By MingMing Yang
Waking up and discovering that what just happened a while ago was only a dream must be an experience that everyone has had. In fact, humans spend about 2 hours each night dreaming, and are thought to dream 3 to 6 times per night, although most dreams cannot be remembered. Dreams can be experienced in all stages of the sleep cycle, but are usually most vivid during rapid-eye-movement sleep(REMS)(Brain Basics: Understanding Sleep. ). They can be entertaining, romantic, frightening, or frequently, bizarre. Scientists and psychologists have long been fascinated by this seemingly “consciousness in sleep” and much research has been undertaken around this area.
REMS and non-REM sleep (NREMS) are two clearly distinguishable stages of sleeps. They can be identified from electroencephalography (EEG) activities, where high-frequency and low-amplitude waves indicate REMS, and high-voltage and low-frequency waves indicate NREMS(Muzur, Pace-Schott & Hobson, 2002). As mentioned, dreaming could occur in all stages of human sleep, but the frequency of successful dream recall upon provoked awakening is more than 80% for REMS, but only about 50% for NREMS due to the differences in brain activities(Gennaro et al., 2011). Certain studies of the neurobiology of dream have found that a deep gray matter structure, the hippocampus, seem to play an important role in the processing of mnestic and emotional sources of dream contents.
In a study(Fell et al., 2006), EEG was recorded during sleep from rhinal and hippocampal depth electrodes implanted in 12 epilepsy patients. Participants were awakened during REMS and asked to recall their dreams. It was found that rhinal–hippocampal connectivity values are approximately twice as large for patients with good dream recall versus those patients with poor recall, and successful memorization of dreams is accompanied by an enhanced rhinal–hippocampal and intrahippocampal EEG coherence. Thus, dream memorization is closely associated with rhinal–hippocampal and intrahippocampal connectivity. Since neuroimaging studies have shown that the activity in the hippocampus, entorhinal cortex, and other peri-hippocampal regions increases during REMS compared with both wakefulness and NREMS(Hobson et al., 1998), and an increased cerebral blood flow was also observed in the hippocampal formation during REMS compared with wakefulness, but no differences between NREMS and wakefulness(Braun et al., 1997), the higher dream recall rate and increasing dream vividness in REMS can be explained.
The activation-synthesis theory, first proposed by Harvard psychiatrists J. Allan Hobson and Robert McCarley, is a neurobiological explanation of why we dream(The brain as a dream state generator: an activation-synthesis hypothesis of the dream process. 1977). According to the theory, circuits in the brain become activated during REM sleep and causes areas of the limbic system involved in emotions, sensations, and memories, including the amygdala and hippocampus, to create an array of electrical brain impulses. The brain then synthesizes and interprets this internal activity and attempts to find meaning in these signals, which we experience as dreams. This model suggests that dreams are a subjective interpretation of these signals generated by the brain during sleep, and our active minds pull together the various images and memory fragments of the dream to create a cohesive narrative when we wake(Experts Weigh in With 7 Theories About Why People Dream. ).
Though studies have shown that dreams are very much associated with REM sleep, they are also distinct from each other. This is confirmed by case study of the Charcot–Wilbrand syndrome (CWS), a distinct neuropsychological dysfunction that denotes the cessation of dreaming as a consequence of focal brain damage(Bischof & Bassetti, 2004). In the study, a 73-year-old woman suffered from CWS after having a stroke that led to complete bilateral occipital lobe damage. It was found that despite normal REM sleep, the patient was not able to recall and report any dream experience spontaneously, not even when awakened from REM sleep. Persistence of dreaming despite loss of REM sleep has also been reported by other patients. Thus, these show that REM sleep and dreams may be originated from different anatomical structures. Another interesting finding from the above case was that this patient’s total dream loss was not associated with neuropsychological deficits that are usually found in patients with CWS (visual irreminiscence, prosopagnosia, and topographagnosia). It was then suggested that deep bilateral occipital lobe damage, including the right inferior lingual gyrus, may represent the “minimal lesion extension” necessary for CWS to arise.
Most of the times when we dream, we are not aware of it. However, we sometimes experience a kind of dream called lucid dream, a special oneiric experience in which one has conscious access to dream content, being able to control or even stop it(Mota-Rolim & Araujo, 1975). Because of the presence of self-awareness, it is thought that learning to lucid dream can be an effective way of treating patients with recurrent nightmares. Previous study has suggested that lucid dreams are derived from non-lucid dreams in REM sleep(LaBerge, Levitan & Dement, 1986). As spontaneous lucidity is quite rare, studies on lucid dreams mostly consist of small effect sizes. In a study(Voss et al., 2009), six student volunteers were trained to become lucid and to signal lucidity through a pattern of horizontal eye movements. During the study, 3 students successfully became lucid, and it was found that when subjects become lucid, they shift their EEG power, especially in the 40-Hz range and especially in the frontolateral and frontal regions of the brain. This led to the prediction that lucid dreaming involves the reactivation of the dorsolateral prefrontal cortex (DLPFC), which is thought to be the site of executive ego function in which its activation is diminished in REM sleep.
Following this hypothesis, Dresler et al. conducted a study(Dresler et al., 2012) with four experienced lucid dreamers, and observed increased activity in the right DLPFC, which is associated with self-focused metacognitive evaluation. They further found that activation in bilateral frontopolar areas, which have been related to the processing of internal states, is also increased during lucid dreaming. Furthermore, the strongest increase in activation during lucid compared to non-lucid REM sleep was observed in the precuneus, a brain region that has been implicated in self-referential processing. Thus, lucid dreaming constitutes a hybrid state of consciousness with definable and measurable differences from waking and from REM sleep.
Clearly, there are still many areas about how and why our brain generates dreams that have yet to be discovered. With the advancement of neuroimaging devices and dedicated researchers, dream study has a promising future that we can all look forward to.
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Imperial Bioscience Review 