Simone Duss

Sleep stage II contributes to the consolidation of declarative memories

Various studies suggest that non-rapid eye movement (NREM) sleep, especially slow-wave sleep (SWS), is vital to the consolidation of declarative memories. However, sleep stage 2 (S2), which is the other NREM sleep stage besides SWS, has gained only little attention. The current study investigated whether S2 during an afternoon nap contributes to the consolidation of declarative memories. Participants learned associations between faces and cities prior to a brief nap. A cued recall test was administered before and following the nap. Spindle, delta and slow oscillation activity was recorded during S2 in the nap following learning and in a control nap. Increases in spindle activity, delta activity, and slow oscillation activity in S2 in the nap following learning compared to the control nap were associated with enhanced retention of face-city associations. Furthermore, spindles tended to occur more frequently during up-states than down-states within slow oscillations during S2 following learning versus S2 of the control nap. These findings suggest that spindles, delta waves, and slow oscillations might promote memory consolidation not only during SWS, as shown earlier, but also during S2.

Unconscious relational encoding depends on hippocampus

See Mayes (doi:10.1093/brain/awu284) for a scientific commentary on this article. The hippocampus is thought to support only conscious memory, while neocortex supports both conscious and unconscious memory. Duss et al. show that amnesic patients with damage to the hippocampal–anterior thalamic axis exhibit a diminished form of unconscious encoding and retrieval, suggesting that certain forms of unconscious memory are hippocampus-dependent.

Formation of semantic associations between subliminally presented face-word pairs

Recent evidence suggests that consciousness of encoding is not necessary for the rapid formation of new semantic associations. We investigated whether unconsciously formed associations are as semantically precise as would be expected for associations formed with consciousness of encoding during episodic memory formation. Pairs of faces and written occupations were presented subliminally for unconscious associative encoding. Five minutes later, the same faces were presented suprathreshold for the cued unconscious retrieval of face-occupation associations. Retrieval instructions required participants to classify the presented individuals according to their putative (1) regularity of income, (2) length of education, and (3) creativity value of occupational activity. The three instructions yielded more classifications consistent with a persons occupation if the person had been subliminally presented with his written occupation versus a meaningless word (control condition). This suggests that consciousness is not necessary to encode, long-term store, and retrieve semantically precise associations between primarily unrelated items.

Integrating Events Across Levels of Consciousness

Our knowledge grows as we integrate events experienced at different points in time. We may or may not become aware of events, their integration, and their impact on our knowledge and decisions. But can we mentally integrate two events, if they are experienced at different time points and at different levels of consciousness? In this study, an event consisted of the presentation of two unrelated words. In the stream of events, half of events shared one component (“tree desk” … “desk fish”) to facilitate event integration. We manipulated the amount of time and trials that separated two corresponding events. The contents of one event were presented subliminally (invisible) and the contents of the corresponding overlapping event supraliminally (visible). Hence, event integration required the binding of contents between consciousness levels and between time points. At the final test of integration, participants judged whether two supraliminal test words (“tree fish”) fit together semantically or not. Unbeknown to participants, half of test words were episodically related through an overlap (“desk”; experimental condition) and half were not (control condition). Participants judged episodically related test words to be closer semantically than unrelated test words. This subjective decrease in the semantic distance between test words was both independent of whether the invisible event was encoded first or second in order and independent of the number of trials and the time that separated two corresponding events. Hence, conscious and unconscious memories were mentally integrated into a linked mnemonic representation.

The role of sleep in recovery following ischemic stroke: A review of human and animal data

