Vocabulary learning and regularity extraction: Temporal dynamics of consolidation and associations with slow-wave sleep and sleep spindles

Fast sleep spindles and slow-wave sleep (SWS) have been linked to memory consolidation, however, their associations with learning and longer term retention of different aspects of language remain unclear. We investigated the temporal dynamics of consolidation of vocabulary and grammar, and their links with these sleep metrics. Young adult participants were trained in the evening on an artificial language that used plural inflections with an underlying morpho-phonological regularity that was not taught explicitly. Some of the words were presented frequently and others infrequently. Polysomnographic measures were collected during the night following learning; participants were tested on the vocabulary, trained inflections, and generalisation to untrained words at four time points across nine days. Accuracy on the vocabulary test improved across the first night following learning, and the change was positively associated with SWS duration. Memory for infrequent words declined towards Day 9, but greater spindle density during the first night was associated with a smaller decline. Although mean group accuracy on trained inflections did not significantly change overnight, individually, the change was negatively correlated with spindle density. Generalisation accuracy showed no change across time and no correlations with sleep characteristics. Overall, the results demonstrate that vocabulary and grammar learning have different temporal dynamics of consolidation and distinct patterns of association with sleep metrics. The findings suggest a protective role of spindles for long-term retention of memory, particularly of weakly encoded items, and emphasise the need to dissociate the benefits of SWS from those of spindles.