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When someone throws a ball at you, you almost immediately know to catch it — even before you consciously realize it. In the past, people thought that in such a situation, the brain functioned like a camera: an image of the flying ball enters through your eyes and is then processed by your brain. The brain then programs an appropriate action to react to it. But doesn’t that process take too long? Would you still be able to catch the ball in time?
Researchers Christian Keysers, Giorgia Silani, and Valeria Gazzola show that this process in the brain works differently. Christian explains: “Your brain doesn’t react to what entered the eye — but it predicts what will happen, based on expectations and previous experiences. By doing so, our actions keep pace with the ball, despite the hundreds of milliseconds it takes the brain to process visual input and move our body. It plans ahead to allow time to execute the action and catch the ball. The image that enters through your eyes is mainly used to check if your expectations match reality. Only when there’s a difference between your expectation and what you see, your brain uses the visual input to nudge its expectations in a more accurate direction.”
Predicting others
Valeria Gazzola: “What’s interesting is that you also use your own motor programs and somatosensory cortices to predict the actions of others. When you perform a physical action, like lifting a carton of milk to pour some of it into your coffee, you have expectations about the weight of the carton, and how it should feel in your hand when you start lifting it. You don’t really notice the weight of the carton consciously, because your brain predicted it. But if someone else already finished the milk, and the carton is so much lighter than you expected, the sudden difference between your expectations and the sensory feedback suddenly grabs your attention.”
“When you see someone else do so, you don’t directly feel the weight of the carton. Still, you can make predictions using your own motor programs, and test them against what you see. So, you still feel surprised if the carton flyes skywards much faster than you expected. We think this has to do with so-called mirror neurons, which are cells within your own motor cortex that become active when you see someone else perform an action. This acts as a sort of ‘shortcut,’ allowing you to use your own motor programs, and the predictive machinery necessary for your own actions to predict the behavior of others.”
But what about emotions? Gazzola explains: “We know that regions in our brain that are involved in our own emotions, become active while we witness the emotions of others. However, how we predict the emotions of others is not fully understood. Revieving the literature revealed that the regions in our brain that are active when we receive reward or punishment also become active when someone else receives reward or punishment. Reward and punishment are therefore valuable predictors for the emotions of others.”
Complex system
Christian Keysers: “Imagine I have a button, and every time I press it, an actor starts screaming in pain. If I do this five times, your expectations change: the first time, it’s totally unexpected, but by the fifth time, you can predict what will happen.”
“According to the traditional theory of perception, where the brain only processes the image you see, you should see the same reaction in the brain each time. But if the outside world primarily serves to test your predictions, you’d expect a strong brain response the first time, and a much smaller response the last time, because you already know what’s going to happen.”
“So what actually happens in the brain? Over many studies we see that it’s quite complex. Some brain areas respond relatively consistently across all five times. At the same time, there are also brain regions where activity changes across the five times. In this review paper, we look at the many studies that have emerged on the topic to propose how these different brain system organize into a coherent predictive brain. For example, if some actions become predictable, your premotor cortex knows how to perform the action in your body. This region will then inhibit visual regions in your brain, leading to less visual input. What you perceive is then no longer what you actually see, but what you expect to see. Only if something unexpected happens, this inhibition becomes ineffective. The visual areas now show a strong respons that is sent forwards to the premotor cortex to revise the predictions.”
Predictions and autism
It’s believed that the predictive system in people with autism is less well-tuned. This makes the world around them more unpredictable, leading to less suppressed stimuli. Christian Keysers: “Imagine you’re standing in a crowded room with many people. Because our brain makes a lot of predictions, we can ignore most stimuli and focus only on what’s important. But when this predictive system doesn’t work well, such a busy environment can suddenly feel overwhelming.”
“The brain is complex and has the unique ability to adapt. It’s interesting to realize that your brain isn’t just a camera simply processing what comes in. Instead, your brain constantly operates based on predictions. Your brain is always ahead and continuously constructs what the world should be.”