Johannes Holz

Learning as a Model for Neural Plasticity in Major Depression

BACKGROUND The neuroplasticity hypothesis of depression proposes that a dysfunction of neural plasticity-the basic ability of living organisms to adapt their neural function and structure to external and internal cues-might represent a final common pathway underlying the biological and clinical characteristics of the disorder. This study examined learning and memory as correlates of long-term synaptic plasticity in humans to further test the neuroplasticity hypothesis of depression. METHODS Learning in three tasks, for which memory consolidation has been shown to depend on local synaptic refinement in areas of interest (hippocampus-dependent declarative word-pair learning, amygdala-dependent fear conditioning, and primary-cortex-dependent visual texture discrimination), was assessed in 23 inpatients who met International Classification of Disease, 10th Revision, criteria for severe unipolar depression and 35 nondepressed comparison subjects. RESULTS Depressed subjects showed a significant deficit in declarative memory consolidation and enhanced fear acquisition as indicated by skin conductance responses to conditioned stimuli, in comparison with nondepressed subjects. Depressed subjects demonstrated impaired visual discrimination at baseline, not allowing for valid group comparisons of gradual improvement, the plasticity-dependent phase of the task. CONCLUSIONS The results of the study are consistent with the neuroplasticity hypothesis of depression, showing decreased synaptic plasticity in a dorsal executive network that comprises the hippocampus and elevated synaptic plasticity in a ventral emotional network that includes the amygdala in depression. Evaluation of further techniques aimed at modulating synaptic plasticity might prove useful for developing novel treatments for major depressive disorder.

Increased EEG sigma and beta power during NREM sleep in primary insomnia.

The hyperarousal model of primary insomnia suggests that a deficit of attenuating arousal during sleep might cause the experience of non-restorative sleep. In the current study, we examined EEG spectral power values for standard frequency bands as indices of cortical arousal and sleep protecting mechanisms during sleep in 25 patients with primary insomnia and 29 good sleeper controls. Patients with primary insomnia demonstrated significantly elevated spectral power values in the EEG beta and sigma frequency band during NREM stage 2 sleep. No differences were observed in other frequency bands or during REM sleep. Based on prior studies suggesting that EEG beta activity represents a marker of cortical arousal and EEG sleep spindle (sigma) activity is an index of sleep protective mechanisms, our findings may provide further evidence for the concept that a simultaneous activation of wake-promoting and sleep-protecting neural activity patterns contributes to the experience of non-restorative sleep in primary insomnia.

EEG sigma and slow‐wave activity during NREM sleep correlate with overnight declarative and procedural memory consolidation

Previous studies suggest that sleep‐specific brain activity patterns such as sleep spindles and electroencephalographic slow‐wave activity contribute to the consolidation of novel memories. The generation of both sleep spindles and slow‐wave activity relies on synchronized oscillations in a thalamo‐cortical network that might be implicated in synaptic strengthening (spindles) and downscaling (slow‐wave activity) during sleep. This study further examined the association between electroencephalographic power during non‐rapid eye movement sleep in the spindle (sigma, 12–16 Hz) and slow‐wave frequency range (0.1–3.5 Hz) and overnight memory consolidation in 20 healthy subjects (10 men, 27.1 ± 4.6 years). We found that both electroencephalographic sigma power and slow‐wave activity were positively correlated with the pre–post‐sleep consolidation of declarative (word list) and procedural (mirror‐tracing) memories. These results, although only correlative in nature, are consistent with the view that processes of synaptic strengthening (sleep spindles) and synaptic downscaling (slow‐wave activity) might act in concert to promote synaptic plasticity and the consolidation of both declarative and procedural memories during sleep.

