We all know people who have poor impulse control. They can't open a bag of chips without eating the whole thing, or they lose their temper over something minor and can't calm down. A new study finds it may involve two major circuits in the basal ganglia. 

The new study didn't start out with impulse control or suppression of it, it started with Parkinson's and Huntington's Disease. These conditions manifest as movement disorders with broadly opposite symptoms. While Huntington's patients suffer from uncontrolled, involuntary movement, Parkinson's patients struggle with action initiation. Peculiarly, both conditions stem from dysfunction of the same brain region: the basal ganglia. How can the same structure support contradictory functions?

Prior studies identified two major circuits in the basal ganglia, the direct and indirect pathways, and it is thought that while the activity of the direct pathway promotes movement, the indirect pathway suppresses it. How has remained unconfirmed. Instead of investigating the basal ganglia during movement, the researchers focused on active action suppression instead. They designed a task where mice had to determine whether an interval separating two tones was longer or shorter than 1.5 seconds. If it was shorter, a reward would be provided on the left side of the box, and if it was longer, it would be available on the right.

"The key was that the mouse had to stay perfectly still in the period between the two tones", said doctoral student Bruno Cruz. "So even if the animal was certain the 1.5-second mark had passed, it needed to suppress the urge to move until after the second tone sounded, and only then go for the reward."

The researchers tracked neural activity of both pathways while the mouse performed the task. As in past studies, activity levels were similar when the mouse was moving. However, things changed during the action-suppression period. Unlike coactivation during movement, activity patterns across the two pathways were strikingly different during the action suppression period. The activity of the indirect pathway was overall higher and it continuously increased while the mouse waited for the second tone. According to the authors, this observation suggests that the indirect pathway flexibly supports the behavioural goals of the animal. "As time passes, the mouse becomes more confident that it’s in a ‘long-interval’ trial. And so its urge to move becomes increasingly more difficult to restrain. It’s likely that this continuous increase in activity reflects this internal struggle," Cruz explained. 

Inspired by this idea, Cruz tested the effect of inhibiting the indirect pathway. This manipulation caused the mice to behave impulsively more often, significantly increasing the number of trials where they darted to the reward port prematurely. With this innovative approach, the team effectively uncovered an “impulsivity switch”.