A more generalized fear response after a daytime nap

Sleep deprivation increases the generalization of perceptual and concept-based fear: An fNIRS study

Insufficient sleep can initiate or exacerbate anxiety by triggering excessive fear generalization. In this study, a de novo paradigm was developed and used to examine the neural mechanisms governing the effects of sleep deprivation on processing perceptual and concept-based fear generalizations. A between-subject design was adopted, wherein a control group (who had a typical night's sleep) and a one-night sleep deprivation group completed a fear acquisition task at 9:00 PM on the first day and underwent a generalization test the following morning at 7:00 AM. In the fear acquisition task, navy blue and olive green were used as perceptual cues (P+ and P-, respectively), while animals and furniture items were used as conceptual cues (C+ and C-, respectively). Generalization was tested for four novel generalized categories (C+P+, C+P-, C-P+, and C-P-). Shock expectancy ratings, skin conductance responses, and functional near-infrared spectroscopy were recorded during the fear acquisition and generalization processes. Compared with the group who had a typical night's sleep, the sleep deprived group showed higher shock expectancy ratings (especially for P+ and C-), increased oxygenated hemoglobin in the dorsolateral prefrontal cortex, and increased activation in the triangular inferior frontal gyrus during the generalization test. These findings suggest that sleep deprivation increases the generalization of threat memories, thus providing insights into the overgeneralization characteristics of anxiety and fear-related disorders.

Fear memory in humans is consolidated over time independently of sleep

Fear memories can be altered after acquisition by processes, such as fear memory consolidation or fear extinction, even without further exposure to the fear-eliciting stimuli, but factors contributing to these processes are not well understood. Sleep is known to consolidate, strengthen, and change newly acquired declarative and procedural memories. However, evidence on the role of time and sleep in the consolidation of fear memories is inconclusive. We used highly sensitive electrophysiological measures to examine the development of fear-conditioned responses over time and sleep in humans. We assessed event-related brain potentials (ERP) in 18 healthy, young individuals during fear conditioning before and after a 2-hour afternoon nap or a corresponding wake interval in a counterbalanced within-subject design. The procedure involved pairing a neutral tone (CS+) with a highly unpleasant sound. As a control, another neutral tone (CS-) was paired with a neutral sound. Fear responses were examined before the interval during a habituation phase and an acquisition phase as well as after the interval during an extinction phase and a reacquisition phase. Differential fear conditioning during acquisition was evidenced by a more negative slow ERP component (stimulus-preceding negativity) developing before the unconditioned stimulus (loud noise). This differential fear response was even stronger after the interval during reacquisition compared with initial acquisition, but this effect was similarly pronounced after sleep and wakefulness. These findings suggest that fear memories are consolidated over time, with this effect being independent of intervening sleep.

The Impact of Sleep on Fear Extinction

Sleep plays a crucial role in the consolidation of memories, including those for fear acquisition and extinction training. This chapter reviews findings from studies testing this relationship in laboratory, naturalistic, and clinical settings. While evidence is mixed, several studies in humans have linked fear and extinction recall/retention to both rapid eye-movement and slow wave sleep. Sleep appears to further aid in the processing of both simulated and actual trauma and improves psychotherapeutic treatment outcomes in those with anxiety and trauma- and stressor-related disorders. This chapter concludes with a discussion of the current challenges facing sleep and emotional memory research in addition to suggestions for improving future research.

Sex differences in the effect of subjective sleep on fear conditioning, extinction learning, and extinction recall in individuals with a range of PTSD symptom severity

Sleep has been found to play a key role in fear conditioning, extinction learning and extinction recall, and sleep disturbances are linked to many mental disorders including post-traumatic stress disorder (PTSD). Previous studies examining associations between sleep and fear or extinction processes primarily focused on objectively measured sleep architecture. Little research has so far focused on subjective sleep measures and particularly in clinical populations, which often experience subjectively poor sleep, including PTSD. Here we investigated whether subjective sleep disturbance, sleep onset latency, wake after sleep onset or sleep efficiency were related to fear conditioning, extinction learning or extinction recall in a large sample of individuals with a range of PTSD symptom severity (n = 248). Overall, we did not find that subjective sleep was associated with fear conditioning or extinction processes. However, exploratory analyses examining the moderating effect of sex found that shorter sleep onset latency and greater sleep efficiency were associated with improved extinction recall in women with higher PTSD symptom severity. This suggests that less time falling asleep and longer time asleep while in bed may be protective in highly symptomatic women against the commonly observed impaired extinction recall in PTSD. More studies are needed to explore sex-specific effects further.

