Degradation of cortical representations during encoding following sleep deprivation

Altered hypothalamic functional connectivity after partial sleep deprivation in young and elderly adults

BACKGROUND

The hypothalamus plays a critical role in sleep-wake regulation and attention control. However, few studies explored the alterations of hypothalamic functional connectivity after sleep deprivation. The purpose of the study is to investigate the influence of partial sleep deprivation on hypothalamic functional connectivity in young and elderly adults and to determine whether age modulates the interactions between partial sleep deprivation and hypothalamic functional connectivity.

METHODS

Data for this study were collected as part of the Stockholm Sleepy Brain Project. Forty-one young adults (aged 20-30) and thirty-six elderly adults (aged 65-75) were finally recruited in the study. Participants underwent resting-state functional magnetic resonance imaging (rs-fMRI) scans following full sleep and partial sleep deprivation (3 h of sleep) in a crossover design. Seed-based functional connectivity analyses were performed for the bilateral hypothalamus using rs-fMRI data.

RESULTS

For young adults, partial sleep deprivation caused enhanced hypothalamic functional connectivity with the inferior frontal gyrus and precuneus. For elderly adults, reduced functional connectivity was observed between the hypothalamus and frontal, parietal, temporal, occipital regions, and cingulate gyrus following partial sleep deprivation. Further, a significant interactive effect between age and partial sleep deprivation on the hypothalamic functional connectivity was observed. Age-related abnormalities of hypothalamic functional connectivity were observed in frontoparietal regions, pallidum, rectus, and superior occipital gyrus.

CONCLUSION

Partial sleep deprivation led to increased hypothalamic functional connectivity in young adults, while decreased hypothalamic functional connectivity in elderly adults. Our results indicate that age modulates the influence of sleep deprivation on intrinsic brain functional connectivity.

Sleep deprivation-induced memory impairment: exploring potential interventions

Sleep's crucial role in memory consolidation is well-established, with neuroimaging and sleep stage analysis revealing the intricate processes involved. Sleep deprivation significantly impairs memory performance and the ability to form new memories, highlighting the need for effective countermeasures. This article concludes that while sleep deprivation significantly impairs memory, the emerging insights into the gut-brain axis offer a promising frontier for developing novel interventions that can mitigate these effects. The review discusses various interventions, ranging from pharmaceutical compounds like donepezil, memantine, and tolcapone, to innovative physical therapy techniques such as transcranial magnetic stimulation (TMS), deep brain stimulation (DBS), and transcranial direct current stimulation (tDCS). Additionally, the emerging role of the gut-brain axis in sleep deprivation-induced memory impairment is examined, shedding light on the complex interplay between gut microbiota and cognitive functions. This comprehensive review explores the multifaceted relationship between sleep deprivation and memory impairment, delving into the neurobiological mechanisms underlying these processes and examining potential interventions.

Hippocampal convergence during anticipatory midbrain activation promotes subsequent memory formation

The hippocampus has been a focus of memory research since H.M's surgery abolished his ability to form new memories, yet its mechanistic role in memory remains debated. Here, we identify a candidate memory mechanism: an anticipatory hippocampal "convergence state", observed while awaiting valuable information, and which predicts subsequent learning. During fMRI, participants viewed trivia questions eliciting high or low curiosity, followed seconds later by its answer. We reasoned that encoding success requires a confluence of conditions, so that hippocampal states more conducive to memory formation should converge in state space. To operationalize convergence of neural states, we quantified the typicality of multivoxel patterns in the medial temporal lobes during anticipation and encoding of trivia answers. We found that the typicality of anticipatory hippocampal patterns increased during high curiosity. Crucially, anticipatory hippocampal pattern typicality increased with dopaminergic midbrain activation and uniquely accounted for the association between midbrain activation and subsequent recall. We propose that hippocampal convergence states may complete a cascade from motivation and midbrain activation to memory enhancement, and may be a general predictor of memory formation.

