Sleep Facilitates Problem Solving With No Additional Gain Through Targeted Memory Reactivation

Abstract

According to the active systems consolidation theory, memories undergo reactivation during sleep that can give rise to qualitative changes of the representations. These changes may generate new knowledge such as gaining insight into solutions for problem solving. targeted memory reactivation (TMR) uses learning-associated cues, such as sounds or odors, which have been shown to improve memory consolidation when re-applied during sleep. Here we tested whether TMR during slow wave sleep (SWS) and/or rapid eye movement (REM) sleep increases problem solving. Young healthy volunteers participated in one of two experiments. Experiment 1 tested the effect of natural sleep on problem solving. Subjects were trained in a video game-based problem solving task until being presented with a non-solved challenge. Followed by a ~10-h incubation interval filled with nocturnal sleep or daytime wakefulness, subjects were tested on the problem solving challenge again. Experiment 2 tested the effect of TMR on problem solving, with subjects receiving auditory TMR either during SWS (SWSstim), REM sleep (REMstim), or wakefulness (Wakestim). In Experiment 1, sleep improved problem solving, with 62% of subjects from the Sleep group solving the problem compared to 24% of the Wake group. Subjects with higher amounts of SWS in the Sleep group had a higher chance to solve the problem. In Experiment 2, TMR did not change the sleep effect on problem solving: 56 and 58% of subjects from the SWSstim and REMstim groups solved the problem compared to 57% from the Wakestim group. These findings indicate that sleep, and particularly SWS, facilitates problem solving, whereas this effect is not further increased by TMR.

Keywords: memory; problem solving; sleep; slow wave sleep; targeted memory reactivation.

Figure 1

Experimental design and procedures. (A) In Experiment 1, subjects in the Sleep group practiced the problem-solving task in the evening, spent a whole night of sleep in the lab and were tested on the task again in the morning. The Wake group practiced the problem-solving task in the morning, spent the day awake and was tested in the evening. In Experiment 2, subjects in the SWSstim group and the REMstim group practiced the task in the evening, slept a whole night in the lab with auditory targeted memory reactivation (stimulation) either during SWS or REM sleep and were tested in the next morning. Subjects in the Wakestim group practiced the task in the morning, spent the day awake while they received targeted memory reactivation, and were tested in the evening. (B) Targeted memory reactivation protocols in the SWSstim group (top), the REMstim group (middle) and the Wakestim group (bottom). (C) Visual depiction of the B-SEM task layout (left). The player controls the movement and actions of a character by mouse to reach a floating balloon that will bring him to the next level. To reach this goal the player needs to command the character to interact with different objects and tools in limited 3D scenarios. The task includes four different scenarios with increasing task difficulty, each with four difficulty levels (right). All subjects played four levels during initial practice in fixed order: level 1 of scenario A, level 1 of scenario B, level 1 of scenario C, level 2 of scenario C. Finally, subjects played level 3 of scenario C, representing the problem solving level. None of the subjects in the final analysis solved this level during practice. At testing, all subjects played two levels: the unsolved level 3 of scenario C from the practice session and the new level 3 of scenario D, with the order of levels being counterbalanced across subjects.

Figure 2

Sleep facilitates problem solving. Experiment 1, percentage of subjects who solved the problem after the sleep and wake incubation interval. Experiment 2, percentage of subjects who solved the problem after the incubation interval with targeted memory reactivation during wakefulness (Wakestim), SWS (SWSstim) and REM sleep (REMstim).

Figure 3

Sleep duration in different sleep stages for subjects from the Sleep group separated by those who solved the problem (Solvers, n = 13) and those who did not solve the problem (Non-Solvers, n = 8). Means ± Standard Errors of the Means are presented for Stage 1 sleep, Stage 2 sleep, slow wave sleep (SWS), rapid-eye movement (REM) sleep, Total Sleep Time (TST), and time awake after sleep onset.