If you have that one pesky habit you just can’t break, a rare type of neuron in your brain may be the culprit.

Duke researchers in the lab of Nicole Calakos, associate professor of neurology, recently identified a fast-spiking interneuron—that makes up around one percent of neurons in the brain—as a key player in habit formation. In the study published in the journal eLife, mice whose interneurons were genetically manipulated to activate or "spike" less often did not form habits as easily as mice with normal, non-inhibited interneurons.

"I think these particular findings are one piece in a puzzle that we're otherwise building out to understand how habits are formed and their specificity," Calakos said. 

The study is the first to identify the specific type of neuron involved in habit formation, although a 2016 study showed that an area of the brain known as the striatum was an integral component in habit formation. 

Graduate student Justin O'Hare, who was the lead author of the latest study, said that the investigation extended the 2016 study to pinpoint the specific region or component of the striatum driving habitual actions.

“If a part of the brain is required for some type of behavior, then there can be basically two possible reasons: one is that information just sort of passively goes through that part of the brain,” he said. “So if you take that part out, the information can’t get through, and your behavior doesn’t work.”

The other possibility, O’Hare said, is that the brain and connections between neurons actually change as the information passes through the brain—a process called a “memory trace.”

To investigate which option was the case, O’Hare and the other researchers inactivated fast-spiking interneurons and observed a surprising result.

“We actually saw that when we turned off these fast-spiking interneurons, it caused this brain region to do everything the exact opposite way from [forming] habits,” he said.

The ultimate proof came when the researchers trained two groups of mice to press a lever, which would cause a sugar pellet to be dispensed. One group of mice had normal fast-spiking interneurons, whereas the other group had the activity of their interneurons decreased with a genetic tool.

O’Hare explained that the team tested habit formation by feeding the mice sugar pellets before placing them in a box with the lever. The mice that did not form a habit would ignore the lever since they had already eaten as much sugar as they wanted. The habit-driven mice, however, repeatedly pressed the lever even though they already had their fill of sugar.

“If the animal is habitual to pressing this lever, then it’s not really thinking about what’s going to be delivered to it,” he said. “It’s not really thinking much about the reward—it sees the lever, and it just sort of has this automated response to start pressing it.”

Other habitual actions such as a mouse reaching out or pressing something with its nose have also been related to the same brain region, suggesting that fast-spiking interneurons may control multiple habit types, O’Hare added. 

He cited a recent study by Merel Kindt, professor of psychology at the University of Amsterdam, who was able to rid people of their phobias by using a simple drug. Subjects who viewed a tarantula in a glass jar after being injected with a beta-blocker—a class of drugs commonly used for anxiety disorders among other conditions—were less scared of the spider compared to other subjects.

O’Hare envisioned a similar use of a drug to target fast-spiking interneurons that control habits. 

“One benefit of identifying one type of cell in the brain is that maybe you can find a noninvasive inroad to manipulating that cell type,” he explained.

Still, Calakos noted that the fast-spiking interneurons are merely one piece in the complex puzzle of habit formation.

"The fast-spiking interneurons were really an unexpected contributor to habits and memory," she said. "But it seems as though we're still missing some pieces of information to help us understand why we're habitual specifically in one task but not another."

For now, treating humans is out of the question until further studies provide more information on the processes at play around fast-spiking interneurons.

“A future study that focuses on what you call the exact ‘plasticity mechanism’ that allows these fast-spiking interneurons to strengthen—I think that could provide a target for drugs and a potential way to help people down the road,” O’Hare said.