Ear Plugs to Lasers: The Science of Concentration

“Multitasking is a myth,” Ms. Gallagher said. “You cannot do two things at once. The mechanism of attention is selection: it’s either this or it’s that.” She points to calculations that the typical person’s brain can process 173 billion bits of information over the course of a lifetime.
“People don’t understand that attention is a finite resource, like money,” she said. “Do you want to invest your cognitive cash on endless Twittering or Net surfing or couch potatoing? You’re constantly making choices, and your choices determine your experience, just as William James said.”

http://www.nytimes.com/2009/05/05/science/05tier.html?em