Sleep-Dependent Consolidation of Probabilistic Sequence Learning in Balanced Bilingual Adults: Behavioral Gains and Spindle Dynamics

Abstract

Sleep is known to support memory consolidation, yet its role in sequence learning among bilingual adults remains undercharacterized. We investigated whether overnight sleep preferentially consolidates probabilistic sequence knowledge in balanced bilinguals and whether spindle dynamics predict behavioral gains. Eighty-four Dutch–English balanced bilinguals completed a serial reaction time task (SRTT) with embedded probabilistic regularities. Participants were randomly assigned to a Sleep group (training in the evening, retest after nocturnal polysomnography) or a Wake group (training in the morning, retest after equivalent daytime interval). Sequence-specific learning was defined as the reaction-time (RT) advantage for high-probability triplets relative to low-probability triplets. Mixed-effects models revealed greater overnight consolidation in the Sleep group (estimate −12.4 ms, SE 3.9, p<.001), corresponding to a medium effect (d=0.56). Within the Sleep group, fast spindle density over central sensors predicted offline improvement (β=0.31, p=.004) controlling for baseline performance, age, and sleep duration. Cross-context generalization from an L1 to L2 cue set (or vice versa) was robust and did not differ between groups. Exploratory phase–amplitude coupling analyses indicated increased N2 spindle–theta coupling preceding trials that later showed the largest gains (p=.021). Findings suggest that bilingual adults exhibit sleep-dependent strengthening of probabilistic sequence representations, with spindles indexing systems-level plasticity consistent with cortico-striatal consolidation.