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Sleep Hypn —5. Lucid dreaming has a special status compared with non-lucid REM and NREM dreaming because it is a skill that needs to be trained and occurs only rarely in untrained individuals. Lucid dreaming itself might occur in different degrees, ranging from pre-lucid reflections i. Since dream lucidity can be trained and signalized in experimental settings by means of the eye-signalling technique LaBerge et al. Many of the typical qualities of dreaming are similar to our waking experience.
Such similarities include the full range of multimodal sensory qualities, colourful visual imagery, occasionally realistic pain perception, as well as almost identical spatial organization i. Interestingly, the phenomenological characteristics of dreams in the various phases of the sleep cycle differ in several ways. Particularly rich, emotional, and perceptually vivid dream experiences have been reported after awakenings from REM sleep e. Foulkes ; Dresler et al. Dreaming during REM sleep typically follows loose, fanciful, and often very bizarre narratives; relates to current concerns; reflects interests, personality, and mood; draws on long-term memory; and involves social interactions Hall and Van de Castle ; Domhoff ; Fox et al.
The dreamer is often uncertain about time, space, and personal identities and typically has the subjective experience of being awake Schwartz and Maquet Reports of dreaming tend to be most elaborate and bizarre after waking up from the last period of REM sleep Hobson Sensation and perception. Dreams share similarities with experiences during wakefulness, since the perceptual modalities that are utilized most during waking hours also dominate during dreaming Hobson Dream experiences are typically characterized by a range of visual and auditory sensations, physical activities such as self-motion or interaction with objects in the environment, and involve written and spoken language Desseilles et al.
Tactile percepts, odours, tastes, as well as pleasure and pain are not as commonly reported following REM sleep awakenings Hall and Van de Castle ; Domhoff ; Hobson ; Foulkes Oftentimes the sensational and perceptual experiences of the dream world are unlike those which occur in the world of wakefulness.
Alterations from waking life experiences include sensory distortions, misidentifications of characters and places, changes in spatio-temporal integration e. Dreamers tend to report elevated levels of joy, surprise, anger, fear, and anxiety Foulkes et al. A possible explanation for this finding might be less critical self-reflection during dreams Hobson et al. Since REM dream reports frequently contain fear- and anxiety-related elements Valli and Revonsuo , it has been suggested that the realistic representation of fear in dreams and nightmares serves as a threat simulation in a harmless environment in order to prepare individuals for dangerous situations in real life Revonsuo ; Valli et al.
It has also been shown that several periods of dreaming during one night may be related to the same emotional conflict Offenkrantz and Rechtschaffen During the sleep-onset phase, individuals frequently experience hypnagogic hallucinations while being unaware that they have already fallen asleep Underwood These experiences share some similarities with dreams during REM sleep in terms of dream bizarreness but are typically characterized by emotional flatness Foulkes and Vogel ; Vogel et al.
They are often static Hobson et al. Activities that were performed before sleeping might influence the content of such hallucinations Stickgold et al. After the sleep-onset, NREM dreams are typically more thought-like, fragmentary, and related to current concerns, unlike the vivid, hallucinatory, and mainly visual content of REM dreams Eiser After awakenings from sleep stage N3 early during the night, reports tend to be short, thought-like, less vivid, less visual, less motorically animated, less emotional, and less emotionally pleasant than REM reports, while being more conceptual, more plausible, more concerned with current issues, and typically involve greater volitional control Rechtschaffen ; Hobson et al.
Interestingly, dream phenomenology has often been compared with psychosis because dreams share large similarities with many of the typical characteristics of psychosis and particularly with the positive symptoms of schizophrenia e.
These similarities range from internally generated, vivid imagery to intensified and often inappropriate affect as well as diminished ego functions i.
Most pronounced are elevated levels and uncritical acceptance of cognitive bizarreness, decreased reality testing, and the delusional belief of being awake while dreaming Limosani et al. Furthermore, the dreamer lacks control of dream events and often shows blunted distinction between first- and third-person perspectives Hobson et al.
These observations are supported by functional magnetic resonance imagining data of psychotic patients, which suggest that dream bizarreness i. In fact, there are remarkable similarities in terms of cognitive bizarreness between the waking thoughts of individuals with psychosis, their dream reports, and the dream reports of healthy individuals e.
Scarone et al. Interestingly though, the psychotic patients differ from the healthy individuals in that they tend to judge their dream reports as less bizarre Lusignan et al. Contrasting the state of lucid dreaming with psychosis, Dresler et al. Taken together, these findings indicate that there might be a shared mechanism responsible for some of the features of dreaming and psychosis Limosani et al.
However, a recent paper by Mota et al. As such, psychosis could potentially amplify the experience of internal stimuli at the expenses of external ones, enabling psychotic patients to better control their internal reality than healthy individuals. These range from no consciousness to simple awareness of perception and emotion i. In the present review, we use the concepts of primary and secondary consciousness to represent the general notions of sensory awareness and higher level of awareness such as reflection, respectively.
