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For the first time, a Cornell University-led study in rats teases apart the role of the hippocampus in two functions of memory — one that remembers associations between time, place and what one did, and another that allows one to predict or plan future actions based on past experiences.
The breakthrough reveals that these two memory tasks, both coded in the hippocampus, can be separated. The finding has important implications for one day treating memory and learning issues found in dementia and Alzheimer’s disease.
The study, published in Science, used advanced optogenetic techniques to disable one type of memory while maintaining the other.
“We uncovered that two different neural codes support these very important aspects of memory and cognition, and can be dissociated, as we did experimentally,” said Antonio Fernandez-Ruiz, assistant professor of neurobiology and behavior.
One type of neural code controls the ability to make associations, such as remembering that apples are sold at the neighborhood grocery store. The other type is predictive and involves the ability to flexibly use memory to plan a new behavior; for example, if you always travel the same route to the store, but one day the road is closed, you can use an internal memorized map of the neighborhood to make a prediction of a new route.
Until now, nobody has known how the hippocampus supports these functions and if there was any relationship between the two.
In the study, rats with perturbed hippocampi had to explore a maze and find a new path every day to collect a reward. With the manipulation, the rats could not remember how to get the reward.
In a second experiment, the rats had to learn to associate a particular location in the environment with a reward. When the predictive capabilities were impaired, this associative memory remained intact. The researchers proved they could decouple these two types of memory.
The findings have implications for treating Alzheimer’s disease and other forms of dementia, where patients experience neural degeneration in the hippocampus as well as memory and navigation problems.
“By looking at which type of memory deficits occur in a patient,” Fernandez-Ruiz said, “we can try to infer what type of underlying neuronal mechanism has been compromised, which will help us develop more targeted interventions.