Menz, M.M., Rihm, J.S., Salari, N., et al. (2013). The role of sleep and sleep deprivation in consolidating fear memories. NeuroImage, 75(1): 87-96.
Research questions: What is the role of sleep and sleep deprivation on the consolidation of fear memories and neuronal mechanisms?
Significance of the study: Due to the critical contribution of fear learning mechanisms to anxiety disorders and the involvement of the amygdala in transforming stressful events into fear memories and anxiety, this study determines the possible influence of sleep and sleep deprivation on the consolidation of fear memories.
Hypotheses: Primary hypothesis: Sleep and sleep deprivation affects fear memory consolidation. Secondary hypotheses: (1) Sleep deprivation affects anxiety and shock expectancy ratings; (2) Sleep deprivation affects skin conductance responses; (3) Neural correlates are involved in successful recall.
Research design: Randomized cohort design
Sample population: The participants are 40 healthy right-handed, males, ages 20 to 35 years (mean: 24.90±3.76). They have normal or corrected-to-normal vision with no self-reported history of psychiatric or neurological disorders. No interest has been shown on the ethnic characteristics of the participants.
Variables: Independent variables: sleep (control variable) and sleep deprivation (intervention variable); Pavlovian conditioning with and without immediate extinction. Dependent variables: anxiety and shock expectancy ratings; skin conductance responses.
A. Selection and preparation: Two to three days prior to the intervention date, all participants are oriented about the procedures, screened by medical questionnaire and physician, and gave written informed consent.
B. Sample assignment: Participants were randomly assigned to one of two sleep intervention groups: sleep group (spent the night after fear conditioning asleep) and wake group (spent the night after fear conditioning awake). The assignment was unknown to the participants until the beginning of the sleep intervention.
C. Intervention: Day 1: The participants underwent Pavlovian fear conditioning with and without immediate extinction (light electrical shock). Then they either slept or remained awake in the night immediately following fear learning. Recall performance was conducted to obtain baseline data. Day 2: All participants slept at home at night (recovery sleep) without sleep monitoring except for the no-nap restriction. Participants in the sleep group were prepared for standard polysomnographic recordings (e.g. of EEG, EOG, and EMG). They were allowed to watch a movie or read before lights turned off (between 11 and 12 p.m.) for ad libitum sleep. Participants in the wake group follow standard activities at night (reading, watching a movie, or playing card games) until 10 a.m. Day 3: Conditioning protocol was repeated without extinction across participants. Recall performance was tested using explicit memory testing (shock expectancy ratings), autonomous responses (skin conductance) and brain activation (functional magnetic resonance imaging). The study provides a detailed procedure of the experimental protocol in the article.
Results: Sleep consolidates fear memories better than sleep deprivation. This means that learned fear are recalled better among those who slept at night than those who did not sleep.
III. Personal reaction
Like or dislike: I like the rich detail in the literature review, which allows readers to understand easily the theoretical bases of the study, and in the methods, which makes it easier for other researchers to replicate the study in the future. The literature review found in the Introduction involved highly organized writing, which started by explaining the significance of emotional value of memories for better recall. It concisely moved to the neurology of fear and fear memory; then, the strong influence of sleep on memory consolidation. In less than a page long, it moved from the theoretical background of the study and into the rationale. Similarly, the methodology is well-organized and crystal clear both in its writing and its details, from participant screening to protocol implementation. The protocol itself is complex and broad. However, the authors managed to make it highly understandable and repeatable.
Personal thoughts of the results and conclusions: I agree with the results and conclusions as discussed. Moreover, the main result surprised me. I used presumed that people becomes more vulnerable to fear memories when they are more sleep deprived than well-slept. The study showed the opposite: recall of fear memories is better among participants who had slept after their exposure to fear-generating stimuli (sleep). Although counterintuitive to me, I can understand the underlying logic, which indicates that sleep deprivation tends to cause ineffective memory consolidation. It has practical implication though. Sleep deprivation can help modulate the negativity of fear exposure, allowing the mind to forget these fearful stimuli.
Confusing or difficult: Nothing in the article that is particularly confusing to or difficult to understand at regular reading speed.
Questions or other thoughts: The most important thought that emerged from the review of this study is strangely how sleep deprivation can defeat unwanted fear stimuli.