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Babies associate words with content — through sleep
While babies sleep, astonishing processes take place in their brains. Scientists at the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) in Leipzig, Germany observed that babies succeed in associating a meaning with a word between the ages of six to eight months. This capability, until now, was only thought to be seen in older children and adults. Memory assigned to the meaning of words, passes through the same stages in sleep that is seen in typical language development. So-called proto-words combine simultaneous visual and acoustic stimuli and become real words when connected to content.
The work is published in Current Biology.
Scientists investigated these relationships by introducing six to eight month old infants to fantasy objects using fantasy names such as "Bofel" or "Zuser". The objects only differed in form or color and were identified by the same name — just as all cats are called "cats" although different in coat details. The fictitious objects were created to make sure that infant participants couldn't access any pre-existing knowledge.
From the infants' brain reaction it was clear the children couldn't connect new objects of the same category with a corresponding name. They did not recognise a new Bofel as a "Bofel" although it was quite similar to the previously seen Bofel version. For the babies, every new object-word pair was unknown and unique, they had not yet built a general relationship between them.
Children developed two different types of knowledge depending on their amount of sleep. After a half-hour nap, they showed a brain reaction which three-month-olds already have — associating a visual stimulus with an acoustic one. During their nap they filtered similar features out of the objects and connected them with the sound of a word. Similar to the three-month-old babies, they perceived the word as a random sound with no meaning.
Unlike the infants who napped for half an hour, babies that slept for about 50 minutes showed a brain reaction that was previously only known for older children and adults. Here, the so-called N400 component occurred, which signals that incongruous meanings were processed in the brain--whether it be in sentences, word pairs, picture stories or object-word pairs. By means of this component the researchers were able to recognise that the young participants in fact learned the meaning of the words.
"Our results demonstrate that children hold real word meanings in their long-term memory much earlier than assumed. Although the brain structures relevant for this type of memory are not fully matured, they can already be used to a distinguishable extent."
In this context, one stage of sleep could be of particular importance: The duration of the second of the four stages of sleep, in particular, seems to have an important influence on the development of lexical or dictionary type of memory. "During this light sleep, the transition from a simple early developing form of lexical memory to an advanced later developing form evidently takes place", says study leader Manuela Friedrich. "These two types of memory which develop during sleep are comparable with those that we know from infant development. Whereas during sleep there are just minutes in between the two types, in typical development there are months." The formation of memory content in sleep clearly takes place quickly.
Manuela Friedrich adds: "In our study, however, the babies received a lot of information they normally pick up over a longer time period. But only during sleep, when the child's brain is disconnected from the outer world, can it filter and save essential relationships. Only during the interaction between awake exploration and ordering processes, while sleeping, can early cognitive and linguistic capabilities develop properly."
•Memory consolidation during infant sleep parallels early stages of lexical development
•The duration of sleep stage 2 determines the developmental stage of new infant memories
•Local sleep spindles in N2 are involved in the formation of lexical-semantic LTM
•Sleep-dependent memory consolidation precedes the normal course of development
From the age of 3 months, infants learn relations between objects and co-occurring words [ 1 ]. These very first representations of object-word pairings in infant memory are considered as non-symbolic proto-words comprising specific visual-auditory associations that can already be formed in the first months of life [ 2–5 ]. Genuine words that refer to semantic long-term memory have not been evidenced prior to 9 months of age [ 6–9 ]. Sleep is known to facilitate the reorganization of memories [ 9–14 ], but its impact on the perceptual-to-semantic trend in early development is unknown. Here we explored the formation of word meanings in 6- to 8-month-old infants and its reorganization during the course of sleep. Infants were exposed to new words as labels for new object categories. In the memory test about an hour later, generalization to novel category exemplars was tested. In infants who took a short nap during the retention period, a brain response of 3-month-olds [ 1 ] was observed, indicating generalizations based on early developing perceptual-associative memory. In those infants who napped longer, a semantic priming effect [ 15, 16 ] usually found later in development [ 17–19 ] revealed the formation of genuine words. The perceptual-to-semantic shift in memory was related to the duration of sleep stage 2 and to locally increased sleep spindle activity. The finding that, after the massed presentation of several labeled category exemplars, sleep enabled even 6-month-olds to create semantic long-term memory clearly challenges the notion that immature brain structures are responsible for the typically slower lexical development.
All authors: Manuela Friedrich, Ines Wilhelm, Matthias Mölle, Jan Born, Angela D. Friederici
Keywords: infants, sleep, memory consolidation, word meanings, object categories, semantic priming, ERPs, NREM sleep, sleep stage 2, sleep spindles
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After a midday nap, babies who fell asleep after their learning phases
could differentiate between the right and wrong term for a new object.
Image credit: Max Plank Institute for Human Cognitive and Brain Sciences (MPI CBS)