Unraveling the mystery of dreams

Abstract

Dreaming has always fascinated philosophers and psychologists. Freud, Jung, and other scientists made numerous speculations about the generation and purpose of dreams. However, even with decades of scientific research, the neural substrates of dreaming remain unclear. Since first described by Aserinsky and Kleitman in 1953 (1), rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep are two clearly distinct stages of sleep. REM sleep is characterized by wake-like brain activity on electroencephalograms (EEG). NREM sleep refers to other sleep stages apart from REM. The dreaming experience (DE) is often similar to perception in the real world, and brain activity during REM sleep is similar to a state of conscious. Since the 1950s, REM sleep has been considered the only sleep stage that can generate dreams (2). Nevertheless, this proposition was refuted by subsequent findings (3). Experimental data suggest that there is no absolute state that is specific to the generation of dreams. REM sleep is neither necessary nor sufficient for dream generation. In an interesting study that was recently published in Nature Neuroscience, Siclari et al. confirmed that dreams can occur during any sleep stage, albeit less often in NREM sleep (4). In their previous review, Nir and Tononi proposed that "It is time we moved beyond sleep stages when trying to link dream consciousness to neuronal events, and focused on more subtle features of brain activity in space and time" (5).This study made substantial efforts to illustrate the neural mechanisms that underlie dreams and provided preliminary evidence of decoding dream content.According to brain lesion studies by Solms, bilateral medial occipital-temporal lesions lead to the full or partial loss of visual imagery in dreams (6). Patients with unilateral hemispheric lesions in the temporo-parieto-occipital region presented a more frequent loss of dream recall than patients with lesions outside this area (7). In addition to brain lesion studies, the neuroimaging and neurological literature suggests that brain networks that are implicated in memory and emotional and reward processes and parieto-occipital regions may be involved in dream generation. A metaanalysis of positron emission tomography studies reported the lower activation of cognitive control networks and greater activation of limbic and prelimbic brain regions, reflecting a decrease in self-awareness (8) and increase in subjective experiences in dreams (5). However, the brain regions that are identified are not always consistent, and the precise neural underpinnings of dreaming remain elusive. Using a serial awakening method, Siclari and colleagues investigated the neural mechanisms of dreams using a within-state model separately for REM and NREM sleep because of the substantial distinctions between these sleep stages. According to self-reports after awakening, the state before waking can be divided into three categories: DE, experiencing something but cannot remember the content, or not experiencing anything (N...