Whenever you shift from one activity to another, say, writing a presentation to checking your e-mail, your brain goes through a four-step process. It makes no difference how quickly you shift your attention; the process is still essentially the same. 1) To begin with, blood rushes to the anterior prefrontal cortex, which notifies the rest of your brain that you’re about to work on your presentation. 2) This all-points bulletin to your brain has two parts. First it searches for the neurons that it’ll need for a particular task, and then it notifies those neurons that they’re needed. These neurons, most of which are capable of taking part in a variety of tasks, are then temporarily deputized to participate in the process of writing your presentation. This entire search and notification routine is fast, but it still takes a few tenths of a second. 3) Then, when you decide to stop writing your presentation and check your e-mail, your brain must first disengage from the current task before moving on to the next. This process is speedy, but as we’ll soon see, it’s not nearly rapid enough. 4) The final step is the same as the first, only this time the anterior prefrontal cortex, instead of telling your brain that you plan to write your presentation, will notify it that you intend to check e-mail. Once again, a few tenths of a second are required.