Rapid eye movements are reduced in blind individuals

The structure of sleep and how it may be altered by visual impairments

Individuals with visual impairments often experience poor sleep health, which may impact brain physiology and function, as evidenced by altered brain activity during sleep. The sleeping brain can be categorized into stages: three non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage, with each stage defined by its structure, that is, the duration and frequency of specific brain oscillations. Research investigating alterations in sleep structure among visually impaired individuals has yielded mixed results: some studies indicate reduced or absent deep sleep (N3), others report longer REM latency (the time until the first REM epoch), while some suggest that circadian dysfunction may play a more significant role than visual impairment itself. Sleep is regulated by two processes: the homeostatic sleep drive, which accumulates during wakefulness and is relieved during sleep, and the circadian process, which describes the 24-hour sleep-wake cycle. The circadian process is particularly vulnerable to disruption by visual impairments, as damage to the retina can alter photic entrainment, the process by which light signals from the retina align the circadian sleep-wake cycle with the solar day. Visually impaired individuals often experience a drifting sleep-wake cycle that misaligns with the light-dark cycle, and during periods of misalignment, sleep quality may be particularly poor, especially REM sleep, which is largely under circadian control. Some causes of visual impairment, such as glaucoma, may be more susceptible to circadian dysfunction than others, as glaucoma affects cells in the retinal layer necessary for photic entrainment, which in turn may increase the risk of changes to sleep structure. Given that abnormal sleep structure is associated with long-term health consequences, including increased risks of depression, anxiety, and cognitive decline, it may contribute to the high prevalence of these issues found among the visually impaired population. Further research is needed to clarify the roles of the causes of visual impairments, circadian misalignment, and the impact on sleep structure. A better understanding of these relationships could help develop targeted interventions to improve sleep and enhance health outcomes for visually impaired individuals.

Do eye movements in REM sleep play a role in overnight emotional processing?

Eye movements (EMs) are a defining feature of rapid eye movement (REM) sleep, yet we are still not clear why they happen. A few hypotheses attempt to explain the possible underlying mechanisms. However, a clear understanding of their functional significance remains lacking. Interestingly, there is an EM component in Eye Movement Desensitization and Reprocessing (EMDR) therapy, that is approved for post-traumatic stress disorder (PTSD). The developer of EMDR, Francine Shapiro described the technique as mimicry of REM. Robert Stickgold built on this by proposing a putative neurobiological model in which the repeated eye movements in EMDR initiate brainstem REM-like mechanisms. In this article, we combine Stickgold's model with the results of a study which showed that alternating bilateral visual stimulation in mice yielded sustained increases in the activities of the Superior Colliculus (SC) and mediodorsal thalamus (MD) which suppressed the activity of basolateral amygdala. We pose a hypothetical question: could EMs during REM sleep similarly inhibit amygdala activity through the SC-MD pathway? And could this be part of the affective modulation mechanisms characteristic of REM sleep?

Dreaming conundrum

Dreaming, a common yet mysterious cognitive phenomenon, is an involuntary process experienced by individuals during sleep. Although the fascination with dreams dates back to ancient times and gained therapeutic significance through psychoanalysis in the early twentieth century, its scientific investigation only gained momentum with the discovery of rapid eye movement (REM) sleep in the 1950s. This review synthesises current research on the neurobiological and psychological aspects of dreaming, including factors influencing dream recall and content, neurophysiological correlates, and experimental models, and discusses the implications for clinical practice.

Mortality risk assessment using deep learning-based frequency analysis of electroencephalography and electrooculography in sleep

STUDY OBJECTIVES

To assess whether the frequency content of electroencephalography (EEG) and electrooculography (EOG) during nocturnal polysomnography (PSG) can predict all-cause mortality.

METHODS

Power spectra from PSGs of 8716 participants, including from the MrOS Sleep Study and the Sleep Heart Health Study, were analyzed in deep learning-based survival models. The best-performing model was further examined using SHapley Additive Explanation (SHAP) for data-driven sleep-stage specific definitions of power bands, which were evaluated in predicting mortality using Cox Proportional Hazards models.

RESULTS

Survival analyses, adjusted for known covariates, identified multiple EEG frequency bands across all sleep stages predicting all-cause mortality. For EEG, we found an all-cause mortality hazard ratio (HR) of 0.90 (CI: 95% 0.85 to 0.96) for 12-15 Hz in N2, 0.86 (CI: 95% 0.82 to 0.91) for 0.75-1.5 Hz in N3, and 0.87 (CI: 95% 0.83 to 0.92) for 14.75-33.5 Hz in rapid-eye-movement sleep. For EOG, we found several low-frequency effects including an all-cause mortality HR of 1.19 (CI: 95% 1.11 to 1.28) for 0.25 Hz in N3, 1.11 (CI: 95% 1.03 to 1.21) for 0.75 Hz in N1, and 1.11 (CI: 95% 1.03 to 1.20) for 1.25-1.75 Hz in wake. The gain in the concordance index (C-index) for all-cause mortality is minimal, with only a 0.24% increase: The best single mortality predictor was EEG N3 (0-0.5 Hz) with a C-index of 77.78% compared to 77.54% for confounders alone.

