New Study Contributes to Further Understanding Hippocampus’s Role in Memory

Melissa Glenn’s research shows how hippocampal damage affects the ability to recall differences between objects

Melissa Glenn, professor of psychology and associate provost for academic programs

By Kardelen Koldas ’15Photography by Gabe Souza

January 12, 2022

What does the hippocampus do? This fancy-named, seahorse-shaped structure is often described as the brain’s memory center. But this prevailing explanation is inaccurate, and a Colby professor’s research is expanding our understanding of the hippocampus.

“If all we think is that the hippocampus equals memory, we overlook the contribution of other neural structures to memory.”

So cautions Melissa Glenn, professor of psychology and associate provost for academic programs, whose research focuses on behavioral neuroscience and the hippocampus. “Having a hyper-focus on the hippocampus is extremely limiting for the field, not just for understanding how memory works but for understanding the implications of that knowledge in medicine and neuroscience.”

Glenn is among the scientists working to broaden that hyper-focus to better grasp how memory works and the hippocampus’s role in it.

Her latest paper, published in Frontiers in Behavioral Neuroscience, investigated how damage to the hippocampus impacts remembering differences between objects—known as visual discrimination memory. It also for the first time examined whether this ability can be strengthened by additional learning, as suggested by the distributed reinstatement theory.

Here’s how the theory works: While the hippocampus plays a key role for some kinds of memories, it isn’t necessary for others. However, it can still overshadow other memory systems and become dominant in memory formation. In these instances, the distributed reinstatement theory proposes that the more something is relearned, the more other systems get a chance to learn that information, making it less likely to be forgotten after the hippocampus is damaged.

“We knew from other studies that damage to the hippocampus could impair visual discrimination memory,” she said. “This paper is unique in that we’re trying to use this kind of memory and very deliberately assess the predicted outcomes of the theory, that visual discrimination memories can be made independent of the hippocampus with additional learning.”

Left: Professor of Psychology Melissa Glenn notates damage to structures in the region of the brain that includes the hippocampus as part of her ongoing research. Right: Glenn holds a slide showing slices of rats’ brains with damage.

Glenn collaborated with researchers from Canada’s Trent University on the project. They executed a series of experiments to evaluate rats’ ability to differentiate between two different objects in a tub of water and analyzed if the distributed reinstatement theory held true for this memory type. Researchers used rats because they’re natural swimmers and excellent analogs to humans in terms of their cognition and memory.

In a round tub, researchers placed two objects—a red buoy and a yellow duck—and separated them by a barrier. For half of the rats, they tied the duck to a platform leading out of the water; for the other half, they tied the buoy. When placed in the tub, the rats had to decide between the two objects with the goal of choosing the one that signaled the location of the platform. The rats could take as many trials as needed to learn the task and consistently swim to the correct object, which, on average, took 12 days of training.

For the first experiment, the rats received 50 additional trials after fully learning the task to reinforce this new information. Without that extra time, the hippocampus would probably be the only structure that had adequate time to learn the task, Glenn explained, referring to the distributed reinstatement theory. Next, the rats underwent hippocampal lesions to see if, and how much, they remembered the task. Back in the tank, the researchers saw that with a damaged hippocampus, rats couldn’t recall the visual discrimination.

“One possibility that we explored is that they didn’t have enough additional learning episodes to support the memory,” said Glenn.

Because of this, in the second experiment, a new group of rats first underwent hippocampal lesions and later were placed in the tank to see how much time the other brain structures needed to learn the task. It turned out they needed more than 200 trials. The rats with hippocampal damage were able to learn it though they were about twice as slow to pick up the task compared to rats without hippocampal damage.

The final experiment drew on findings from the previous two and gave rats more than 230 trials to learn the task before undergoing the hippocampal lesion. Then, researchers tested how well these rats remembered the task and also had them relearn it. Surprisingly, the rats learned it much faster than they did before the surgery.

“They aren’t starting over from zero,” said Glenn. “Clearly, there’s some savings of the memory in these structures outside of the hippocampus supporting this faster learning.”

This experiment demonstrated the distributed reinstatement theory’s relevancy for visual discrimination memory, she explained. Moreover, the findings emphasize the importance of looking at memory systems other than the hippocampus.

“These findings are telling us something about what these other systems are,” she said, “so we can start to understand those.”