Effects of sleep and number of repetitions on novel spoken word learning: fMRI evidence.

The brain mechanisms underlying our ability to learn words are still not well understood. Previous evidence indicates that the newly learnt items acquire lexical status only after a period of sleep, during which cortical consolidation processes are believed to occur. Recent findings, however, indicate that many exposures to a novel word during a short time may lead to a faster formation of cortical memory traces. To investigate the effects of sleep and number of exposures on word acquisition, we presented our volunteers with spoken familiar words and novel pseudowords during two behavioural training sessions taking place on two consecutive days (day1, day2). The number of times each item was repeated (20 vs 150) varied orthogonally to the day-of-training. After the training (on day 2), we used fMRI to measure brain responses to these trained and, as a further control, to previously unheard (untrained) items. We found that brain responses to words and pseudowords in the left STG were differentially modulated by day/training. While word responses were generally smaller and mostly unaffected by training, we found that untrained, day2-trained, and day1-trained pseudowords exhibited increasingly smaller responses (i.e., gradually becoming more and more like real word ones). This is line with previous results, indicating that sleep leads to consolidation of newly acquired representations. When pulling apart data for 20- and 150-times-repeated pseudowords, however, the effects of sleep appeared limited mostly to the latter items, suggesting a critical role for the number of repetitions required to set off consolidation and word learning processes