Coordination of slow waves with sleep spindles predicts sleep-dependent memory consolidation in schizophrenia

Study Objectives:
Schizophrenia patients have correlated deficits in sleep spindle density and sleep-dependent memory consolidation. In addition to spindle density, memory consolidation is thought to rely on the precise temporal coordination of spindles with slow waves (SWs). We investigated whether this coordination is intact in schizophrenia and its relation to motor procedural memory consolidation.
Methods:
Twenty-one chronic medicated schizophrenia patients and 17 demographically matched healthy controls underwent two nights of polysomnography, with training on the finger tapping motor sequence task (MST) on the second night and testing the following morning. We detected SWs (0.5–4 Hz) and spindles during non-rapid eye movement (NREM) sleep. We measured SW–spindle phase–amplitude coupling and its relation with overnight improvement in MST performance.
Results:
Patients did not differ from controls in the timing of SW–spindle coupling. In both the groups, spindles peaked during the SW upstate. For patients alone, the later in the SW upstate that spindles peaked and the more reliable this phase relationship, the greater the overnight MST improvement. Regression models that included both spindle density and SW–spindle coordination predicted overnight improvement significantly better than either parameter alone, suggesting that both contribute to memory consolidation.
Conclusion:
Schizophrenia patients show intact spindle–SW temporal coordination, and these timing relationships, together with spindle density, predict sleep-dependent memory consolidation. These relations were seen only in patients suggesting that their memory is more dependent on optimal spindle–SW timing, possibly due to reduced spindle density. Interventions to improve memory may need to increase spindle density while preserving or enhancing the coordination of NREM oscillations.