The findings show some of the most fascinating things that our brain does.

A new study published in the journal Nature today reveals how our brain prevents us from acting hastily. The findings of the study discovered two distinct brain regions: one that motivates action and the other that inhibits that motivation. 

And could also be used “trigger impulsive behaviour by manipulating neurons in these areas,” according to Joe Paton, the study’s senior author.

FIXING A PROBLEM

Paton’s team set out to solve a mystery caused in part by Parkinson’s and Huntington’s Disease. These conditions show up as movement disorders with symptoms that are mostly the opposite of each other. Parkinson’s patients have trouble initiating actions, but Huntington’s patients experience uncontrollable, involuntary movement. Unusually, both disorders are brought on by the same brain area’s dysfunction: the basal ganglia. How can the same structure serve two different purposes?

Paton claims that previous research that distinguished the direct and indirect routes as the two main circuits in the basal ganglia provided an important clue. The indirect pathway is supposed to restrict movement while the direct pathway’s activity promotes it. Nonetheless, the specific nature of this interaction remained largely unknown.

A TIMING TASK WITH A TWIST

Paton approached the issue creatively. Paton’s team concentrated on active action suppression as opposed to basal ganglia during movement, which was the subject of earlier studies.

The group created a challenge in which mice were required to decide if the time between two tones was greater or shorter than 1.5 seconds. A prize would be offered on the left side of the box if it were shorter and on the right if it were longer.

Bruno Cruz, a doctorate student in the lab, said that the mouse had to remain perfectly still in the pause between the two tones. Therefore, even if the animal knew the 1.5-second mark had passed, it had to control its want to move until the second tone was played, and only then should it go for the treat.

A “SWITCH” FOR IMPULSIVITY

During the mouse’s performance of the challenge, the researchers monitored the brain activity of both circuits. Similar to earlier studies, the mouse’s activity levels increased as it moved. Nevertheless, during the action-suppression phase, things altered.

“Interestingly, unlike the coactivation we and others have observed during movement, activity patterns across the two pathways were strikingly different during the action suppression period,” says Cruz, adding “the activity of the indirect pathway was overall higher and it continuously increased while the mouse waited for the second tone.”

This observation, in the authors’ opinion, implies that the indirect pathway adaptably serves the animal’s behavioral objectives. “As time passes, the mouse becomes more confident that it’s in a ‘long-interval’ trial,” explains Cruz, “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 investigated the impact of blocking the indirect pathway after being inspired by this concept. This change made the mice act more impulsively, which greatly increased the number of times they went to the reward port before they were ready. With this creative approach, the team was able to find a switch for impulsivity.

According to Paton, “this discovery has broad implications. In addition to the clear relevance for Parkinson’s and Huntington’s Disease, it also provides a unique opportunity to investigate conditions of impulse control, such as addiction and Obsessive-Compulsive Disorder.”

IN SEARCH OF MOTIVATING FACTORS

The team discovered a part of the brain that actively inhibits the urge to act, but where does that urge come from? The initial suspect was the direct channel of the same location because it is believed that direct pathways encourage action. However, when the researchers restricted the mouse, its behavior remained essentially unaltered.

“We knew the mice were experiencing a strong drive to act because removing suppression promoted impulsive-like action,” Paton recalls, “But it wasn’t immediately clear where else the site of action promotion could be. To answer this question, we decided to turn to computational modelling.”

“Mathematical models are extremely useful for making sense of complex systems, such as this one,” says Gonçalo Guiomar, a doctoral student in the lab, adding that “we took accumulated knowledge about the basal ganglia, formulated it mathematically, and tested how the system processes information. We then combined the model’s prediction with evidence from previous studies and identified a promising new candidate: the dorsomedial striatum.”

The team’s theory was accurate. In this novel location, inhibiting the direct pathway’s neurons was sufficient to change the mouse’s behavior. Both of the locations we recorded from are found in the striatum, a portion of the basal ganglia. According to Guiomar, the first area is in charge of so-called “low-level” motor-sensory processes, whereas the second area is responsible for “high-level” activities like decision-making.

ACTIVITY, TEMPTATION, AND MORE

The authors claim that this model provides a fresh view of how the basal ganglia function by running counter to the conventional wisdom about the region’s centralized nature.

The findings of the study show, according to Paton, “that there are potentially multiple neural circuits in the brain that are constantly competing over which action to execute next. This insight is important for understanding more deeply how this system works, which is imperative for treating certain movement disorders, but it goes even further.”

“Observations from neuroscience,” he adds, “are at the core of many machine learning and AI techniques. The idea that decision-making can happen through the interaction of numerous parallel circuits within the same system might prove useful for designing new types of intelligent systems.”

Lastly, Paton says that the study’s ability to get at inner cognitive experiences may be one of its most unique parts.

“Impulsivity, temptation… These internal processes,” according to Paton, “are some of the most fascinating things that the brain does, because they reflect our inner life.”

“But they are also the hardest to study, because they don’t have a lot of outward signs that we can measure. Setting up this new method was challenging,” Paton concludes, “but now we have a powerful tool to investigate internal mechanisms, such as those that are involved in resisting and succumbing to temptation.”

Image Credit: Getty

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