Imagine one day being able to consult with a doctor about "switching off" your smoking habit with a day of outpatient surgery.
That's the possibility raised by a new study conducted by MIT neuroscientists aimed at finding the master switch in the brain that controls habits. Researchers 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.
"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," says Institute Professor Ann Graybiel, a member of the McGovern Institute for Brain Research at MIT. "However, it doesn't free up all of it. There's some piece of your cortex that's still devoted to that control."
As we're all well aware, old habits die hard, and that's because they are deeply wired into our brains. That's great in some cases because it allows our brain to expend energy on other things while a habitual behavior, such as driving to work, occurs with very little thought required. But in other cases habits can wreak havoc with our lives, as with obsessive-compulsive disorder. And sometimes what was once a beneficial habit continues even though it no longer benefits us.
The MIT team simulated that scenario 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 behavior was habitual, the researchers eventually stopped giving the trained rats any rewards, and found that they continued running the maze flawlessly. The researchers then offered the rats chocolate milk in their cages, but mixed it with lithium chloride, which causes light nausea. The rats still continued to run left when cued to do so, but 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 behavior 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 behaviors 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.
Immediately, the rats stopped running to the left (the side with the tainted chocolate milk). 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," according to Kyle Smith, lead author of the paper.
Once broken of the habit of running left, the rats soon formed a new habit, running to the right side every time, even when cued to run left. The researchers showed that they could break this new habit by once again inhibiting the IL cortex with light. To their surprise, they found that these rats immediately regained their original habit of running left when cued to do so.
"This habit was never really forgotten," Smith says. "It's lurking there somewhere, and we've unmasked it by turning off the new one that had been overwritten."
So what does this mean for us? First, it appears that old habits can be broken but they aren't forgotten. They're replaced by new habits, but the old habit is still lurking. And it seems that the IL cortex (the "master switch") favors new habits over old ones.
We are, of course, a long way from testing this technique in humans. But eventually, according to Graybiel, it's possible the technology will evolve to the point where it might be a feasible option for treating disorders involving overly repetitive or addictive behavior.
The study was published in the Proceedings of the National Academy of Sciences.