University Distinguished Professor of Psychology, Northeastern University; Research Neuroscientist, Massachusetts General Hospital; Lecturer in Psychiatry, Harvard Medical School
The Predictive Brain

Your brain is predictive, not reactive. For many years, scientists believed that your neurons spend most of their time dormant and wake up only when stimulated by some sight or sound in the world. Now we know that all your neurons are firing constantly, stimulating one another at various rates. This intrinsic brain activity is one of the great recent discoveries in neuroscience. Even more compelling is what this brain activity represents: millions of predictions of what you will encounter next in the world, based on your lifetime of past experience.

Many predictions are at a micro level, predicting the meaning of bits of light, sound, and other information from your senses. Every time you hear speech, your brain breaks up the continuous stream of sound into phonemes, syllables, words, and ideas by prediction. Other predictions are at the macro level. You’re interacting with a friend and, based on context, your brain predicts that she will smile. This prediction drives your motor neurons to move your mouth in advance to smile back, and your movement causes your friend’s brain to issue new predictions and actions, back and forth, in a dance of prediction and action. If predictions are wrong, your brain has mechanisms to correct them and issue new ones.

If your brain didn’t predict, sports couldn’t exist. A purely reactive brain wouldn’t be fast enough to parse the massive sensory input around you and direct your actions in time to catch a baseball or block a goal. You also would go through life constantly surprised.

The predictive brain will change how we understand ourselves, since most psychology experiments still assume the brain is reactive. Experiments proceed in artificial sequences called “trials,” where test subjects sit passively, are presented with images, sounds, words, etc., and make one response at a time, say, by pressing a button. Trials are randomized to keep one from affecting the next. In this highly controlled environment, the results come out looking like the subject’s brain makes a rapid, automatic response, followed by a controlled choice about 150 milliseconds later, as if the two responses came from distinct systems in the brain. These experiments fail to account for a predicting brain, which never sits awaiting stimulation, but continuously prepares multiple, competing predictions for action and perception, while actively collecting evidence to select between them. In real life, moments or “trials” are never independent because each brain state influences the next. Most psychology experiments are therefore optimized to disrupt the brain’s natural process of prediction.

The predictive brain presents us with an unprecedented opportunity for new discoveries about how a human brain creates a human mind. New evidence suggests that thoughts, feelings, perceptions, memories, decision-making, categorization, imagination, and many other mental phenomena, which historically are treated as distinct brain processes, can all be united by a single mechanism, prediction. Even our theory of human nature is up for grabs, as prediction deprives us of our most cherished narrative: the epic battle between rationality and emotions to control behavior.