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Researchers from Arizona State University and Stanford University analyzed patterns of brain activity in humans and discovered a previously unknown role for the hippocampus, a brain area important to memory, in forming associations during learning.
“This study has important implications for understanding how the brain’s memory systems contribute to learning and decision-making” said Vishnu Murty, an assistant professor of psychology at Temple University who was not involved with the research. “The findings could help us understand the complexity of decision-making deficits in populations where the focus has mostly been on memory deficits, like Alzheimer’s disease, other psychopathology disorders and normal aging.”
When people learn, they build associations among features—like the time of day and mood of a toddler—to predict an outcome, like whether or not roughhousing will end in tears. “These associations have a big influence on behavior,” said Ian Ballard, who recently earned his doctorate in neuroscience from Stanford University and is first author on the paper. “But in the real world, objects or events are defined by more than one feature or combinations of features, and we wanted to understand how the brain builds associations over similar configurations of features.”
To unpack how the brain might handle the problem of building associations over such complex, real-world information, the research team focused on a small brain structure that looks like a snail shell. The hippocampus is approximately one-third the size of a gumball, and this small brain structure is crucial for memory formation. Without it, people cannot form new memories about facts or events, like what day it is or the names of coworkers. During memory formation, the hippocampus represents individual details of an event, like where you parked your car, to be as different as possible from each other.
“One challenge with memory is that it is hard to distinguish similar experiences. So, if you use the same parking garage at work every day, you have to remember which floor and space to go to at the end of the day,” said Samuel McClure, associate professor of psychology at ASU and senior author on the paper. “The problem is that it is easy to confuse where you parked on different days. The hippocampus is critical for remembering the combination of where and when.”
How the hippocampus forms memories provided a mechanism for how complex combinations of features could be represented in the brain, but whether the small, coiled structure actually contributed to how people learn about the world was an open question.
It was only the combinations of two stimulus pictures, like a face paired with a townhouse, that that could be used to predict when a target would appear. The individual stimuli, like the face image, were not useful for predicting by themselves.
A new way the hippocampus contributes to learning. When the research team looked at how the patterns of activity in the hippocampus were related to other brain areas, they found hippocampal activity was tightly correlated with activity in the striatum. Nestled beneath the cortex, the striatum consists of three separate structures—the caudate, putamen and nucleus accumbens—and plays an important role in learning what predicts desirable outcomes.
How the striatum handles information during learning is well known, but where all the information comes from is an open question. Ballard added that this study starts to chip away at that question by showing the hippocampus provided information about combinations of features to the striatum and that information was used to learn how to succeed at the task. Until recently, the brain was thought to have separate learning systems, but the findings suggest the hippocampal memory system and striatal reinforcement learning system are interrelated.