How the brain forms habits and why it is so difficult to change them

A discovery made at the Sainsbury Wellcome Centre at University College London is changing the way we form habits. Experts have discovered that the brain uses two systems at once to learn through trial and error. The first, already known, is responsible for evaluating rewards, and the second, newly discovered, helps to consolidate repeated actions, forming habits.

Imagine you go to the same café every day. At first you spend time choosing what to order, evaluating whether you’ll like the food. But over time, you start taking the same dish automatically, without thinking about it. That’s the workings of a new system the researchers called the action prediction error (APE) signal. It allows the brain to store patterns of behaviour, freeing up resources for other tasks.

Dopamine, a chemical linked to pleasure, plays a key role in this process. Previously, it was thought that dopamine mainly signalled whether the outcome of an action was better or worse than expected. Now scientists have found that in a particular area of the brain – the tail of the striatum – dopamine works differently, helping us remember how often we perform an action rather than how valuable it is.

To test this theory, the team conducted experiments on mice. The animals were trained to distinguish between sounds: a high tone signalled movement to the left, a low tone signalled movement to the right. When the researchers temporarily switched off the striatum tail, the mice continued to learn, but were unable to quickly consolidate the habit. This showed that without the APE, the brain relies solely on assessing the value of an action, which slows down the formation of sustained behaviour.

Why is this important for ordinary life? The new system explains how we move from conscious choices to automatic actions. For example, when you learn to drive, at first you focus on every movement. But over time, driving becomes a habit, and you may be carrying on a conversation or thinking about other things at the same time. APE allows your brain to switch to autopilot, conserving energy for more complex decisions.

For people with addictions, this discovery could be the key to new treatments. Because habits such as smoking are perpetuated through the APE, replacing a harmful action with a beneficial one – such as chewing gum instead of a cigarette – could help reprogramme the brain. If you repeat the new action often enough, it will become a habit on its own, suppressing the old one.

Interestingly, the finding also sheds light on Parkinson’s disease. In this disease, dopamine neurons associated with movement die, which can impair habitual actions such as walking. However, more flexible movements, such as skating, remain accessible because they utilise a different brain system.

The team now plans to further explore how the two systems – reward and action – interact with each other. Their goal is to understand how this knowledge can be used to combat bad habits and improve the quality of life for people with neurological disorders.