A new MIT study may explain why people have less motivation to learn and experience new things as they get older. Using a mouse model, the scientists have discovered a brain circuit that may be the source of our desire to learn, and this neural network appears to shut down with age.
According to the experts, the brain circuit identified in the study is especially important for weighing the cost versus the reward of a particular decision.
The study revealed that reactivating this brain circuit boosted the motivation for learning and evaluating decisions among older mice. When the circuit was suppressed, however, so was the motivation to learn.
“As we age, it’s harder to have a get-up-and-go attitude toward things,” said study senior author Professor Ann Graybiel. “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.”
Professor Graybiel’s lab has been studying striosomes, clusters of cells in the striatum, for several decades. The striatum is part of a network of brain regions that are linked to habit formation, voluntary movement, emotion, and addiction.
The experts have found that striosomes play an important role in a type of decision-making known as approach-avoidance conflict. These decisions involve weighing options that have both positive and negative elements. For example, you could be offered a new job with better pay that would require you to move away from family and friends.
In a previous study, the MIT team found that stress plays a major role in this type of emotional decision making. The researchers demonstrated that stressed mice were far more likely to choose high-risk, high-payoff options. However, this behavior could be altered by manipulating the circuit.
“A person, or in 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 study co-lead author Emily Hueske.
For the current study, the researchers set out to investigate what happens in striosomes as mice were presented with these types of choices. The researchers found evidence that striosomes could be critical for assigning subjective value to a particular outcome.
“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,” said study co-lead author Alexander Friedman.
The researchers found that, among older mice, engagement in learning this type of cost-benefit analysis went down. At the same time, their striosomal activity declined compared to younger mice.
The team found a similar loss of motivation in a mouse model of Huntington’s disease, a neurodegenerative disorder that affects the striatum and its striosomes.
When the experts used genetically targeted drugs to boost activity in the striosomes, the mice became more engaged in learning the task.
The researchers are currently working to develop drug treatments with the potential to stimulate this circuit. They also speculate that patients could be trained to boost activity in this circuit.
“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 Friedman.
The study is published in the journal Cell.