Sleep restriction over several days does not affect long-term recall of declarative and procedural memories in adolescents

OBJECTIVES There is broad evidence that sleep as opposed to waking facilitates the consolidation of both declarative and procedural memory. The current study addressed the question whether different extents of sleep restriction after learning would impair long-term memory consolidation in adolescents. METHODS Eighty-eight healthy adolescents were randomized to five different sleep protocols with 9, 8, 7, 6 or 5 h of time in bed for four consecutive nights under controlled conditions that excluded daytime sleep. Declarative (word-pair task) and procedural memory (mirror tracing task) encoding was assessed prior to the sleep restriction protocol. Recall was assessed after two recovery nights following the sleep protocol and 4 weeks later. RESULTS Sleep diaries and actigraphy data demonstrated that the participants closely followed the sleep protocols. There were no differences in demographic parameters or memory encoding at baseline. In contrast to the initial prediction, restriction of nocturnal sleep over four consecutive nights had no significant impact on declarative or procedural memory consolidation. Polysomnographic monitoring after sleep restriction demonstrated a high preservation of the amount of slow wave sleep in the restricted conditions. CONCLUSIONS The results suggest that adolescents show a high resilience of memory consolidation to substantial sleep curtailment across four nights that might be promoted by increased sleep intensity under conditions of sleep restriction.

The impact of increasing sleep restriction on cortisol and daytime sleepiness in adolescents

Sleep restriction is a widespread phenomenon, specifically in adolescents. This study investigated the impact of increasing sleep restriction in adolescents on cortisol levels and daytime sleepiness. Eighty-eight healthy adolescents were randomized to five sleep restriction protocols (four consecutive nights with 9, 8, 7, 6, or 5 h time in bed). Polysomnography (baseline and last experimental night) and multiple sleep latency test (day 6) data were obtained. Saliva cortisol levels were assessed half-hourly in the evening before and in the morning after the baseline and the last experimental night. Four nights of sleep restriction in healthy adolescents lead to a linear increase of objective sleepiness, but had no significant effect on evening or morning cortisol levels. The lack of detrimental effects of sleep restriction on cortisol levels might be due to compensation mechanisms during sleep.

The Timing of Learning before Night-Time Sleep Differentially Affects Declarative and Procedural Long-Term Memory Consolidation in Adolescents

Sleep after learning has been shown to foster the consolidation of new memories. However, fundamental questions on the best timing of learning before night-time sleep persist. We tested the hypothesis that learning directly prior to night-time sleep compared to 7.5 hrs prior to night-time sleep provides better conditions for the consolidation of declarative and procedural memories. Fifty healthy female adolescents (aged 16–17 years) were trained on a declarative word-pair and a procedural finger-tapping task at 3 pm (afternoon group, n = 25) or at 9 pm (evening group, n = 25), followed by a sleep laboratory night. Retrieval was assessed 24 hours and 7 days after initial training. Subjects trained in the afternoon showed a significantly elevated retention rate of word-pairs compared to subjects trained in the evening after 24 hours, but not after 7 days. In contrast, off-line gains in finger-tapping performance were significantly higher in subjects trained in the evening compared to those trained in the afternoon after both retention intervals. The observed enhanced consolidation of procedural memories after training in the evening fits to current models of sleep-related memory consolidation. In contrast, the higher retention of declarative memories after encoding in the afternoon is surprising, appeared to be less robust and needs further investigation.

The effect of sleep-specific brain activity versus reduced stimulus interference on declarative memory consolidation.

Studies suggest that the consolidation of newly acquired memories and underlying long‐term synaptic plasticity might represent a major function of sleep. In a combined repeated‐measures and parallel‐group sleep laboratory study (active waking versus sleep, passive waking versus sleep), we provide evidence that brief periods of daytime sleep (42.1 ± 8.9 min of non‐rapid eye movement sleep) in healthy adolescents (16 years old, all female), compared with equal periods of waking, promote the consolidation of declarative memory (word‐pairs) in participants with high power in the electroencephalographic sleep spindle (sigma) frequency range. This observation supports the notion that sleep‐specific brain activity when reaching a critical dose, beyond a mere reduction of interference, promotes synaptic plasticity in a hippocampal‐neocortical network that underlies the consolidation of declarative memory.

