Washington – A new study from MIT neuroscientists has found that a small region of the brain’s prefrontal cortex, where most thought and planning occurs, is responsible for moment-by-moment control of which habits are switched on at a given time.
The new study offers hope for those trying to kick bad habits, said Institute Professor Ann Graybiel, a member of the McGovern Institute for Brain Research at MIT.
“We’ve always thought – and I still do – that the value of a habit is you don’t have to think about it. It frees up your brain to do other things. However, it doesn’t free up all of it. There’s some piece of your cortex that’s still devoted to that control,” said Graybiel, senior author of the new study.
It shows that though habits may be deeply ingrained, the brain’s planning centres can shut them off. It also raises the possibility of intervening in that brain region to treat people who suffer from disorders involving overly habitual behaviour, such as obsessive-compulsive disorder.
Kyle Smith, a McGovern Institute research scientist is the lead author of the paper. Other authors are recent MIT graduate Arti Virkud and Karl Deisseroth, a professor of psychiatry and behavioural sciences at Stanford University.
Habits often become so ingrained that we keep doing them even though we’re no longer benefiting from them. The MIT team experimentally simulated this situation with rats trained to run a T-shaped maze. As the rats approached the decision point, they heard a tone indicating whether they should turn left or right. When they chose correctly, they received a reward – chocolate milk (for turning left) or sugar water (for turning right).
To show that the behaviour was habitual, the researchers eventually stopped giving the trained rats any rewards, and found that they continued running the maze correctly. The researchers then went a step further, offering the ratschocolate milk in their cages but mixing it with lithium chloride, which causes light nausea. The rats still continued to run left when cued to do so, although they stopped drinking the chocolate milk.
Once they had shown that the habit was fully ingrained, the researchers wanted to see if they could break it by interfering with a part of the prefrontal cortex known as the infralimbic (IL) cortex. Although the neural pathways that encode habitual behaviour appear to be located in deep brain structures known as the basal ganglia, it has been shown that the IL cortex is also necessary for such behaviours to develop.
Using optogenetics, a technique that allows researchers to inhibit specific cells with light, the researchers turned off IL cortex activity for several seconds as the rats approached the point in the maze where they had to decide which way to turn.
Almost instantly, the rats dropped the habit of running to the left (the side with the now-distasteful reward). This suggests that turning off the IL cortex switches the rats’ brains from an “automatic, reflexive mode to a mode that’s more cognitive or engaged in the goal – processing what exactly it is that they’re running for,” Smith said.
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