Large-scale structure and individual fingerprints of locally coupled sleep oscillations
Slow oscillations and sleep spindles, the canonical electrophysiological oscillations of nonrapid eye movement sleep, are thought to gate incoming sensory information, underlie processes of sleep-dependent memory consolidation, and are altered in various neuropsychiatric disorders. Accumulating evidence of the predominantly local expression of these individual oscillatory rhythms suggests that their cross-frequency interactions may have a similar local component. However, it is unclear whether
... ocally coordinated sleep oscillations exist across the cortex, and whether and how these dynamics differ between fast and slow spindles, and sleep stages. Moreover, substantial individual variability in the expression of both spindles and slow oscillations raises the possibility that their temporal organization shows similar individual differences. Using two nights of multichannel electroencephalography recordings from 24 healthy individuals, we characterized the topography of slow oscillation-spindle coupling. We found that while slow oscillations are highly restricted in spatial extent, the phase of the local slow oscillation modulates local spindle activity at virtually every cortical site. However, coupling dynamics varied with spindle class, sleep stage, and cortical region. Moreover, the slow oscillation phase at which spindles were maximally expressed differed markedly across individuals while remaining stable across nights. These findings both add an important spatial aspect to our understanding of the temporal coupling of sleep oscillations and demonstrate the heterogeneity of coupling dynamics, which must be taken into account when formulating mechanistic accounts of sleep-related memory processing. Statement of Significance Nonrapid eye movement sleep is a state marked by unique brain rhythms, most notably slow oscillations and both fast and slow sleep spindles. These neural oscillations interact such that spindles preferentially occur in a specific phase of the slow oscillation, possibly underlying the reorganization of memories. Importantly, this oscillatory coupling has been presumed to be similar across brain areas and individuals. We found that the precise phase of the slow oscillation at which spindles are expressed varies considerably from person to person, but also with spindle class (slow or fast), sleep stage, and brain region. These findings provide new insights into how oscillatory activity of the sleeping brain is organized and how this may relate to sleep-dependent memory processing.