We tend to think that habits like eating junk food are a matter of willpower–too little willpower, too much sugar and fat in our diets. But the more we learn about the malleability of the brain, the more we know that “willpower” is a far too easy explanation for what’s really going on. The truth is that habits change how our brains work. What begins as a behavior morphs into changes in brain circuitry, and with repetition and time those changes strengthen and endure.
A new study by Duke University researchers helps clarify the matter by showing how a sugar habit changes specific brain circuits, and how those changes produce cravings that reinforce the habit.
The research team began by getting a group of healthy mice hooked on sugar. Similar to classic studies on drug addiction, the mice in this study were trained to press a tiny lever to receive doses of sweets. Once the mice were hooked, they continued pressing the lever even when the sweets were removed. So that was step one, establishing a behavioral pattern to get the goods.
Then the researchers compared the brains of the sugar-dependent mice to another group of healthy mice not hooked on sugar, focusing particularly on the network of brain areas known as the basal ganglia. In studies of drug addiction, electrical activity in this brain network is traceable along two main pathways: one that carries a “go” signal, which triggers action to pursue the object of the addiction, and one that carries a “stop” signal, which puts the brakes on the pursuit. It's the give and take between these pathways that regulates how we pursue just about anything (food, sex, goals, name it).
In the brains of the mice hooked on sugar, the researchers found stronger, more active go and stop signals than in the mice not hooked on sugar. And they found that in the sugar-dependent group, the go signal consistently appeared before the stop signal. In the non-dependent group, the stop signal appeared before the go signal. In other words, the brains of the sugar-hooked mice lost the braking capacity to regulate their behavior.
These brain circuitry changes were so strong, the researchers could predict which mice became hooked on sugar just by examining samples of their brains without knowing which mouse group the sample came from.
What this tells us is that once a behavioral pattern was established and reinforced (pressing the lever to get more sugar), the brains of the mice changed in a significant and enduring way. Both “stop” and “go” signals in their basal ganglia networks became hyper-charged, but they also flipped, with the go signal bumping out the stop signal.
The result of those changes is that the mice continued pursuing sugar even when it was gone. The circuitry changes in their brains produced a strong signal to get more sweets. We humans call that signal a craving. Filling the craving reinforces the very system that’s propelling the brain to want more.
“One day, we may be able to target these circuits in people to help promote habits that we want and kick out those that we don’t want,” said the study’s senior investigator Nicole Calakos, M.D., Ph.D., an associate professor of neurology and neurobiology at the Duke University Medical Center.
The study was published in the journal Neuron.