Cross-Night Modulation of Change Detection ERPs to Foreign Speech Sound Features During N2 Sleep

The sleeping brain can discriminate speech sounds, but evidence for cross-night changes in neural responses to foreign speech in adults remains limited. Here, Finnish-speaking adults naïve to tonal languages were exposed during Stage N2 of non-rapid eye movement (NREM) sleep to small and large deviations in tone (pitch contour) embedded in vowel /a/ across four consecutive nights, while electroencephalography was recorded. Event-related potentials (ERPs) P2 and P450 revealed robust change detection responses to tone deviations during sleep, and importantly, P450 amplitude decreased from night 1 to night 4. No evidence of perceptual learning was found in waking change detection ERPs or in behavioural performance measured with the same stimuli before and after the four-night sleep exposure. In conclusion, the sleeping brain remained sensitive to deviations in vowel tone, and the amplitude reduction of the late ERP deflection suggests a change in auditory processing across nights of exposure. However, in the absence of transfer effects to wakeful measures, these findings do not provide evidence for phonetic learning-related plasticity, and the extent to which the effects are speech-specific remains unclear. These findings provide new insight into the dynamics of auditory processing during sleep and contribute to broader neuroscientific understanding of sensory processing in altered states of consciousness.