Daytime slow-wave sleep promotes memory consolidation for novel morphology

Morphemes are defined as the smallest meaningful units of a language that can be used to form new words (derivation, e.g., suffix -ness in darkness) or convey grammatical information (inflection, e.g., plural affix -s in houses). Acquiring novel morphology thus represents an essential but also a challenging aspect of language learning. Emerging neurocognitive evidence suggests that memory traces for novel suffixes can be formed even during a brief learning session, as reflected by rapid neural activation enhancement to the newly learnt items. However, the generalized use of the new suffix may require longer time and involve offline consolidation. While recent evidence emphasizes that memory consolidation considerably benefits from even a short period of sleep, there is scarce evidence of daytime napping effects on the neural correlates of novel morphology acquisition. Addressing this was the goal of the present study, in which we recorded ERPs to novel suffixes during the learning process and introduced a daytime nap period into the learning session.
In a within-subject design containing two separate sessions, we trained 21 native Finnish-speaking participants with novel suffixes through a word-picture association task. After this 10-minute training, we used EEG to record brain responses to semantically trained and untrained suffixes, combined with previously unpresented real and pseudoword stems in a 22-minute auditory passive listening task. Additionally, existing Finnish suffixes served as controls. Passive exposure was followed by a 90-minute retention period, containing napping in one session and wakefulness in the other one, after which an 8-minute passive-listening EEG block was repeated.
We compared the ERP responses to trained, untrained, and real suffixes before and after the retention period to investigate the effect of sleep versus wakefulness on the consolidation of novel suffixes. Response amplitudes for semantically trained novel suffixes significantly increased after 90 minutes of napping, but only in cases when slow-wave sleep (SWS) occurred. Furthermore, we found that longer time spent in SWS was associated with a larger response enhancement.
Overall, our results suggest that a short period of daytime sleep can be beneficial to the acquisition of novel suffixes and highlight the importance of SWS for memory consolidation.