INFANTS CREATE NEW KNOWLEDGE WHILE SLEEPING
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 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 generalizing 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 Max Planck
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. Some objects were similar
to each other, varying only in their proportions, colours or in certain
details. 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 categorization, 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.
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
the objects. "The infants who slept after the training session assigned
new objects to the names of similar-looking objects," says Manuela
Friedrich of the Max Planck Institute for Human Cognitive and Brain Sciences.
"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."
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 generates 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 time 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," explains Friederici.
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