CONCLUSIONS

Spectral power features, possibly reflecting abnormal sleep microstructure, are associated with mortality risk. These findings add to a growing literature suggesting that sleep contains incipient predictors of health and mortality.

Mental Imagery in Dreams of Congenitally Blind People

It is unclear to what extent the absence of vision affects the sensory sensitivity for oneiric construction. Similarly, the presence of visual imagery in the mentation of dreams of congenitally blind people has been largely disputed. We investigate the presence and nature of oneiric visuo-spatial impressions by analysing 180 dreams of seven congenitally blind people identified from the online database DreamBank. A higher presence of auditory, haptic, olfactory, and gustatory sensation in dreams of congenitally blind people was demonstrated, when compared to normally sighted individuals. Nonetheless, oneiric visual imagery in reports of congenitally blind subjects was also noted, in opposition to some previous studies, and raising questions about the possible underlying neuro-mechanisms.

Visuo-spatial imagery in dreams of congenitally and early blind: a systematic review

Background

The presence of visual imagery in dreams of congenitally blind people has long been a matter of substantial controversy. We set to systematically review body of published work on the presence and nature of oneiric visuo-spatial impressions in congenitally and early blind subjects across different areas of research, from experimental psychology, functional neuroimaging, sensory substitution, and sleep research.

Methods

Relevant studies were identified using the following databases: EMBASE, MEDLINE and PsychINFO.

Results

Studies using diverse imaging techniques and sensory substitution devices broadly suggest that the "blind" occipital cortex may be able to integrate non-visual sensory inputs, and thus possibly also generate visuo-spatial impressions. Visual impressions have also been reported by blind subjects who had near-death or out-of-body experiences.

Conclusion

Deciphering the mechanistic nature of these visual impression could open new possibility in utilization of neuroplasticity and its potential role for treatment of neurodisability.

Dissociation between dreams and wakefulness: Insights from body and action representations of rare individuals with massive somatosensory deafferentation

The realism of body and actions in dreams is thought to be induced by simulations based on internal representations used during wakefulness. As somatosensory signals contribute to the updating of body and action representations, these are impaired when somatosensory signals are lacking. Here, we tested the hypothesis that individuals with somatosensory deafferentation have impaired body and actions in their dreams, as in wakefulness. We questioned three individuals with a severe, acquired sensory neuropathy on their dreams. While deafferented participants were impaired in daily life, they could dream of themselves as able-bodied, with some sensations (touch, proprioception) and actions (such as running or jumping) which had not been experienced in physical life since deafferentation. We speculate that simulation in dreams could be based on former, "healthy" body and action representations. Our findings are consistent with the idea that distinct body and action representations may be used during dreams and wakefulness.

The vision of dreams: from ontogeny to dream engineering in blindness

The mechanisms involved in the origin of dreams remain one of the great unknowns in science. In the 21st century, studies in the field have focused on 3 main topics: functional networks that underlie dreaming, neural correlates of dream contents, and signal propagation. We review neuroscientific studies about dreaming processes, focusing on their cortical correlations. The involvement of frontoparietal regions in the dream-retrieval process allows us to discuss it in light of the Global Workspace theory of consciousness. However, dreaming in distinct sleep stages maintains relevant differences, suggesting that multiple generators are implicated. Then, given the strong influence of light perception on sleep regulation and the mostly visual content of dreams, we investigate the effect of blindness on the organization of dreams. Blind individuals represent a worthwhile population to clarify the role of perceptual systems in dream generation, and to make inferences about their top-down and/or bottom-up origin. Indeed, congenitally blind people maintain the ability to produce visual dreams, suggesting that bottom-up mechanisms could be associated with innate body schemes or multisensory integration processes. Finally, we propose the new dream-engineering technique as a tool to clarify the mechanisms of multisensory integration during sleep and related mental activity, presenting possible implications for rehabilitation in sensory-impaired individuals. The Theory of Proto-consciousness suggests that the interaction of brain states underlying waking and dreaming ensures the optimal functioning of both. Therefore, understanding the origin of dreams and capabilities of our brain during a dreamlike state, we could introduce it as a rehabilitative tool.

CITATION

Vitali H, Campus C, De Giorgis V, Signorini S, Gori M. The vision of dreams: from ontogeny to dream engineering in blindness. J Clin Sleep Med. 2022;18(8):2051-2062.

