Habits are a powerful thing. Once formed, they require little conscious thought, which can be a good thing, like driving a manual transmission while being able to pay attention to traffic, or a bad one, like lighting a cigarette. Bad habits can be hard to break, but researchers modified some rats so that they could switch off a habit with little more than a bit of light. What they found, however, was that breaking one habit caused the rats to simply revert to a previous one.

The rats were trained to find food in a T-shaped enclosure. Starting at the base, they'd be given one of two tones, with the different sounds indicating which of the arms of the T would contain the food; one turn told them to turn right, the other left.

With 40 trials a day for 10 days, the rats reached an accuracy of about 90 percent, consistently choosing the correct path to the food based on the tone they heard. To demonstrate this was a habit, the researchers put a noxious substance in one of the food sources. But in response to the tone, the rats kept going to it even after the unpleasant experience, although at a slightly lower rate.

To rewire this habit, the researchers relied on a technique called optogenetics. This involves expressing a bacterial gene that encodes a light-activated ion channel in nerve cells. When exposed to light of the right wavelengths, the channel opens up, allowing ions to flow and triggering a change in voltage, which sets off a nerve impulse.

In this case, the researchers injected the virus into the brains of rats, then implanted a fiber optic cable to let them trigger neural activity. To get at habits in particular, they targeted the infralimbic cortex, which exerts control over habitual activities. Now, when light was sent into the brain of these rats, the activity of the infralimbic cortex would be disrupted.

Back in the maze, the light had a dramatic effect. The rats should have been creatures of habit, going to the food the tone indicated even after it started causing nausea. But, with the light switched on, the rats quickly stopped responding to the tone they associated with an unpleasant meal. In other words, their previous habit was easy to break when the light was shining. But it didn't disrupt all habits, as the second tone, which consistently sent them to a safe food source, continued to trigger that behavior. In fact, after a few trials, the rats started responding to both tones by running down the part of the cage that had a safe food source, even though no food appeared there in response to one of the tones. This became their new habit.

And, once the original habit was broken, it stayed broken. The researchers could shut the lights off, and the rats wouldn't go anywhere near the food that caused nausea.

But the original habit hadn't gone away. If the researchers waited over a week to let the new habit solidify, and then turned the light back on, a strange thing happened: the original habit returned. Faced with the tone that sent them towards noxious food, the rats happily returned to running down the arm of the T that led to it. And, once again, the original habit persisted even after the light was switched off.

We tend to think of habits as being present or not. But this work suggests that we may have a large collection of habits, and can even maintain more than one that applies to a single situation. In the rats, the infralimbic cortex seems to determine whether the animal performs a habitual behavior or not. But it also appears to be able to select from a menu of possible behaviors in response to a trigger.

Could this apply to humans? Absolutely, but it's probably a better idea to test this further in rats. Implanting fiber optic cables in their brains takes a lot of surgical skill, so this study was done with only a handful of rats (it got its statistical significance by putting each of them through a lot of trials). The first experiment I'd like to see done is adding a third habit to the infralimbic cortex's menu of possible behaviors.

PNAS, 2012. DOI: 10.1073/pnas.1216264109 (About DOIs).