Covering the Gap Between Sleep and Cognition – Mechanisms and Clinical Examples

A growing number of studies have shown the strong relationship between sleep and different cognitive processes, especially those that involve memory consolidation. Traditionally, these processes were attributed to mechanisms related to the macroarchitecture of sleep, as sleep cycles or the duration of specific stages, such as the REM stage. More recently, the relationship between different cognitive traits and specific waves (sleep spindles or slow oscillations) has been studied. We here present the most important physiological processes induced by sleep, with particular focus on brain electrophysiology. In addition, recent and classical literature were reviewed to cover the gap between sleep and cognition, while illustrating this relationship by means of clinical examples. Finally, we propose that future studies may focus not only on analyzing specific waves, but also on the relationship between their characteristics as potential biomarkers for multiple diseases.

Altered insula-prefrontal functional connectivity correlates to decreased vigilant attention after total sleep deprivation

BACKGROUND

Sleep deprivation can robustly affect vigilant attention. The insula is a key hub of the salience network that mediates shifting attention between endogenous and exogenous states. However, little is known regarding the involvement of insular functional connectivity in impaired vigilant attention after total sleep deprivation (TSD). The purpose of this study is to explore the alterations in insular functional connectivity and its association with vigilant attention performance following TSD.

METHODS

Twenty-six adult men were enrolled in the study. Participants underwent two counterbalanced resting-state functional magnetic resonance imaging (rs-fMRI) scans, once in rested wakefulness (RW) and once after 36 h of TSD. Seed-based functional connectivity analysis was performed using rs-fMRI data for the left and right insula. The vigilant attention was measured using a psychomotor vigilance test (PVT). Furthermore, Pearson correlation analysis was conducted to investigate the relationship between altered insular functional connectivity and PVT performance.

RESULTS

Compared to RW, enhanced functional connectivity was observed between the insula and prefrontal cortex and anterior cingulate cortex, while reduced functional connectivity was observed between the insula and temporal, parietal, and occipital regions following TSD. Moreover, altered insular functional connectivity with the prefrontal cortex, ie superior frontal gyrus and middle frontal gyrus, and inferior temporal gyrus was correlated with PVT performance after TSD.

CONCLUSION

Our results suggest that insular coupling with the prefrontal cortex and inferior temporal gyrus may act as neural indicators for vigilant attention impairment, which further reveals the critical role of the salience network in cognitive decline following TSD.

Sleep Deprivation Impairs Binding of Information with Its Context

Binding information to its context in long-term memory is critical for many tasks, including memory tasks and decision making. Failure to associate information to its context could be an important aspect of sleep deprivation effects on cognition, but little is known about binding problems from being sleep-deprived at the time of encoding. We studied how sleep deprivation affects binding using a well-established paradigm testing the ability to remember auditorily presented words (items) and their speakers (source context). In a laboratory study, 68 healthy young adults were randomly assigned to total sleep deprivation or a well-rested control condition. Participants completed an affective item and source memory task twice: once after 7h awake during baseline and again 24h later, after nearly 31h awake in the total sleep deprivation condition or 7h awake in the control condition. Participants listened to negative, positive, and neutral words presented by a male or female speaker and were immediately tested for recognition of the words and their respective speakers. Recognition of items declined during sleep deprivation, but even when items were recognized accurately, recognition of their associated sources also declined. Negative items were less bound with their sources than positive or neutral items,but sleep deprivation did not significantly affect this pattern.Our findings indicate that learning while sleep-deprived disrupts the binding of information to its context independent of item valence. Such binding failures may contribute to sleep deprivation effects on tasks requiring the ability to bind new information together in memory.

A daytime nap restores hippocampal function and improves declarative learning

STUDY OBJECTIVES

Daytime naps can confer benefits on subsequent declarative learning, but the physiological correlates of this improvement are less well studied. We examined learning following a daytime nap compared with an equivalent waking period using fMRI and polysomnography.

METHODS

Forty healthy young adults who slept normally the previous night encoded word pair lists in an MRI scanner at 13:00 and 16:30. Between sessions, participants either stayed awake and watched a documentary (Wake Group; N = 20) or had a 90-minute nap opportunity (Nap Group; N = 20) monitored by polysomnography. Approximately 40 minutes after completing each encoding session, memory for learned words was assessed using cued-recall.

