From The Dana Foundation:
Studies about the brain usually focus on neural activity during the completion of a specific task—remembering a series of words, for example. But over the last 20 years, researchers have been interested in what the brain does during periods of supposed inactivity. They discovered that when someone appears to be doing nothing at all, a network of brain regions—named the default network—is hard at work, allowing for the rich inner lives inside our heads. Applying what is known about the default network to diseases like Alzheimer’s allows for new possibilities for diagnosis and evaluation of treatments.
You’re lying in a brain scanner in the dark, looking up at a small white crosshair, left alone with your thoughts for the next six minutes. What goes through your mind? Perhaps you think about why you volunteered for this, or what you’ll do with the money you earn from this experiment. Perhaps you plan out the rest of your day, or start replaying a conversation from yesterday. New techniques in neuroimaging are helping scientists understand how your brain represents such internally directed and spontaneous thoughts.