Can a nap opportunity help protect weak memories from being forgotten?

Details

Each day we encode countless pieces of information, raising the question of how the brain selects memories for long-term storage. Sleep is critical for memory consolidation, and some memories appear to be prioritized for this benefit. Initial memory strength appears to be an important condition in determining which memories are consolidated during sleep. But the role of consolidation-related oscillations, including sleep spindles and slow oscillations, in this preferential consolidation had not been explored.

The researchers had 54 human participants, 76% of whom were female, study word pairs to three distinct encoding strengths. Recall was tested immediately after learning and again six hours later. After learning, 36 participants had a two-hour nap opportunity, while the remaining 18 stayed awake throughout.

Over six hours awake, weakly encoded memories deteriorated fastest. In the nap group, this effect was attenuated: forgetting rates were equivalent across encoding strengths. Compared with staying awake, sleep was associated with better memory for weakly encoded information, suggesting that sleep helps reduce the forgetting of weak memory traces.

Within the nap group, consolidation of weakly encoded items was associated with the density of fast sleep spindles during non-rapid eye movement sleep. Fast sleep spindles are a hallmark oscillation of this sleep stage and mediate consolidation processes. Spindles coupled to slow oscillations predicted consolidation of weak memories independently of uncoupled spindles. These relationships were unique to weakly encoded items: spindles did not correlate with memory for intermediate or strongly encoded items.

The findings suggest that sleep spindles facilitate memory consolidation, guided partly by memory strength. They provide new evidence for preferential sleep-based consolidation and identify a physiological correlate of this benefit.