问答题 What are you doing when you aren't doing anything at all? If you said "nothing," then you have just passed a test in logic and failed a test in neuroscience. (46)When people perform mental tasks, different areas of their brains become active, and brain scans show these active areas as brightly colored squares on an otherwise dull gray background. But researchers have recently discovered that when these areas of our brains light up, other areas go dark. This dark network is off when we seem to be on, and on when we seem to be off. When we appear to be doing nothing, we are clearly doing something. But what?
The answer, it seems, is time travel. (47)The human body moves forward in time at the rate of one second per second whether we like it or not, but the human mind can move through time in any direction and at any speed it chooses. Why did evolution design our brains to go wandering in time? Perhaps it's because an experi- ence is a terrible thing to waste.
(48)Time travel allows us to pay for an experience once and then have it again and again at no additional charge, learning new lessons with each repetition. Animals learn by trial and error, and the smarter they are, the fewer trials they need. Traveling backward buys us many trials for the price of one, but traveling forward allows us to dispense with trials entirely. (49)Just as pilots practice flying in flight simulators, the rest of us practice living in life simulators, and our ability to simulate future courses of action and preview their consequences en- ables us to learn from mistakes without making them. The dark network allows us to visit the future, but not just any future: only when we move ourselves through time does it come alive.
Perhaps the most startling fact about the dark network isn't what it does but how often it does it. Neurosci- entists refer to it as the brain's default mode, which is to say that we spend more of our time away from the present than in it. (50)People typically overestimate how often they are in the moment because they rarely take notice when they take leave; it is only when the environment demands our attention that our mental time ma- chines switch themselves off. We stay just long enough to take a message and then we slip off again to the land of Elsewhen, our dark networks awash in fight.