RESULTS

A significant Session × Group interaction effect (p < 0.001) was observed in which memory was significantly improved in the Nap but not in the Wake group (p < 0.001). There was also a Session × Run × Group interaction effect in the left hippocampus (p = 0.001), whereby activation during word pair encoding increased only following the nap. Both performance improvement (rs = 0.46, p = 0.04) and nap-related increase in hippocampal activation (rs = 0.46, p = 0.04) were correlated with nap spindle count (12-15 Hz) but not with slow oscillation power (p's ≥ 0.18).

CONCLUSIONS

After a habitual nocturnal sleep, participants who had a 90-minute afternoon nap encoded word pairs better than a comparable group who stayed awake. Increases in hippocampal activation following the nap suggest restored hippocampal function. Naptime spindles may contribute to improved memory.

Decreased P2 Waveform Reflects Impaired Brain Executive Function Induced by 12 h of Low Homeostatic Sleep Pressure: Evidence From an Event-Related Potential Study

Homeostatic sleep pressure can cause cognitive impairment, in which executive function is the most affected. Previous studies have mainly focused on high homeostatic sleep pressure (long-term sleep deprivation); thus, there is still little related neuro-psycho-physiological evidence based on low homeostatic sleep pressure (12 h of continuous wakefulness) that affects executive function. This study aimed to investigate the impact of lower homeostatic sleep pressure on executive function. Our study included 14 healthy young male participants tested using the Go/NoGo task in normal resting wakefulness (10:00 am) and after low homeostatic sleep pressure (10:00 pm). Behavioral data (response time and accuracy) were collected, and electroencephalogram (EEG) data were recorded simultaneously, using repeated measures analysis of variance for data analysis. Compared with resting wakefulness, the participants' response time to the Go stimulus was shortened after low homeostatic sleep pressure, and the correct response rate was reduced. Furthermore, the peak amplitude of Go-P2 decreased significantly, and the peak latency did not change significantly. For NoGo stimulation, the peak amplitude of NoGo-P2 decreased significantly (p < 0.05), and the peak latency was significantly extended (p < 0.05). Thus, the P2 wave is likely related to the attention and visual processing and reflects the early judgment of the perceptual process. Therefore, the peak amplitude of Go-P2 and NoGo-P2 decreased, whereas the peak latency of NoGo-P2 increased, indicating that executive function is impaired after low homeostatic sleep pressure. This study has shown that the P2 wave is a sensitive indicator that reflects the effects of low homeostatic sleep pressure on executive function, and that it is also an important window to observe the effect of homeostatic sleep pressure and circadian rhythm on cognitive function.

Splitting sleep between the night and a daytime nap reduces homeostatic sleep pressure and enhances long-term memory

Daytime naps have been linked with enhanced memory encoding and consolidation. It remains unclear how a daily napping schedule impacts learning throughout the day, and whether these effects are the same for well-rested and sleep restricted individuals. We compared memory in 112 adolescents who underwent two simulated school weeks containing 8 or 6.5 h sleep opportunities each day. Sleep episodes were nocturnal or split between nocturnal sleep and a 90-min afternoon nap, creating four experimental groups: 8 h-continuous, 8 h-split, 6.5 h-continuous and 6.5 h-split. Declarative memory was assessed with picture encoding and an educationally realistic factual knowledge task. Splitting sleep significantly enhanced afternoon picture encoding and factual knowledge under both 6.5 h and 8 h durations. Splitting sleep also significantly reduced slow-wave energy during nocturnal sleep, suggesting lower homeostatic sleep pressure during the day. There was no negative impact of the split sleep schedule on morning performance, despite a reduction in nocturnal sleep. These findings suggest that naps could be incorporated into a daily sleep schedule that provides sufficient sleep and benefits learning.

A split sleep schedule rescues short-term topographical memory after multiple nights of sleep restriction

Study Objectives

Chronic sleep restriction in adolescents is widespread, yet we know little about how to apportion the limited amount of sleep obtained to minimize cognitive impairment: should sleep occur only nocturnally, or be split across separate nocturnal and daytime nap periods? This is particularly relevant to hippocampal-dependent cognitive functions that underpin several aspects of learning.

