — the irrelevant information.
So, too, the multitaskers took longer than non-multitaskers to switch among tasks, like differentiating vowels from consonants and then odd from even numbers. The multitaskers were shown to be less efficient at juggling problems. (Play a game testing how well you switch between tasks.)
Your Brain on Computers – Attached to Technology and Paying a Price – NYTimes.com