While infants sleep they are reprocessing what they have learned, a new study has found.
Working with researchers from the University of Tubingen, scientists from the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig have discovered that babies of the age from 9 to 16 months remember the names of objects better if they had a short nap.
And only after sleeping can they transfer learned names to similar new objects. The infant brain thus forms general categories during sleep, converting experience into knowledge.
The researchers also showed that the formation of categories is closely related to a typical rhythmic activity of the sleeping brain called sleep spindles: Infants with high sleep spindle activity are particularly good at generalising their experiences and developing new knowledge while sleeping.
Sleep means much more than just relaxation for our brain. The flow of information from the sensory organs is largely cut off while we sleep, but many regions of the brain are especially active.
Most brain researchers today believe that the sleeping brain retrieves recent experiences, thereby consolidating new knowledge and integrating it into the existing memory by strengthening, re-linking or even dismantling neuronal connections. This means that sleep is indispensable for memory.
The researchers have found this to be the case even in infants and toddlers. In order to study the impact of sleep on infant memory, they invited parents to attend a study with their 9- to 16-month-old children.
During the training session, the infants were repeatedly shown images of certain objects while hearing the fictitious names assigned to the objects.
The similar objects, which belonged to the same category according to their shapes, were always given the same names. During this process, the researchers recorded the infants' brain activity using electroencephalography (EEG).
One group of infants spent the next one to two hours sleeping in their prams while an electroencephalogram (EEG) was recorded, while the others remained awake, going for a walk in their prams or playing in the examination room.
In the subsequent testing session, the researchers again presented the infants with picture-word pairs - this time both in the same combinations as in the learning session and in new combinations - and again measured their brain activity while doing so.
The analysis of brain activity showed that the infants had learned the names of the individual objects during the training session, irrespective of their age.
The situation with categorisation, however, was different: At the end of the training session, they were unable to assign new objects to the names of similar objects which they had heard several times.
