Activation for Newly Learned Words in Left Medial-Temporal Lobe During Toddlers’ Sleep is Associated with Word Learning

Infants and toddlers amass a lexicon at a rapid pace and the extent of this development is a strong predictor of important future outcomes, including educational achievements. However, little is known about the neural substrates that support early vocabulary learning. Hippocampal and anterior medial-temporal (MTL) processes have been recently hypothesized to support memory for the initial association between new words and their referents, but this hypothesis has not been tested during early child development, when a more dominant role of cortical learning has also been proposed. We assessed 2-year-olds (N = 38) for their memory of newly learned pseudo-words associated with novel objects and puppets. We tested memory for these associations after a 20-minute and a one-week delay. We then played these pseudo-words, previously known words, and completely novel pseudo-words during natural nocturnal sleep, while collecting functional magnetic resonance imaging data. We found that activation in the left hippocampus and left anterior MTL for newly learned words was associated with memory for puppet names at the 20-minute delay, and activation in these same regions for known words compared to novel words was associated with memory for the puppet names at the one-week delay. In addition, activation for newly learned words was associated with productive vocabulary, underscoring that existing vocabulary might support the engagement of hippocampal and MTL mechanisms during learning of new words.