Prolonged Sleep under Stone Age Conditions

STUDY OBJECTIVES We report on a unique experiment designed to investigate the impact of prehistoric living conditions on sleep-wake behavior. METHODS A group of five healthy adults were assessed during life in a Stone Age-like settlement over two months. RESULTS The most notable finding was that nocturnal time in bed and estimated sleep time, as measured by actigraphy, markedly increased during the experimental period compared to the periods prior to and following the experiment. These increases were primarily driven by a phase-advance shift of sleep onset. Subjective assessments of health and functioning did not reveal any relevant changes across the study. CONCLUSIONS Our observations provide further evidence for the long-held belief that the absence of modern living conditions is associated with an earlier sleep phase and prolonged sleep duration. COMMENTARY A commentary on this article appears in this issue on page 723.

Brief periods of NREM sleep do not promote early offline gains but subsequent on-task performance in motor skill learning

HighlightsSleep modulates motor learning.Brief periods of NREM sleep do not promote early offline gains in motor learning.Brief periods of NREM sleep improve subsequent on‐task performance in motor learning.Our findings inform the optimization of sleep‐wake schedules for motor performance.Future work is needed to clarify sleep’s impact on training‐induced performance gains.Future work is needed to specify sleep’s impact on detrimental on‐task processes. ABSTRACT Sleep modulates motor learning, but its detailed impact on performance curves remains to be fully characterized. This study aimed to further determine the impact of brief daytime periods of NREM sleep on ‘offline’ (task discontinuation after initial training) and ‘on‐task’ (performance within the test session) changes in motor skill performance (finger tapping task). In a mixed design (combined parallel group and repeated measures) sleep laboratory study (n = 17 ‘active’ wake vs. sleep, n = 19 ‘passive’ wake vs. sleep), performance curves were assessed prior to and after a 90 min period containing either sleep, active or passive wakefulness. We observed a highly significant, but state‐ (that is, sleep/wake)‐independent early offline gain and improved on‐task performance after sleep in comparison to wakefulness. Exploratory curve fitting suggested that the observed sleep effect most likely emerged from an interaction of training‐induced improvement and detrimental ‘time‐on‐task’ processes, such as fatigue. Our results indicate that brief periods of NREM sleep do not promote early offline gains but subsequent on‐task performance in motor skill learning.

Auswirkungen von Medienkonsum auf Schlaf bei Kindern und Jugendlichen

ZusammenfassungDie vorliegende Arbeit gibt eine Übersicht über die Zusammenhänge von Medienkonsum und Schlaf bei Kindern und Jugendlichen. Bei dem Großteil der verfügbaren Studien handelt es sich um Querschnittsuntersuchungen, von denen die Mehrheit einen signifikant negativen Zusammenhang zwischen Medienkonsum und Schlafqualität feststellen konnte. Experimentelle Studien, die Aufschluss über kausale Zusammenhänge geben könnten, sind jedoch bislang kaum vorhanden. Anhand der vorliegenden Ergebnisse werden mögliche Gründe und Folgen der Beeinträchtigung von Schlaf durch Fernsehen, Computer- und Internetnutzung sowie Handygebrauch zusammengefasst und diskutiert. In Anbetracht der Aktualität des Themas besteht weiterer Forschungsbedarf, um Eltern und die Öffentlichkeit zu sensibilisieren und um wissenschaftlich fundierte Ratschläge geben zu können.AbstractThis article provides an overview of the current literature on the relationships between media consumption and sleep in children and adolescents. To date, the majority of studies are cross-sectional studies reporting a significant negative correlation between media consumption and the quality of sleep. Interventional studies further investigating the cause–effect chain are rare. Potential reasons and consequences of sleep disruptions are reviewed and discussed. Given the high prevalence of media consumption in children and adolescents, future studies are needed to increase awareness in parents and society at large and to provide advice and guidance on acceptable amounts of media consumption.