The influence of sleep on fear extinction in trauma-related disorders

In Posttraumatic Stress Disorder (PTSD), fear and anxiety become dysregulated following psychologically traumatic events. Regulation of fear and anxiety involves both high-level cognitive processes such as cognitive reattribution and low-level, partially automatic memory processes such as fear extinction, safety learning and habituation. These latter processes are believed to be deficient in PTSD. While insomnia and nightmares are characteristic symptoms of existing PTSD, abundant recent evidence suggests that sleep disruption prior to and acute sleep disturbance following traumatic events both can predispose an individual to develop PTSD. Sleep promotes consolidation in multiple memory systems and is believed to also do so for low-level emotion-regulatory memory processes. Consequently sleep disruption may contribute to the etiology of PTSD by interfering with consolidation in low-level emotion-regulatory memory systems. During the first weeks following a traumatic event, when in the course of everyday life resilient individuals begin to acquire and consolidate these low-level emotion-regulatory memories, those who will develop PTSD symptoms may fail to do so. This deficit may, in part, result from alterations of sleep that interfere with their consolidation, such as REM fragmentation, that have also been found to presage later PTSD symptoms. Here, sleep disruption in PTSD as well as fear extinction, safety learning and habituation and their known alterations in PTSD are first briefly reviewed. Then neural processes that occur during the early post-trauma period that might impede low-level emotion regulatory processes through alterations of sleep quality and physiology will be considered. Lastly, recent neuroimaging evidence from a fear conditioning and extinction paradigm in patient groups and their controls will be considered along with one possible neural process that may contribute to a vulnerability to PTSD following trauma.

The association between mnemonic discrimination ability and differential fear learning

BACKGROUND AND OBJECTIVES

It is important to be able to learn which stimuli in our surroundings predict aversive outcomes. To maintain emotional well-being, it is similarly important to be able to learn which stimuli predict safety. The ability to discriminate between stimuli that predict danger and safety has been suggested to not only have an emotional component, but also a cognitive one. One such candidate mechanism is mnemonic discrimination (MD), the ability to differentiate between two memories that are similar but not identical. In the present study, we wanted to examine if MD performance helps to explain inter-individual differences in the ability to acquire a differentiated fear response during fear conditioning.

METHODS

Participants performed a task assessing MD ability, and then underwent a fear conditioning procedure. Fear responses were measured using skin conductance responses (SCRs).

RESULTS

Results revealed no support for MD ability being associated with to which degree a differentiated fear response was acquired, or with the time needed to acquire such a response.

LIMITATIONS

Our only outcome measurement was SCRs. Future studies need to include fear ratings, expectancy ratings and neural responses. Future studies also need to examine this using a stimulus material where the conditioned stimulus and the safety stimulus are more difficult to distinguish from each other.

CONCLUSIONS

If MD ability has a role in inhibiting overgeneralization of fear learning, this does not seem to be driven by MD already during the initial learning.

Sleep Deprivation During Memory Consolidation, but Not Before Memory Retrieval, Widens Threat Generalization to New Stimuli

Past aversive experiences shape our ability to deal with future dangers, through the encoding of implicit and explicit memory traces and through the ability to generalize defensive reactions to new stimuli resembling learned threats. Numerous evidence demonstrate that sleep is important for the consolidation of memories related to threatening events. However, there is a lack of studies examining the effects of sleep deprivation on the retrieval of consolidated threat memories, and previous studies on the role of sleep in threat generalization have produced mixed results. To address these issues, here we adopted a differential threat conditioning and a delayed (second half of the night) sleep deprivation during the first or the seventh night after learning. We found no effects of sleep deprivation on either implicit or explicit threat memories, regardless of its occurrence timing. Conversely, implicit but not explicit responses to novel cues similar to a learned threat displayed a widened generalization pattern, but only if sleep deprivation took place during the first night after conditioning and not if it occurred during the seventh night after conditioning. Therefore, we propose that sleeping after exposure to danger may support optimal implicit discrimination processes to evaluate new signals in the future and that even a brief period of sleeplessness may widen threat generalization to new stimuli, which is a hallmark of several threat-related disorders.

Influence of Perceptual and Conceptual Information on Fear Generalization: A Behavioral and Event-Related Potential Study

Learned fear can be generalized through both perceptual and conceptual information. This study investigated how perceptual and conceptual similarities influence this generalization process. Twenty-three healthy volunteers completed a fear-generalization test as brain activity was recorded in the form of event-related potentials (ERPs). Participants were exposed to a de novo fear acquisition paradigm with four categories of conditioned stimuli (CS): two conceptual cues (animals and furniture); and two perceptual cues (blue and purple shapes). Animals (C+) and purple shapes (P+) were paired with the unconditioned stimulus (US), whereas furniture (C-) and blue shapes (P-) never were. The generalized stimuli were thus blue animals (C+P+, determined danger), blue furniture (C-P+, perceptual danger), purple animals (C+P-, conceptual danger), and purple furniture (C-P-, determined safe). We found that perceptual cues elicited larger fear responses and shorter reaction times than did conceptual cues during fear acquisition. This suggests that a perceptually related pathway might evoke greater fear than a conceptually based route. During generalization, participants were more afraid of C+ exemplars than of C- exemplars. Furthermore, C+ trials elicited greater N400 amplitudes. Thus, participants appear able to use conceptually based cues to infer the value of the current stimuli. Additionally, compared with C+ exemplars, we found an enhanced late positive potential effect in response to C- exemplars, which seems to reflect a late inhibitory process and might index safety learning. These findings may offer new insights into the pathological mechanism of anxiety disorders.

