Scientists from the US have discovered that young children are able to (at least briefly) remember visual stimuli – and this process is accompanied by the activity of hippocampal neurons. Thus, the causes of infantile amnesia are probably not that the immature hippocampus cannot encode memories, but that these memories then become inaccessible for retrieval. The work is published in the journal Science.
People usually do not remember what happened to them in early childhood. This phenomenon is called infantile amnesia and is also characteristic of many other mammals, but its mechanism is not fully understood. All theories and explanations come down to three: the child's brain is unable to form episodic memories (i.e., encode them); memories are formed, but are not retained until adulthood; memories are formed and retained, but access to them is lost (the retrieval process is disrupted).
The latter theory is supported by rodents, who can recall forgotten infant memories by placing them in the same context or by optogenetically activating hippocampal neurons that were active at the time of memorization. And human infants can memorize general patterns as early as three months—this learning depends on the hippocampus. However, encoding of episodic memories depends on a trisynaptic circuit in the hippocampus, whereas memorization of general patterns relies on a monosynaptic pathway that develops earlier. Thus, the question of when the infant hippocampus begins encoding episodic memories remains open.
Tristan Yates of Columbia University and his colleagues from various universities in New York conducted an experiment with 26 infants from four months to two years old. Half of the children were over a year old, and half were younger. They were placed in an fMRI scanner and shown images of objects, faces, and landscapes. Periodically, instead of a new picture, a pair of images appeared on the screen - one seen 20-100 seconds ago and a new one. If the child looked longer at the familiar image, the scientists concluded that he remembered it. At the same time, the researchers recorded brain activity in order to later assess the involvement of the hippocampus in memorization.
Overall, the researchers found no significant preference for familiar images across the entire sample. However, some children recognized previously seen images throughout the experiment. The activity of the hippocampus during the first image presentation was significantly higher in such cases, especially the posterior hippocampus, which in adults is responsible for encoding episodic memories. This effect was observed mainly in children over one year old, although children of all ages paid equal attention to the stimuli. In addition, the orbitofrontal cortex was additionally involved in older children during memorization. The effect was also stronger when less time passed between the first and repeated image presentations.
Since preference for familiar objects is traditionally associated with incomplete encoding, the results may reflect the still-developing function of the hippocampus. However, they do indicate that the hippocampus of children as young as twelve months can encode memories. This means that infantile amnesia is likely due to failures in the storage or retrieval of memories.
The method used did not allow the scientists to assess the neural mechanisms of memory retrieval, since in the test trials the children were shown two pictures at once - a familiar and a new one. Future studies could test other schemes where only one object is presented, as well as test other forms of memory - for example, free recall or associative inference.
Previously, scientists found that the hippocampus of two-year-old children responds to familiar songs, even while they are sleeping.
