Old Habits Gone But Not Forgotten

Scientists have identified a small region of the rat brain that seems to control whether certain habits will continue or be replaced by new habits. The finding offers insights into the brain pathways that help us shift between fixed and flexible behaviors.

Habits are a powerful force in our lives. Repeated activities—like tooth brushing or driving the same route to work—can become ingrained behaviors performed with little conscious thought. Such automatic habits can be helpful. Others can become harmful routines, such snacking on sweets, smoking, or abusing drugs or alcohol.

Scientists have long searched for the brain circuitry that creates, controls and overwrites our habits. Earlier research hinted that a specific brain area—known as the infralimbic cortex in rats—plays an important role.

To learn more, Drs. Ann Graybiel and Kyle Smith of the Massachusetts Institute of Technology and their colleagues used a technique called optogenetics to turn the infralimbic cortex off and on in rats with ingrained habits. The approach uses flashes of laser light to temporarily shut down genetically altered cells in specific brain regions. The study was supported in part by NIH’s National Institute of Mental Health (NIMH).

As reported in the November 13, 2012, edition of the Proceedings of the National Academy of Sciences, the scientists trained rats to run a T-shaped maze. As rats approached the crossroad, differing audio tones signaled which of 2 possible rewards awaited—sugar water might lay in one direction or chocolate milk in the other. Choosing the wrong direction meant no reward. Eventually, running the maze correctly became nearly automatic for the rats.

To confirm that the behavior had become a habit, the scientists then “devalued” one of the rewards by pairing it with an unpleasant substance. Despite their new aversion to one of the rewards, the rats continued to run the maze as before, turning toward the now-distasteful reward when cued by a certain tone.

Using optogenetics, the scientists then briefly disabled the infralimbic cortex as rats approached the crossroad. With that brain area offline, the rats appeared to act more thoughtfully, turning away from the unpleasant reward and toward the untainted treat. Over time, they continued this new habit, automatically turning in the same direction regardless of the cue or reward.

Once the new habit was firmly established, the infralimbic cortex was again disrupted via optogenetics. Surprisingly, the new habit was blocked, and the rats returned to their original habit of following the audio cues, even if it led to an unpleasant reward.

“This habit was never really forgotten,” Smith says. “It's lurking there somewhere, and we've unmasked it by turning off the new one.”

These results suggest that the brain can quickly toggle between old and new habits, and that the infralimbic cortex plays a key role in mediating these behaviors. “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.

—by Vicki Contie