Despite advancements in understanding the pathophysiology of stroke and the state of the art in acute management of afflicted patients as well as in subsequent neurorehabilitation training, stroke remains the most common neurological cause of long-term disability in adulthood. To enhance stroke patients’ independence and well-being it is necessary, therefore, to consider and develop new therapeutic strategies and approaches. We postulate that sleep might play a pivotal role in neurorehabilitation following stroke. Over the last two decades compelling evidence for a major function of sleep in neuroplasticity and neural network reorganization underlying learning and memory has evolved. Training and learning of new motor skills and knowledge can modulate the characteristics of subsequent sleep, which additionally can improve memory performance. While healthy sleep appears to support neuroplasticity resulting in improved learning and memory, disturbed sleep following stroke in animals and humans can impair stroke outcome. In addition, sleep disorders such as sleep disordered breathing, insomnia, and restless legs syndrome are frequent in stroke patients and associated with worse recovery outcomes. Studies investigating the evolution of post-stroke sleep changes suggest that these changes might also reflect neural network reorganization underlying functional recovery. Experimental and clinical studies provide evidence that pharmacological sleep promotion in rodents and treatment of sleep disorders in humans improves functional outcome following stroke. Taken together, there is accumulating evidence that sleep represents a “plasticity state” in the process of recovery following ischemic stroke. However, to test the key role of sleep and sleep disorders for stroke recovery and to better understand the underlying molecular mechanisms, experimental research and large-scale prospective studies in humans are necessary. The effects of hospital conditions, such as adjusting light conditions according to the patients’ sleep-wake rhythms, or sleep promoting drugs and non-invasive brain stimulation to promote neuronal plasticity and recovery following stroke requires further investigation.

Rapid eye movements sleep as a predictor of functional outcome after stroke: a translational study

Study Objectives Sleep disturbances are common in acute stroke patients and are linked with a negative stroke outcome. However, it is also unclear which and how such changes may be related to stroke outcome. To explore this link, we performed a sleep electroencephalogram (EEG) study in animals and humans after ischemic stroke. Methods (1) Animal study: 12 male rats were assigned to two groups: ischemia (IS) and sham surgery (Sham). In both groups, sleep architecture was investigated 24 h before surgery and for the following 3 days. (2) Human study: 153 patients with ischemic stroke participating in the SAS-CARE prospective, multicenter cohort study had a polysomnography within 9 days after stroke onset. Functional stroke outcome was assessed by the modified Rankin Scale (mRS) at hospital discharge (short-term outcome) and at a 3-month follow-up (long-term outcome). Results (1) Animal study: rapid eye movement (REM) sleep was significantly reduced in the IS group compared to the Sham group. (2) Human study: patients with poor short-term functional outcome had a reduction of REM sleep and prolonged REM latency during the acute phase of stroke. REM latency was the only sleep EEG variable found to be significantly related to short- and long-term functional impairment in a multiple linear regression analysis. Conclusions Acute ischemic stroke is followed by a significant reduction of REM sleep in animals and humans. In humans, this reduction was linked with a bad stroke outcome; in addition, REM latency was found to be an independent predictor of stroke evolution. Potential explanations for this role of REM sleep in stroke are discussed. Clinical Trial Registration http://clinicaltrials.gov. Unique identifier: NCT01097967.

Sleep-Wake Disorders in Stroke—Increased Stroke Risk and Deteriorated Recovery? An Evaluation on the Necessity for Prevention and Treatment

Purpose of ReviewSleep-wake disorders (SWD) are common not only in the general population but also in stroke patients, in whom SWD may be pre-existent or appear “de novo” as a consequence of brain damage. Despite increasing evidence of a negative impact of SWD on cardiocerebrovascular risk, cognitive functions, and quality of life, SWD are insufficiently considered in the prevention and management of patients with stroke. This narrative review aims at summarizing the current data on the bidirectional link between SWD and stroke.Recent FindingsSeveral studies have demonstrated that sleep-disordered breathing (SDB) is an independent risk factor for stroke and has a detrimental effect on stroke recovery. Short and long sleep duration and possibly other SWD (e.g., insomnia, circadian rhythm disorders) may also increase the risk of stroke and influence its outcome. Data on SDB treatment increasingly indicate a benefit on stroke risk and evolution while treatment of other SWD is still limited.SummaryA systematic search for SWD in stroke patients is justified due to their high frequency and their negative impact on stroke outcomes. Clinicians should actively consider available treatment options.

Die Rolle des Schlafes in der Neurorehabilitation nach einem Hirninfarkt und traumatischen Hirnverletzungen

Zusammenfassung. Der Artikel gibt eine kurze Ubersicht uber die bisherigen Erkenntnisse zum Einfluss des Schlafes im Wiederherstellungsprozess eingeschrankter motorischer und kognitiver Funktionen ...