While sleeping, babies’ brains actively categorize the information they have learned while awake, converting experience into knowledge. This is the key finding of a new study from scientists at Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany.

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The researchers used electroencephalography to monitor the brain activity of their infant participants.
Image credit: Max Planck Institute

Medical News Today recently reported on a study published in the Proceedings of the National Academy of Sciences that suggested daytime naps boost infants’ learning skills.

The Max Planck study – published in Nature Communications – supports the findings of that study, but also suggests a mechanism behind this consolidation of information.

The Max Planck researchers recruited 90 infants between the ages of 9 and 16 months for the study. First, the infants were repeatedly shown images of unfamiliar objects with fictitious names.

The objects were categorized by their shapes, although some shapes belonging to the same category varied slightly in proportions, colors or other details. Shapes that were similar were always given the same names, which the infants heard every time they saw a shape.

While this familiarization activity was happening, the researchers used electroencephalography (EEG) to monitor the brain activity of their infant participants.

By analyzing the EEG records, the researchers deduced that the infants had learned the names of the objects during the training session. However, the participants had more problems with categorization – they were not able to connect new objects with the names of similar objects that they were already familiar with.

Next, one group of infants napped for 1-2 hours in their baby carriages – again, with their brain activity recorded by EEG. The other infants stayed awake and variously went for a walk in their baby carriages or played in the examination room.

The infants were then shown the picture-word pairs for a second time while their brain activity was measured. As well as showing the participants the same combinations as in the previous session, the researchers this time also presented similar but new objects.

The team found significant differences between in the brain activity of the babies who slept and the babies who stayed awake inbetween sessions. The infants who stayed awake had a harder time remembering the names of the individual objects, but the infants who had slept remembered them easily.

There were also “radical differences” in the categorization abilities of the two groups. Study author Manuela Friedrich explains:

The infants who slept after the training session assigned new objects to the names of similar-looking objects. They were not able to do that before their nap, and nor were the ones who stayed awake able to do it. This means that the categories must have been formed during sleep.”

According to the researchers, the children’s age did not influence the results, but a particular brain function did. “Sleep spindles” are brainwaves that occur when bundles of nerves located between the thalamus and cerebral cortex generate rhythmic activity of 10-15 cycles per second.

Sleep spindles have been shown to influence memory consolidation in adults, and Friedrich says the results of her study show that the “greater an infant’s spindle activity, the better it can assign category names to new objects after sleep.”

Therefore, while the infants quickly forget new words while awake, during sleep their brains organize the associations of words and objects, imprinting the new information in their memories. In this study, sleep also enabled the brains of the infants to pool together similar meanings in a way that the awake babies could not.

“In this way, sleep bridges the gap between specific objects and general categories, thus transferring experience into knowledge”, explains Angela Friederici, study head and director of the Max Planck Institute.