Sleep is commonly regarded as a state of disconnection from the environment. Yet, instances of external sensory stimuli affecting the course of dreams have been reported for centuries. Importantly, understanding the impact of external stimuli on dreams could shed light on the origin and generation of dreams, the functional mechanisms that preserve sleep continuity, and the processes that underlie conscious awareness. Moreover, the possibility of using sensory stimuli for dream engineering could potentially benefit patients suffering from alterations in the intensity or content of sleep conscious experiences. Here, we performed a systematic review following PRISMA guidelines to evaluate the robustness of the current evidence regarding the influence of external sensory stimulation during sleep on dreams experiences. In a literature search using PsycNET, PubMed, ScienceDirect, and Scopus, we selected any experimental work presenting dream data obtained from a confirmed sleep episode during which visual, auditory, olfactory, gustatory, or somatosensory stimulation was administered. A methodological assessment of the included studies was performed using an adapted version of the Downs and Black’s (1998) checklist. Fifty-one publications met the inclusion criteria, of which 21 reported data related to auditory stimulation, 10 to somatosensory stimulation, 8 to olfactory stimulation, 4 to visual stimulation, 2 to vestibular stimulation, and 1 to multi-modal stimulation (audio-visual). Furthermore, 8 references involved pre-conditioned associative stimulation procedures: 6 relied on targeted memory reactivation protocols and 3 on targeted lucid reactivation protocols. The reported frequency of stimulus-dependent dream changes across studies ranged from 0% to ~90%. Such a variability likely reflects the considerable heterogeneity of experimental and methodological approaches. Overall, the literature analysis identified a lack of substantial understanding of the key mechanisms, functions, and correlates of stimulus-dependent dream changes. We believe that a paradigm shift is required for meaningful and significant advancement in the field. We hope that this review will serve as a starting point for such a shift.
IntroductionIdiopathic/isolated rapid eye movement (REM) sleep behavior disorder (iRBD) is considered the prodromal stage of alpha‐synucleinopathies. Thus, iRBD patients are the ideal target for disease‐modifying therapy. The risk FActoRs PREdictive of phenoconversion in iRBD Italian STudy (FARPRESTO) is an ongoing Italian database aimed at identifying risk factors of phenoconversion, and eventually to ease clinical trial enrollment of well‐characterized subjects.MethodsPolysomnography‐confirmed iRBD patients were retrospectively and prospectively enrolled. Baseline harmonized clinical and nigrostriatal functioning data were collected at baseline. Nigrostriatal functioning was evaluated by dopamine transporter‐single‐photon emission computed tomography (DaT‐SPECT) and categorized with visual semi‐quantification. Longitudinal data were evaluated to assess phenoconversion. Cox regressions were applied to calculate hazard ratios.Results365 patients were enrolled, and 289 patients with follow‐up (age 67.7 ± 7.3 years, 237 males, mean follow‐up 40 ± 37 months) were included in this study. At follow‐up, 97 iRBD patients (33.6%) phenoconverted to an overt synucleinopathy. Older age, motor and cognitive impairment, constipation, urinary and sexual dysfunction, depression, and visual semi‐quantification of nigrostriatal functioning predicted phenoconversion. The remaining 268 patients are in follow‐up within the FARPRESTO project.ConclusionsClinical data (older age, motor and cognitive impairment, constipation, urinary and sexual dysfunction, depression) predicted phenoconversion in this multicenter, longitudinal, observational study. A standardized visual approach for semi‐quantification of DaT‐SPECT is proposed as a practical risk factor for phenoconversion in iRBD patients. Of note, non‐converted and newly diagnosed iRBD patients, who represent a trial‐ready cohort for upcoming disease‐modification trials, are currently being enrolled and followed in the FARPRESTO study. New data are expected to allow better risk characterization.
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