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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
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Home | Pregnancy Timeline | News Alerts |News Archive Feb 3, 2015

While babies sleep their brains reprocesses what they have learned previously.
© Manuela Friedrich




Sleep improves infant memory

There is no rest for a baby's brain - not even in sleep. While infants sleep they are reprocessing what they have learned.

Working with researchers from the University of Tübingen, scientists from the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig have discovered that babies aged from 9 to 16 months remember the names of objects better if they had a short nap. Only after sleeping can they transfer learned names to similar new objects. In this way, the infant brain forms general categories during sleep, converting experience into knowledge.

Results of the research are published in Nature Communications.

The formation of categories is closely related to rhythmic activity in the sleeping brain called sleep "spindles." Infants with high sleep spindle activity are good at generalizing their experiences and developing new knowledge while sleeping.

Sleep means much more than just relaxation for our brain. The flow of information from our sensory organs is largely cut off as we sleep, but many regions of the brain are very active. Today, most brain researchers believe our sleeping brain retrieves recent experience, consolidating new experience into existing memory by strengthening, re-linking or even dismantling neural connections. This means sleep is indispensable to memory.

Max Planck researchers found this to be true even in infants and toddlers. To study the impact of sleep on infant memory, parents attended a study with their 9- to 16-month-olds. During their training, infants were shown images of objects while hearing fictitious names given to those objects. Some were similar to each other, varying slightly in details. Similar objects according to shape, were always given the same names.

During the process, infants' brain activity was recorded using electroencephalography (EEG).

One group of infants spent the next one to two hours sleeping, while an electroencephalogram (EEG) recorded their brain activity, the other children remained awake, going for a walk or playing in the exam room. In subsequent testing, researchers again presented the infants with picture-word pairs in the same combinations as in the training session but added new combinations. Their brain patterns were measured again while doing so.

An analysis of brain activity showed that the infants had learned the names of the individual objects during the training session, irrespective of the age of the infant. But the ability to categorize, however, was different.

During the subsequent testing session, the brain activity of the infants who had slept after the training session was markedly different from that of the group who had stayed awake. While the group who had stayed awake had forgotten the names of the individual objects, the children in the sleep group remembered the object-word mappings. There were also radical differences in their abilities to categorize objects.

"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."

Manuela Friedrich PhD, Max Planck Institute for Human Cognitive and Brain Sciences.

While the children's age had no effect, a particular type of brainwave called the sleep spindle has a significant impact on learning outcomes. Sleep spindles occur when nerve bundles between the thalamus and the cerebral cortex generate rhythmic activity of 10 to 15 cycles per second. They are known to influence memory consolidation in adults. "The greater an infant's spindle activity, the better it can assign category names to new objects after sleep," explains Friedrich.

These results show that sleep significantly affects memory organization even in the infant brain - and at a developmental period when memory is growing on a massive scale. "The waking infant brain quickly forgets newly learned names, but during sleep, words are more durably linked to objects and imprinted", says Angela Friederici, Director at the Leipzig-based Max Planck Institute and head of the study. Sleep and sleep spindles also enable the infant brain to pool similar meanings.

Apparently, when the brain is largely cut off from outside influences, it can organize its experiences and form new generalizations.

"In this way, sleep bridges the gap between specific objects and general categories, thus transferring experience into knowledge", explained Friederici.

Sleep consolidates memory and promotes generalization in adults, but it is still unknown to what extent the rapidly growing infant memory benefits from sleep. Here we show that during sleep the infant brain reorganizes recent memories and creates semantic knowledge from individual episodic experiences. Infants aged between 9 and 16 months were given the opportunity to encode both objects as specific word meanings and categories as general word meanings. Event-related potentials indicate that, initially, infants acquire only the specific but not the general word meanings. About 1.5 h later, infants who napped during the retention period, but not infants who stayed awake, remember the specific word meanings and, moreover, successfully generalize words to novel category exemplars. Independently of age, the semantic generalization effect is correlated with sleep spindle activity during the nap, suggesting that sleep spindles are involved in infant sleep-dependent brain plasticity.

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