When did you first have a memory? Chances are it was after you were three. We very rarely remember anything from the first years of our lives... This phenomenon even has a name: infantile amnesia. Is episodic memory, the memory that relates to personally experienced events, deficient in babies? Researchers at Yale University (United States) have decided, thanks to a study based on MRI scans of children. Their results were published in the journal Science.
The hippocampus, seat of memory
The observation of childhood amnesia has led scientists to question the development of a brain region essential to episodic memory: the hippocampus. The equivalent of a large library of lived experiences. In the cartoon Inside Out, it is symbolized by huge shelves on which memories are carefully arranged. So isn't the hippocampus fully operational in babies? Could storage be deficient? This was the preferred approach until now.However, rodent studies have shown that young mice are able to form memories in the hippocampus from a very early age," reports Nick Turk-Browne, a neuroscientist at Yale University, to Science and Future.
Even more astonishing, the traces of this memorization, known as engrams—the physical manifestation of memories—persist even after individuals reach maturity, and can be artificially reactivated. What if memories were also correctly encoded in babies? This is what Nick Turk-Browne's team has succeeded in demonstrating. The explanation for infantile amnesia therefore lies elsewhere…
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The preferential gaze method
“ Our work shows that episodic memories can indeed form in the infant's hippocampus! the researcher is delighted. Thanks to MRI scans of 26 infants aged 4 to 25 months, he directly monitored the blood oxygenation rate in their brains. "This allowed us to monitor their neural activity,” he adds. During the medical imaging examination, the researchers showed different images to the babies, before carrying out a memory test, based on the preferential gaze method.
“ This technique involves simultaneously presenting an old image and a new image to the child, measuring eye movements towards both images, says Nick Turk-Browne. When babies can differentiate between the photos, they look at the old image longer. We can then conclude that they remember it." According to the results of the test, coupled with MRI scans, from the age of one, the hippocampus has the ability to record memories of individual experiences.
So, if storage is effective, how can we explain that we don't remember the first years of our lives?
Failing memory recovery?
Since memories are encoded in the hippocampus as early as 12 months, the researchers hypothesize that it is the retrieval of the memory that is defective.It is thought that the hippocampus is not receiving the right information to bring up old memories, perhaps due to changes in the way the brain processes sensory experiences," suggests Nick Turk-Browne. The sensory system evolves quickly, and could therefore process similar events in a completely different way in babies. In other words, over time, the hippocampus would end up no longer receiving the right "keywords" to retrieve these memories.
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The Yale University team is now working on tracking memories in young children, aged 2 to 3, and hopes to continue tracking them longer, until they are 9, in order to shed light on the mechanisms involved in childhood amnesia.
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