Living in the present is fairly difficult for adults and children alike. But according to a new Yale study, baby rats have no trouble with it. In fact, that’s all they can do.
A team of researchers at the Yale School of Medicine surgically implanted tiny wires into rat brains to measure how well rats could remember running on a linear track. By recording their brain activity while they slept, researchers found that rats were unable to start forming memories until they were three weeks old and that memory formation occurred in distinct developmental stages. Until their brains are developed enough to make memories, rats live moment by moment, researchers found. The study was published in the journal Science on Jan. 11.
“It’s as if they’re living in the present for a week or so in their life. They cannot form memories,” said first author and graduate student in the School of Medicine’s Interdepartmental Neuroscience Program Usman Farooq GRD ’20.
To see if the rats were able to remember the track, researchers recorded which neurons fired during their run and while they slept. Farooq said that each memory an organism forms is represented by a unique sequence of place cells, or specialized neurons, in the brain. Therefore, he said, if the same place cells are activated at both points, a memory is formed.
According to the study, neurons in adult rat brains have innate connections that facilitate the creation of memories. But these connections — called “preplay” — aren’t used by baby rats to make memories because they are still developing. Farooq noted that rats can only make memories — that is, establish a specific order of neurons — once the rat is capable of harnessing this preplay, which only comes at about the fourth week of the rat’s life.
“The animals were just completely representing the present by not even using past experience but also by not even incorporating anything from the future. There was nothing which was used from the internal connectivity of the brain,” Farooq said.
Until the fourth week — which is considered the point at which rats have adultlike memory function — rats undergo two other distinct stages of development. At the first stage, neurons in the rat’s brain fire, but memories fail to form during sleep. Later, at stage two, the rat starts to interact with preplay. By providing a specific timeline for brain development, Farooq said that his team provides answers for neurological questions that had not been addressed before.
“There wasn’t much known about the development of the system which ultimately enables memory formation,” he said, “but now we have uncovered these basic principles of how the system develops to allow memory formation.”
According to second author and professor of psychiatry and neuroscience George Dragoi, rats were the perfect subjects for this study because their memories are stored sequentially, one after another, just like humans. In addition, rats grow far more quickly than humans or other animals, and their brains are simple enough for their activity to be easily measured.
The similarities between rats and humans extend even further: Farooq said that the first few weeks of a rat’s life are equivalent to a human phenomenon called “infantile amnesia,” where humans cannot remember the experiences they had as infants. Just as rats must reach a certain stage of development in order to form memories, humans must do so as well.
However, Farooq noted, there is still work to be done to see exactly how much of his team’s findings map to the human brain. While the basic principles of neurons and memory formation are the same, he said, the developmental stages may differ between species. Researchers will have to rely on indirect methods to bridge the gap between rat brains and human brains because of the ethical implications that come with implanting sensors into human babies.
Despite this, Farooq said the study’s findings are incredibly useful for researchers who study how neural developmental disorders like autism can affect memory formation in the early stages of mental maturity.
The hippocampus is responsible for spatial memory in the brain.
Matthew Kristoffersen | matthew.kristoffersen@yale.edu