Method

We assessed hippocampal function in four groups by evaluating short-term topographical memory with the Four Mountains Test (4MT). All participants began with 9 hours nocturnal time-in-bed (TIB) for 2 days before following different sleep schedules over the next 3 days. Each day, one group had 5 hours nocturnal TIB (5.0h; n = 30), another, 6.5 hours nocturnal TIB (6.5h; n = 29), and a third had 6.5 hours split into 5 hours nocturnal TIB and a 1.5 hour TIB daytime nap (5.0 + 1.5h; n = 29). A control group maintained 9 hours nocturnal TIB (9.0h; n = 30). The 4MT was administered mid-afternoon (1.5 hours after awakening for those who napped).

Results

Performance of the 5.0h and 6.5h nocturnal TIB groups was significantly impaired relative to the 9.0h control group. Performance of participants on the split- sleep schedule (5.0 + 1.5h) did not significantly differ from controls.

Conclusions

These findings suggest that hippocampal function is sensitive to moderate multi-night sleep restriction, but deficits can be ameliorated by splitting sleep, at least for a period after waking from a daytime nap. While this split sleep schedule should not be considered a replacement for adequate nocturnal sleep, it appears to benefit the cognitive and neurophysiological functions that underpin learning in those who are chronically sleep deprived.

Weakly encoded memories due to acute sleep restriction can be rescued after one night of recovery sleep

Sleep is thought to play a complementary role in human memory processing: sleep loss impairs the formation of new memories during the following awake period and, conversely, normal sleep promotes the strengthening of the already encoded memories. However, whether sleep can strengthen deteriorated memories caused by insufficient sleep remains unknown. Here, we showed that sleep restriction in a group of participants caused a reduction in the stability of EEG activity patterns across multiple encoding of the same event during awake, compared with a group of participants that got a full night's sleep. The decrease of neural stability patterns in the sleep-restricted group was associated with higher slow oscillation-spindle coupling during a subsequent night of normal sleep duration, thereby suggesting the instantiation of restorative neural mechanisms adaptively supporting cognition and memory. Importantly, upon awaking, the two groups of participants showed equivalent retrieval accuracy supported by subtle differences in the reinstatement of encoding-related activity: it was longer lasting in sleep-restricted individuals than in controls. In addition, sustained reinstatement over time was associated with increased coupling between spindles and slow oscillations. Taken together, these results suggest that the strength of prior encoding might be an important moderator of memory consolidation during sleep. Supporting this view, spindles nesting in the slow oscillation increased the probability of correct recognition only for weakly encoded memories. Current results demonstrate the benefit that a full night's sleep can induce to impaired memory traces caused by an inadequate amount of sleep.

Multi-Night Sleep Restriction Impairs Long-Term Retention of Factual Knowledge in Adolescents

PURPOSE

Sleep deprivation is associated with increased forgetting of declarative memories. Sleep restriction across multiple consecutive nights is prevalent in adolescents, but questions remain as to whether this pattern of sleep impairs memory for material typically learned in the classroom and the time course of retention beyond a few days.

METHODS

Adolescents aged 15-18 years (n = 29) were given 5 hours sleep opportunity each night for 5 consecutive nights (sleep restricted group; SR), simulating a school week containing insufficient sleep. After the fourth night of restriction, participants learned detailed facts about different species of arthropod across a 6-hour period. Retention was tested 30 minutes and 3 days after learning and contrasted with a control group (n = 30) who had 9 hours sleep opportunity every night of the study. A subset of participants (SR, n = 14; control, n = 22) completed a surprise test 42 days after learning.

RESULTS

Memory was significantly impaired in the SR group relative to controls, with 26% increased forgetting at the 30-minute test (t(57) = 2.54, p = .014, d = .66), 34% at the Day 3 test (t(57) = 2.65, p = .010, d = .69), and 65% at the Day 42 test (t(34) = 3.22, p = .003, d = 1.17). Vigilance was also significantly impaired after 4 nights of restricted sleep (p < .05), but did not correlate significantly with memory (p > .05).

