The Role of Context in Sleep-related Memory Reactivation in Humans: Reinstatement of Temporal Context During Sleep and Its Subsequent Effect on Memory

Each participant will be run in a single afternoon, which includes a 90-minute nap opportunity. This study will be divided into three continuous sessions. The first and third sessions will be done inside the MRI scanner, whereas the middle session will be conducted outside of the scanner, while recording electrophysiological data. The crucial manipulation involves the careful control of temporal context, which is accomplished by preserving the temporal order of trials during training and using spatial scenes as markers of context.

This is a within-subjects study. The main manipulation is the unobtrusive presentation of sounds during sleep, a technique called targeted memory reactivation (TMR). All participants will hear these sounds, but the specific sounds each one will hear will be different. The results will then be compared within participant, not between different groups or individuals. Appropriate statistical methods for such analyses include paired t-test and repeated measures analysis of variance. The choice of which sounds will be presented to each participant will be made based on their performance in the pre-sleep test. This will be done in an attempt to balance pre-sleep scores between presented and unpresented stimuli to remove statistical noise. Both the participant and the experimenter will be blind to which sounds will be presented, and the selection will be automatically made by the computer. This technique has been extensively used and has no known risks.

There are two main reasons that using a within-subject design reduces the required sample size. First, the lack of a between-subject independent variable intuitively requires less participants. Second, the level of statistical noise due to individual differences is reduced (i.e., because each participant is compared with their own scores). Previous TMR studies, which have found significant cuing effects, commonly used 20-25 participants. I plan to include at least 30 participants in this study, after omitting participants who could not complete the task and those who were not sufficiently cued during sleep. Having 30 participants will allow the use of more powerful statistical methods (in accordance with the common rule of thumb derived from the central limit theorem, which states that means based on sample sizes of more than 30 participants can be assumed to follow a normal distribution). I expect the context-related TMR effect (see summary) to be smaller in magnitude relative to the common effect size observed in spatial learning TMR studies (Hedge's g = 0.39 based on a recent meta-analysis15). This is why I included a higher sample size. It is important to note that even if this benefit will be of a smaller magnitude, as I expect, it will still be indicative of the underlying neurocognitive process and therefore extremely valuable for our mechanistic understanding of the role of context in sleep. Aiming at a sample size of at least 30 participants and assuming an omission rate of 80%, I therefore plan to have 38 participants altogether.

Here is a brief summary of the procedure:

Stimuli: 12 images of spatial scenes (e.g., a beach) will each be arbitrarily associated with five smaller images of objects or animals. As described later, half of the scenes will be associated with a right hand motion and half with the left - and half of each of these groups will be cued during sleep. Each of the 60 images will each have a unique 2D position on a circular grid presented on the screen and will be accompanied by a specific, congruent sound (e.g., cat - meow).