The Effects of Direct Context Reactivation During Sleep on Memory
Condition: Sleep; Consolidation
Study type: Interventional

Summary: 
      The context in which memories are encoded is a major factor influencing how memories are
      organized. Individual memories are bound to the context (e.g., the location, time and state
      of mind in which they are encoded) and this context is later reinstated to recall the details
      related to the memory. Although the role of context has been explored with regard to memory
      encoding and retrieval, its role during sleep-related memory consolidation has not been
      explored. Memories are thought to be reactivated during sleep, subsequently benefitting from
      the process. This study will use encephalography (EEG) in humans to consider several
      competing hypotheses regarding context's role in sleep reactivation, thereby enhancing the
      current understanding of how reactivation of memory over sleep relates to models of context
      and memory. Participants will learn to associate pictures of scenes to different sounds and
      to smaller images of items and animals, and then learn the spatial locations of these smaller
      images on a grid. Crucially, for half of these scenes, the sounds themselves will then also
      be linked directly to some of images during training. The associated sounds will then be
      unobtrusively presented during sleep, in a manner that has been shown to improve associated
      memories. The subsequent memory benefits will reveal whether (1) all images associated with
      the cued scene will benefit from cuing, demonstrating a context-reactivation effect; (2) only
      the images directly associated with presented sound will benefit from the cuing,
      demonstrating a item-reactivation effect; or (3) some composite of these two models.
      Regardless of which hypothesis is correct, the results will expand our current understanding
      regarding the role context plays in sleep consolidation.
    

Detailed description: 
      Each participant will be run in a single afternoon, which includes a 90-minute nap
      opportunity. Before the nap, participants will go through training and test sessions - and
      after it they will partake of final memory tests. Neural activity will be continuously
      monitored and recorded throughout the task using electrophysiological equipment.

      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-analysis). 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: 16 images of spatial scenes (e.g., a beach) will each be arbitrarily associated with
      a sound and with four smaller images of objects or animals. Half of the scenes will be
      randomly designated to the context-reactivation (CR) condition and half to the
      item-reactivation (IR) condition (see below). The 64 images will each have a unique 2D
      position on a circular grid presented on the screen.

      Training: Participants will first learn to associate each scene with the paired sound up to
      criterion. Next, they will learn to associate the scene with its four images up to criterion.
      The last part of training will include two type of learning blocks that will be interspersed.
      During the spatial-training blocks, in each trial participants will have to place a single
      image in its correct location. They will then receive feedback to improve. The scene
      associated with the image will be presented while they learn, but crucially the sound will
      never be presented for the CR condition scenes. For the IR scenes, the sound will be
      presented while learning two of the items, but never for the other two.

      An alternative design might have divorced the cued items in the CR condition from the scenes
      altogether; the items could have been associated with novel sounds (i.e., that were not
      connected to the scene) and not presented along with their scene. However, using such a
      design would have introduced a confounding factor. The novel sound may have still been
      associated, to an unknown degree, not only to the item but also to the context to which it
      belongs. The degree to which this novel sound would be associated with the context would
      therefore remain uncontrolled and may vary between participants and scenes. Sounds used for
      item in the IR condition are always additionally associated with the scene. By always having
      the sounds be associated both with context and - in the IR condition - additionally to items,
      I substantially reduce any interpretation issues.

      During the Sound-scenes blocks, which do not include a spatial component or the smaller
      images at all, the scenes will be presented with the sounds only for the CR condition scenes
      (i.e., to balance the number of sound presentations between conditions). These blocks will
      repeat in an interleaved manner until each participant will reach the pre-set learning
      criterion on the spatial-task.

      Pre-sleep test: After training, participants will be tested on their spatial-memory for all
      items without exposure to sounds or scenes.

      Sleep: During NREM (non-rapid eye movement) sleep, the sounds associated with half of the CR
      condition scenes and half of the IR condition scenes will be presented unobtrusively. The
      choice of which sounds to present will be made in a manner that will balance pre-sleep
      results and therefore enhance the contrast between sleep-related effects for cued and
      non-cued images.

      Post-sleep test: At least 10 minutes after the end of the nap, participants will undergo a
      test identical to the pre-sleep one. Immediately after, they will be tested on the scene-item
      and scene-sound associations using both a free-recall and a recognition test. Participants
      will then be thanked, debriefed, paid and dismissed.
    

Intervention name(s): Targeted memory reactivation (sounds)
Intervention type(s): Behavioral
Intervention description(s): I will unobtrusively and repeatedly present learning-related sounds during sleep using speakers. This method was shown to improve memory in various tasks. The sounds will be presented several seconds apart and the volume will be so adjusted as not to disturb the participant's sleep. The sounds will be presented during non-rapid eye movement sleep (sleep stage 2 and slow wave sleep). The sounds presented will be non-congruently related to the scenes in the previous learning task. This manipulation is within-subject - all participants will get it, but different specific sounds will be presented for each individual participant.
Intervention model: Single Group Assignment
Masking: None (Open Label)
Primary purpose: Basic Science