CONCLUSION

Long-term retention of classroom material is significantly compromised when adolescents learn after being sleep restricted, reinforcing the importance of keeping good sleep habits to optimize learning.

Does splitting sleep improve long-term memory in chronically sleep deprived adolescents?

Sleep aids the encoding and consolidation of declarative memories, but many adolescents do not obtain the recommended amount of sleep each night. After a normal night of sleep, there is abundant evidence that a daytime nap enhances the consolidation of material learned before sleep and also improves the encoding of new information upon waking. However, it remains unclear how learning is affected when sleep is split between nocturnal and daytime nap periods during a typical school week of restricted sleep. We compared long-term memory in 58 adolescents who underwent two simulated school weeks of suboptimal continuous (6.5 h nocturnal sleep opportunity) or split sleep (5 h nocturnal sleep +1.5 h daytime nap at 14:00). In the first week, participants encoded pictures in the late afternoon on Day 5 and were tested after 2-nights of recovery sleep. On 3 consecutive days in the second week, participants learned about six species of amphibians in the morning, and six different amphibians in the late afternoon. Testing was performed in the evening following a night of recovery sleep. In the first week, the split sleep group recognized more pictures. In the second week, they remembered more facts about species learned in the afternoon. Groups did not differ for species learned in the morning. This suggests that under conditions of sleep restriction, a split sleep schedule benefits learning after a nap opportunity without impairing morning learning, despite less preceding nocturnal sleep. While not replacing adequate nocturnal sleep, a split sleep schedule may be beneficial for chronically sleep restricted learners.

Sleep deprivation, effort allocation and performance

Sleep deprivation causes physiological alterations (e.g., decreased arousal, intrusion of micro-sleeps), that negatively affect performance on a wide range of cognitive domains. These effects indicate that cognitive performance relies on a capacity-limited system that may be more challenged in the absence of sleep. Additionally, sleep loss can result in a lower willingness to exert effort in the pursuit of performance goals. Such deficits in motivation may interact with the effects of capacity limitations to further stifle cognitive performance. When sleep-deprived, cognitive performance is experienced as more effortful, and intrinsic motivation to perform dwindles. On the other hand, increasing motivation extrinsically (e.g., by monetary incentives) can inspire individuals to allocate more task-related effort, and can partially counter performance deficits associated with sleep deprivation. In this chapter, we review current research on the interplay between sleep deprivation, effort and performance. We integrate these findings into an effort-based decision-making framework in which sleep-related performance impairments may result from a voluntary decision to withdraw effort. We conclude with practical implications of this framework for performance in healthy populations (e.g., work productivity) and clinical conditions.

The impact of sleep deprivation on declarative memory

Sleep plays a crucial role in memory stabilization and integration, yet many people obtain insufficient sleep. This review assesses what is known about the level of sleep deprivation that leads to impairments during encoding, consolidation and retrieval of declarative memories, and what can be determined about the underlying neurophysiological processes. Neuroimaging studies that deprived sleep after learning have provided some of the most compelling evidence for sleep's role in the long-term reorganization of memories in the brain (systems consolidation). However, the behavioral consequences of losing sleep after learning-shown by increased forgetting-appear to recover over time and are unaffected by more common forms of partial sleep restriction across several nights. The capacity to encode new memories is the most vulnerable to sleep loss, since long-term deficits have been observed after total and partial sleep deprivation, while retrieval mechanisms are relatively unaffected. The negative impact of sleep loss on memory has been explored extensively after a night of total sleep deprivation, but further research is needed on the consequences of partial sleep loss over many days so that impairments may be generalized to more common forms of sleep loss.

Sleep and mindfulness meditation as they relate to false memory

By a systematic analysis of the current literature, we compare two states of sleep and meditation in terms of their role in the formation or suppression of Deese-Roediger-McDermott (DRM) false memory. We aim to suggest that the occurrence of false memory under these two states is a result of reinforcing some abilities and changes in cognitive systems which can ultimately improve some aspects of cognitive functions. In our analogy, we propose that: (1) both sleep and meditation may improve source monitoring ability whose failure is one of the most important mechanisms in producing false memories, and (2) despite improvement in source monitoring ability, adaptive cognitive processes, as mechanisms which are common in sleep and meditation, can still produce false memories. In conclusion, we propose that in spite of their contribution to false memory through adaptive processes, the beneficial role of sleep and meditation in cognition may be more prominent than their harmful role.

