The Sleeping Infant Brain Anticipates 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.