A new path to understanding decision making

As we progress through the academic year, motivation can start to wane for many students at Warwick. However, this often becomes more problematic as we age, due to the deterioration of the brain’s reward pathway. A team of scientists at the Massachusetts Institute of Technology (MIT) have identified a brain circuit crucial for maintaining motivation, which has the potential to be manipulated by drugs.

Recently, the team have finally discovered the function of striosomes, a cluster of cells distributed throughout the striatum

The brain circuit discovered is especially important for approach-avoidance conflict decision-making, which requires the evaluation of cost and reward associated with particular actions. An example of such decision-making includes whether taking a higher paid job is worth relocating away from family and friends. The striatum is heavily implicated in this decision-making, which is contained within the basal banglia, a group of subcortical nuclei within the brain linked to habit formation, voluntary movement control, emotion, and addiction.  Recently, the team have finally discovered the function of striosomes, a cluster of cells distributed throughout the striatum. 

The team, led by Graybriel, an institute professor at MIT, investigated changes in striosome activity during the decision-making process in mice by analysing activity as mice learned to choose between positive and negative outcomes. Two different tones were played to the mice, each of which was accompanied by either a reward or a mildly aversive stimulus. Gradually, mice learned that if they licked a spout more during the first tone, they would receive more of the reward (sugar water), and if they licked less, they would get less of the aversive stimulus (bright light). Such learning requires assigning value to costs and reward. 

Striosome activity increased as mice learned the task in comparison to other parts of the striatum, suggesting they’re involved in assigning value to outcomes. The researchers discovered that inhibitory neurons which relay signals from the prefrontal cortex help striosomes enhance their signal-to-noise ratio, helping generate the strong signals seen when mice evaluate a high cost or reward option.

When administered with genetically targeted drugs to boost activity, mice became more engaged, whereas the suppression of activity decreased engagement

Recently published in Cell, it was discovered that the mice’s ability to learn this cost-benefit analysis decreased in older mice, also correlating with a decrease in striosomal activity. When administered with genetically targeted drugs to boost activity, mice became more engaged, whereas the suppression of activity decreased engagement. By reactivating the circuit, researchers were able to improve older mice’s motivation to increase learning, and decrease motivation by suppressing the circuit. The circuit is especially important for social well being and learning, since, as said by Ann Graybiel, an Institute professor at MIT, “It’s tough to learn if you aren’t attending and engaged.”

While the circuit is implicated in age-related decline, several mental health disorders such as anxiety and depression also affect the ability to evaluate the costs and rewards associated with an action. For example, someone suffering with depression may undervalue potentially rewarding experiences, whereas an individual with addiction may overvalue drugs, and undervalue aspects such as family. Alexander Friedman, the paper’s lead author, highlights the importance of assigning value to costs and rewards, saying “In order to survive, in order to do whatever you are doing, you constantly need to be able to learn. You need to learn what is good for you, and what is bad for you.”

Thanks to the team of scientists at MIT, there may be the eventual discovery of drugs able to stimulate the circuit, and increase motivation in humans. This has the potential to be beneficial to both the elderly, and those suffering with mental health disorders. The prevalence of mental health disorders is rising, which is particularly common among students. Perhaps, a drug stimulating the circuit will improve the mental health for students of the future.