However, the brain's executive command center does not completely relinquish control of habitual behavior. 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.
"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."
The new study offers hope for those trying to kick bad habits, says Graybiel, senior author of the new study, which appears this week in the Proceedings of the National Academy of Sciences. It shows that though habits may be deeply ingrained, the brain's planning centers 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 behavior, such as obsessive-compulsive disorder.
Lead author of the paper is Kyle Smith, a McGovern Institute research scientist. Other authors are recent MIT graduate Arti Virkud and Karl Deisseroth, a professor of psychiatry and behavioral sciences at Stanford University.
Old habits die hard
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 behavior 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 rats chocolate 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 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.
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 says.
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."
The findings suggest that the IL cortex is responsible for determining, moment-by-moment, which habitual behaviors will be expressed. "To us, what's really stunning is that habit representation still must be totally intact and retrievable in an instant, and there's an online monitoring system controlling that," Graybiel says.
The study also raises interesting ideas concerning how automatic habitual behaviors really are, says Jane Taylor, a professor of psychiatry and psychology at Yale University. "We've always thought of habits as being inflexible, but this suggests you can have flexible habits, in some sense," says Taylor, who was not part of the research team.
It also appears that the IL cortex favors new habits over old ones, consistent with previous studies showing that when habits are broken they are not forgotten, but replaced with new ones.
Although it would be too invasive to use optogenetic interventions to break habits in humans, Graybiel says it is possible the technology will evolve to the point where it might be a feasible option for treating disorders involving overly repetitive or addictive behavior.
In follow-up studies, the researchers are trying to pinpoint exactly when during a maze run the IL cortex selects the appropriate habit. They are also planning to specifically inhibit different cell types within the IL cortex, to see which ones are most involved in habit control.
The research was funded by the National Institutes of Health, the Stanley H. and Sheila G. Sydney Fund, R. Pourian and Julia Madadi, the Defense Advanced Research Projects Agency, and the Gatsby Foundation.