Right-lateralized sleep spindles are associated with neutral over emotional bias in picture recognition: An overnight study

Sleep is especially important for emotional memories, although the mechanisms for prioritizing emotional content are insufficiently known. As during waking, emotional processing during sleep may be hemispherically asymmetric; right-lateralized rapid-eye movement (REM) sleep theta (~4–7 Hz) is reportedly associated with emotional memory retention. No research exists on lateralized non-REM sleep oscillations. However, sleep spindles, especially when coupled with slow oscillations (SOs), facilitate off-line memory consolidation. Our primary goal was to examine how the lateralization (right-to-left contrast) of REM theta, sleep spindles, and SO-spindle coupling is associated with overnight recognition memory in a task consisting of neutral and emotionally aversive pictures. Thirty-two healthy adults encoded 150 target pictures before overnight sleep. The recognition of target pictures among foils (discriminability, d’) was tested immediately, 12 hours, and 24 hours after encoding. Recognition discriminability between targets and foils was similar for neutral and emotional pictures in immediate and 12-h retrievals. After 24 hours, emotional pictures were less accurately discriminated (p < 0.001). Emotional difference at 24-h retrieval was associated with right-to-left contrast in frontal fast spindle density (p < 0.001). The lateralization of SO-spindle coupling was associated with higher neutral versus emotional difference across all retrievals (p = 0.004). Our findings contribute to a largely unstudied area in sleep-related memory research. Hemispheric asymmetry in non-REM sleep oscillations may contribute to how neutral versus emotional information is processed. This is presumably underlain by both mechanistic offline memory consolidation and a trait-like cognitive/affective bias that influences memory encoding and retrieval. Methodological choices and participants’ affective traits are likely involved. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.3758/s13415-023-01113-4.