A new study from Massachusetts Institute of Technology (MIT) neuroscientists found that a brain circuit that is critical for maintaining motivation may be responsible for a decline in learning as people age, according to a press release.

The circuit is important for learning to make decisions that require evaluating the cost and reward that come with a particular action, according to the study. In a study involving mice, the researchers observed that they could boost older mice’s motivation to engage in this type of learning by reactivating this circuit. Further, this could also decrease motivation by suppressing the circuit.

“As we age, it’s harder to have a get-up-and-go attitude toward things,” said Ann Graybiel, PhD, an Institute Professor at MIT and member of the McGovern Institute for Brain Research, in a press release. “This get-up-and-go, or engagement, is important for our social well-being and for learning—it’s tough to learn if you aren’t attending and engaged.”

The study set out to investigate what happens in striosomes—a collection of brain centers linked to habit formation, control of voluntary movement, emotion, and addiction—to learn how to make certain decisions. The researchers measured and analyzed the activity of striosomes as mice learned to choose between positive and negative outcomes.

The mice heard 2 different tones, 1 of which was accompanied by a reward (a sugar wafer) and another that was paired with a mildly aversive stimulus (bright light). The mice gradually learn that if they licked a spout more when they heard the first tone, they would get more of the sugar water, and if they licked less during the second, the light would not be as bright, according to the study.

The researchers found that as the mice learned the task, striosomes showed higher activity than other parts of the striatum. This activity correlated with the mice’s behavioral responses to both of the tones, suggesting that striosomes could be critical for assigning subjective value to a particular outcome, according to the study.

“A person, or this case a mouse, may value a reward so highly that the risk of experiencing a possible cost is overwhelmed, while another may wish to avoid the cost to the exclusion of all rewards. And these may result in reward-driven learning in some and cost-driven learning in others,” said MIT research scientist Emily Hueske, PhD, in a press release.

Further, the study also found that inhibitory neurons that relay signals from the prefrontal cortex help striosomes to enhance their signal-to-noise ratio, helping to generate the strong signals that are seen when the mice evaluate a high-cost or high-reward option.

In older mice, the engagement in learning this type of cost-benefit analysis went down, while their striosomal activity declined compared with younger mice. A similar loss of motivation was also discovered in a mouse model of Huntington disease, a neurodegenerative disorder that affects the striatum and its striosomes, according to the study.

When the researchers used genetically targeted drugs to boost activity in the striosomes, they found that the mice became more engaged in performance of the task, whereas suppressing striosomal activity led to disengagement.

The researchers are planning toward possible drug treatments that could stimulate this circuit, suggesting that training patients to enhance activity in this circuit through biofeedback could offer another potential way to improve their cost-benefit evaluations.

“If you could pinpoint a mechanism which is underlying the subjective evaluation of reward and cost, and use a modern technique that could manipulate it, either psychiatrically or with biofeedback, patients may be able to activate their circuits correctly,” said study author Alexander Friedman, former MIT research scientist, in a press release.

REFERENCE
Study helps explain why motivation to learn declines with age. MIT News. https://news.mit.edu/2020/why-learn-motivate-age-decline-1027. Published October 27, 2020. Accessed October 29, 2020.