Detecting lucid dreams only by submentalis electromyography

Lucid dreams (LDs) occur when people become aware that they are dreaming. This phenomenon has a wide range of possible applications from the perspectives of psychology, training physical movements, and controlling computers while asleep, among others. However, research on LDs might lack efficiency because the standard LD verification protocol uses polysomnography (PSG), which requires an expensive apparatus and skilled staff. The standard protocol also may reduce LD-induction efficiency. The current study examines whether humans can send phasic signals through submentalis electromyography (EMG) during muscle atonia via pre-agreed chin movements (PACM). This ability would manifest both REM sleep and consciousness, which are the main features of LDs. In laboratory conditions volunteers were instructed to open their jaws three times while in an LD right after the standard verification protocol to achieve the research goal. Results: 4 of 5 volunteers proved to be in an LD using the standard protocol, and then all of them made PACM. The outcomes show that dream signals cannot be blocked in the submentalis area during muscle atonia. Also, this finding can be considered to develop a simplified, reliable LD protocol that needs only one EMG sensor. The cost of this protocol could be only a small percentage of the current protocol, making it more convenient for researchers and volunteers. It can also be used remotely by inbuilt in wearable gadgets. Considering PACM could speed up LD research and provide many discoveries and new opportunities. Also, it can be used in sleep paralysis studies.

Association of neurocognitive functioning with sleep stage dissociation and REM sleep instability in medicated patients with schizophrenia

Many patients with schizophrenia present with impaired cognitive functioning and sleep disturbances. Dissociated stages of sleep represent instability within distinct sleep regulatory cerebral networks. Previous studies found increased rates of rapid eye movement (REM) sleep abnormalities in patients with schizophrenia and a positive association with psychopathology. In this study, we examined if sleep stage dissociation and REM sleep instability was associated with neurocognitive functioning in a sample of medicated patients with schizophrenia. The analyses were performed on 31 baseline polysomnographic recordings as well as baseline data on neurocognitive performance. Regression models were built with the cognitive composite score as primary dependent variable and measures of sleep stage dissociation, including REM sleep without atonia (RSWA), REM sleep without eye movements, non-REM sleep with eye movements, REM sleep percentage in REM periods and REM sleep stability as independent variables. Analyses were adjusted for age, gender, total antipsychotic dose, total benzodiazepine dose, and symptom severity. After correction for multiple testing, we found that the neurocognitive composite score was inversely associated with the degree of RSWA. Exploratory analyses with the cognitive sub scores as dependent variables showed that RSWA was associated with cognitive performance across several sub domains. Dissociated sleep stages, specifically the RSWA feature, might represent a new treatment target for improving cognitive impairment in patients with schizophrenia.

Nocturnal eye movements in patients with idiopathic rapid eye movement sleep behaviour disorder and patients with Parkinson's disease

Patients with idiopathic rapid-eye-movement (REM) sleep behaviour disorder (iRBD) have a high risk of converting into manifest α-synucleinopathies. Eye movements (EMs) are controlled by neurons in the lower brainstem, midbrain and frontal areas, and may be affected by the early neurodegenerative process seen in iRBD. Studies have reported impairment of the oculomotor function in patients with Parkinson's disease (PD) during wakefulness, but no studies have investigated EMs during sleep. We aimed to evaluate nocturnal EMs in iRBD and PD, hypothesizing that these patients present abnormal EM distribution during sleep. Twenty-eight patients with periodic limb movement disorder (PLMD), 24 iRBD, 23 PD without RBD (PDwoRBD), 29 PD and RBD (PDwRBD) and 24 controls were included. A validated EM detector automatically identified EM periods between lights off and on. The EM coverage was computed as the percentage of time containing EMs during stable wake after lights off, N1, N2, N3 and REM sleep. Between-group comparisons revealed that PDwRBD had significantly less EM coverage during wake and significantly higher EM coverage during N2 compared to controls and PLMD patients. PDwoRBD showed significantly less EM coverage during wake compared to controls and higher EM coverage during N2 compared to controls and PLMD. Finally, iRBD showed less coverage of EM during wake compared to controls. The same trend was observed between iRBD and controls in N2 but was not significant. The different profiles of EM coverage in iRBD and PD with/without RBD may mirror different stages of central nervous system involvement across neurodegenerative disease progression.

Applications in sleep: How light affects sleep

Sleep is an active physiological state that plays a critical role in our physical and mental health and well-being. It is generated by a complex interplay between two oscillators, namely, the circadian oscillator and the sleep-wake homeostat. Sleep propensity is a function of wakefulness, that is, the longer one is awake the greater the homeostatic sleep pressure. Sleep onset occurs as a wake promoting circadian signal subsides, coinciding with an evening rise in melatonin and drop in core temperature. Light is one of the strongest time signals for the circadian oscillator. Poor sleep is a prevalent complaint today, attributable, in part, to our easy access to artificial light, especially after dusk. This non-visual effect of light is mediated by a multi-component photoreceptive system, consisting of rods, cones and melanopsin-expressing intrinsically-photosensitive retinal ganglion cells (ipRGC). Perhaps, with this available biological knowledge we can engineer artificial light to minimize its disruptive effect on sleep. We will highlight this by discussing circadian photoreception and its effect on sleep, in the blind population.