Background: Schizophrenia is a neurodegenerative disorder with immense cognitive, psychosocial, and economic costs [Antonucci 2019, Insel 2009, Cloutier 2016]. Cognitive effects and memory consolidation impairments in schizophrenia have been linked to disrupted sleep oscillations [Manoach 2019,Mylonas 2020,Manoach 2014, Wamsley 2012, Goder 2015].
Cortical slow oscillations, sleep spindles, and hippocampal ripples have all been implicated in sleep-dependent memory consolidation. However, currently there are no neuroimaging methods able to capture the transient, subcortical nature of hippocampal ripples in humans. By instead using invasive recordings in epilepsy patients, we will demonstrate, for the first time in humans, that hippocampal ripples coupled with slow oscillations and spindles are associated with memory consolidation.
Methods: We investigate overnight memory consolidation by asking patients to perform a well- validated procedural memory task, the motor sequence task (MST), prior to and following a night of sleep (“memory night”). We then examine sleep-dependent memory consolidation (as indicated by task performance post-sleep relative to initial performance) in relation to sleep spindles and their co-occurrence with hippocampal ripples and slow oscillations.
Results: Our preliminary analysis showed increased ripple density, amplitude, and peak frequency in the memory night compared to a baseline night in the two patients with completed ripple analysis. An additional four patients await analysis but show detectable hippocampal ripples.
Conclusions: Altered ripple composition in memory versus baseline nights implies that increased ripple density, along with spindles and slow oscillations, may play a role in memory consolidation in humans. As we collect more data, we seek to determine how ripples are coordinated with spindles and slow oscillations and whether coupled oscillations correlate with sleep-dependent memory consolidation in humans. This will help determine if coupled oscillations form a better biomarker and future treatment target in patients with schizophrenia than sleep spindles alone.