Auditory stimulation of sleep slow oscillations modulates subsequent memory encoding through altered hippocampal function

Study Objectives

Slow oscillations (SO) during sleep contribute to the consolidation of learned material. How the encoding of declarative memories during subsequent wakefulness might benefit from their enhancement during sleep is less clear. In this study, we investigated the impact of acoustically enhanced SO during a nap on subsequent encoding of declarative material.

Methods

Thirty-seven healthy young adults were studied under two conditions: stimulation (STIM) and no stimulation (SHAM), in counter-balanced order following a night of sleep restriction (4 hr time-in-bed [TIB]). In the STIM condition, auditory tones were phase-locked to the SO up-state during a 90 min nap opportunity. In the SHAM condition, corresponding time points were marked but tones were not presented. Thirty minutes after awakening, participants encoded pictures while undergoing fMRI. Picture recognition was tested 60 min later.

Results

Acoustic stimulation augmented SO across the group, but there was no group level benefit on memory. However, the magnitude of SO enhancement correlated with greater recollection. SO enhancement was also positively correlated with hippocampal activation at encoding. Although spindle activity increased, this did not correlate with memory benefit or shift in hippocampal signal.

Conclusions

Acoustic stimulation during a nap can benefit encoding of declarative memories. Hippocampal activation positively correlated with SO augmentation.

Neural Pattern Similarity in the Left IFG and Fusiform Is Associated with Novel Word Learning

Previous studies have revealed that greater neural pattern similarity across repetitions is associated with better subsequent memory. In this study, we used an artificial language training paradigm and representational similarity analysis to examine whether neural pattern similarity across repetitions before training was associated with post-training behavioral performance. Twenty-four native Chinese speakers were trained to learn a logographic artificial language for 12 days and behavioral performance was recorded using the word naming and picture naming tasks. Participants were scanned while performing a passive viewing task before training, after 4-day training and after 12-day training. Results showed that pattern similarity in the left pars opercularis (PO) and fusiform gyrus (FG) before training was negatively associated with reaction time (RT) in both word naming and picture naming tasks after training. These results suggest that neural pattern similarity is an effective neurofunctional predictor of novel word learning in addition to word memory.

Memory encoding is impaired after multiple nights of partial sleep restriction

Sleep is important for normative cognitive functioning. A single night of total sleep deprivation can reduce the capacity to encode new memories. However, it is unclear how sleep restriction during several consecutive nights affects memory encoding. To explore this, we employed a parallel-group design with 59 adolescents randomized into sleep-restricted (SR) and control groups. Both groups were afforded 9 h time in bed (TIB) for 2 baseline nights, followed by 5 consecutive nights of 5 h TIB for the SR group (n = 29) and 9 h TIB for the control group (n = 30). Participants then performed a picture-encoding task. Encoding ability was measured with a recognition test after 3 nights of 9 h TIB recovery sleep for both groups, allowing the assessment of encoding ability without the confounding effects of fatigue at retrieval. Memory was significantly worse in the sleep-restricted group (P = 0.001), and this impairment was not correlated with decline in vigilance. We conclude that memory-encoding deteriorates after several nights of partial sleep restriction, and this typical pattern of sleep negatively affects adolescents' ability to learn declarative information.

The sleep-deprived human brain

How does a lack of sleep affect our brains? In contrast to the benefits of sleep, frameworks exploring the impact of sleep loss are relatively lacking. Importantly, the effects of sleep deprivation (SD) do not simply reflect the absence of sleep and the benefits attributed to it; rather, they reflect the consequences of several additional factors, including extended wakefulness. With a focus on neuroimaging studies, we review the consequences of SD on attention and working memory, positive and negative emotion, and hippocampal learning. We explore how this evidence informs our mechanistic understanding of the known changes in cognition and emotion associated with SD, and the insights it provides regarding clinical conditions associated with sleep disruption.