Sleep to remember, sleep to forget: Rapid eye movement sleep can have inverse effects on recall and generalization of fear memories

Rapid Eye Movement (REM) sleep has been shown to modulate the consolidation of fear memories, a process that may contribute to the development of Post-Traumatic Stress Disorder (PTSD). However, contradictory findings have been reported regarding the direction of this modulation and its differential effects on recall versus generalization. In two complementary experiments, we addressed this by employing sleep deprivation protocols together with a novel fear-conditioning paradigm that required the discrimination between coexisting threat and safety signals. Using skin conductance responses and functional imaging (fMRI), we found two opposing effects of REM sleep: While REM impaired recall of the original threat memories, it improved the ability to generalize these memories to novel situations that emphasized the discrimination between threat and safety signals. These results, as well as previous findings in healthy participants and patients diagnosed with PTSD, could be explained by the degree to which the balance between threat and safety signals for a given stimulus was predictive of threat. We suggest that this account can be integrated with contemporary theories of sleep and fear learning, such as the REM recalibration hypothesis.

A daytime nap does not increase mnemonic discrimination ability

It has been proposed that sleep readies the brain for novel learning, and previous work has shown that sleep loss impairs the ability to encode new memories. In the present study, we examined if a daytime nap would increase mnemonic discrimination (MD) performance. MD is the ability to differentiate between memories that are similar but not identical. Participants performed the Mnemonic Similarity Task (MST) twice, once in the morning and once in the afternoon. The goal of this task is to distinguish stimuli that have been seen before from novel stimuli that are similar but not identical. After the morning MST, participants were randomly allocated into either a sleep or a wake group. The sleep group had a 2-hr nap opportunity, whereas the wake group spent a similar amount of time passively resting. All participants then performed a second MST in the afternoon with a novel set of images. Results did not show any support for increased MD ability after a nap. There was, however, a correlation showing that an increase in sleepiness between sessions predicted a decrease in MD performance. Future work must systematically examine how strong sleep manipulations that are needed for sleep to have an effect on encoding ability, as well as which kind of memory tasks that are sensitive to sleep manipulations. More knowledge about the relationship between sleep and the ability to differentiate similar memories from each other is important because impaired MD ability has previously been reported in various groups in which sleep disturbances are also common.

Sleep deprivation increases threat beliefs in human fear conditioning

Sleep disturbances and anxiety disorders exhibit high comorbidity levels, but it remains unclear whether sleep problems are causes or consequences of increased anxiety. To experimentally probe the aetiological role of sleep disturbances in anxiety, we investigated in healthy participants how total sleep deprivation influences fear expression in a conditioning paradigm. In a fear conditioning procedure, one face stimulus (conditioned stimulus [CS+]) was paired with electric shock, whereas another face stimulus was not (unpaired stimulus [CS-]). Fear expression was tested the next morning using the two face stimuli from the training phase and a generalization stimulus (i.e. a morph between the CS+ and CS- stimuli). Between fear conditioning and test, participants were either kept awake in the laboratory for 12 hr (n = 20) or had a night of sleep at home (n = 20). Irrespective of stimulus type, subjective threat expectancies, but not skin conductance responses, were enhanced after sleep deprivation, relative to regular sleep. These results suggest that sleep disturbances may play a role in anxiety disorders by increasing perceived threat.

The fluctuations of metabotropic glutamate receptor subtype 5 (mGluR5) in the amygdala in fear conditioning model of male Wistar rats following sleep deprivation, reverse circadian and napping

Sleep is involved in metabolic system, mental health and cognitive functions. Evidence shows that sleep deprivation (SD) negatively affects mental health and impairs cognitive functions, including learning and memory. Furthermore, the metabotropic glutamate receptor subtype 5 (mGluR5) is a metabolic biomarker, which is affected by various conditions, including stress, sleep deprivation, and cognitive and psychiatric disorders. In this research, we investigated the effect of SD and reverse circadian (RC), and two models of napping (continuous and non-continuous) combined with SD or RC on fear-conditioning memory, anxiety-like behavior and mGluR5 fluctuations in the amygdala. 64 male Wistar rats were used in this study. The water box apparatus was used to induce SD/RC for 48 h, and fear-conditioning memory apparatus was used to assess fear memory. The results showed, fear-conditioning memory was impaired following SD and RC, especially in contextual stage. However, anxiety-like behavior was increased. Furthermore, mGluR5 was increased in the left amygdala more than the right amygdala. Additionally, continuous napping significantly improved fear-conditioning memory, especially freezing behavior. In conclusion, following SD and RC, fear-conditioning memory in contextual stage is more vulnerable than in auditory stage. Furthermore, increase in anxiety-like behavior is related to increase in the activity of left amygdala and mGluR5 receptors.

The role of sleep in fear learning and memory

During the last 10 years, a large body of studies have used fear conditioning paradigms to study the role of sleep in the consolidation of fear and safety learning. This line of research could allow us to answer if it is adaptive or not to sleep in the aftermath of a negative experience, and if sleep has a role in consolidating extinction learning. This field has so far produced several contrasting findings. Thus, this review will not deliver many clear conclusions, but will instead be an attempt to summarize what we know at the moment, to describe the potential clinical applications of this research, and to discuss where to go from here.