One can generally differentiate between consciousness during wakefulness, consciousness during dreaming, and non-consciousness, with the possibility of intermediate states being present Limosani et al. Consciousness during waking hours is characterized by the awareness of the external world, our bodies, and our selves. When people are dreaming, they are to some extent consciously aware of their internal world, have sensory, perceptual, and emotional experiences but typically fail to recognize their own condition, the bizarre features of the dream world, their poor memory access, and their limited thought capabilities Hobson Dream consciousness and waking consciousness may differ in terms of their origin i.
In line with this notion, it has been suggested that dreams may be seen as a purer form of consciousness, which is free of the constraints imposed by the perception of, and interaction with, physical environments Revonsuo Early reports suggested a single-mindedness and isolation during dreaming, which refers to the dreamer being absorbed in the dream world without being aware of an alternative reality Rechtschaffen However, the nature, level, and quality of conscious experience during sleep show large variability Nir and Tononi To some extent, this might be comparable to the multiplicity of conscious substates that occur during wakefulness e.
Consciousness is clearly not an all-or-nothing phenomenon but a multifaceted concept with aspects varying across species, vigilance states i. Although there is likely a consensus that consciousness exists while individuals are dreaming, there is an ongoing debate as to whether consciousness exists during dreamless sleep as well Windt et al. Some authors argue that consciousness continues throughout dreamless sleep, provided that one remains aware of having slept. The topic of dreamless sleep is beyond the scope of this paper for further discussions on this issue, please see Thompson ; see also Windt Dreams that occur during REM sleep show mostly aspects of primary but not of secondary consciousness.
During REM sleep, the dreamer tends to have less metacognitive activity i. The dreamer has only limited access to information about the past and anticipated future, and typically concerns him or herself exclusively with the present content of the dream narrative Fox et al.
Goodenough et al. Lucid dreaming is a hybrid state of consciousness with features of both waking and dreaming e. Dresler et al. They found that levels of self-determination i. Furthermore, planning ability i. However, this may be because it is not necessary to plan during dreams and spontaneous execution of intentions is simply more common. Intention enactment i.
This seems plausible because the lucid dreamer is aware that obstacles in dreams are not real and can easily be overcome Dresler et al. In line with this, it has been suggested that restored access to metacognitive abilities and memory functions during lucid dreaming enable the dreamer to execute his or her intentions Metzinger ; Windt and Metzinger Contrasting lucid dreaming and non-lucid dreaming mirrors contrasting primary and secondary consciousness Dresler et al.
Non-lucid REM sleep dreams lack those very features of secondary consciousness, which are the defining characteristic of dream lucidity. These include insight, control over though and actions, as well as logical thought Voss et al. Interestingly, lucid dreaming may be the only phenomenon that can be utilized to examine changes in primary and secondary consciousness in the same vigilance level Spoormaker et al. Establishing a link between dreaming and its underlying neurofunctional changes constitutes a major challenge for researchers Limosani et al.
Dream research typically aims to retrospectively correlate neural activity with the dream characteristics that are common to all dreams e. Periods of REM sleep, NREM sleep, and lucid dreaming are characterized by patterns of regional brain activity that are both similar and distinct from those observed during wakefulness.
The REM sleep phase has most clearly been defined in terms of neurofunctional activation Fig. Schematic brain images showing increased and decreased activity of different brain areas during rapid eye movement REM sleep. The left panel shows a mid-line incision looking at the right hemisphere and the right panel shows a lateral surface of the brain. Areas highlighted with dashed borders are deeper structures. Similarities with wakefulness. The EEG signal during REM sleep shares large similarities with that of wakefulness, and positron emission tomography PET studies have shown that global brain metabolism tends to be very similar as well Hobson et al.
Several brain regions become particularly active during REM sleep. There is strong metabolic activity in higher-order occipito-temporal visual association areas, which might be responsible for the often very vivid visual dream imagery during REM sleep Braun et al. Hyperactivity in motor regions such as the primary motor and premotor cortices, the cerebellum, and the basal ganglia may account for the frequently reported motor content of dreams Braun et al.
Furthermore, increased levels of activity have been observed in the pontine tegmentum, the thalamus, the basal forebrain, as well as in limbic and paralimbic structures e. These brain regions are associated with emotional processing and might be responsible for the often very intense emotional aspects of REM sleep dreaming Maquet and Phillips ; Hobson et al. There is also increased activity in other regions such as the medial prefrontal cortex, circuits of the medial temporal lobe region, and the posterior cingulate cortex Maquet et al.
In fact, there is striking overlap between the default mode network i. This network may play a key role in both mind-wandering and dreaming and possibly represents a shared neural substrate of the two phenomena Domhoff and Fox Even though several brain regions become hyperactive during REM sleep, a number of structures show decreased levels of activity. Among these structures is the right inferior parietal cortex, which is involved in waking volition Goldberg et al.
Decreased activity of the right inferior parietal cortex Maquet et al. Moreover, there is deactivation of executive regions of the prefrontal cortex such as the dorsolateral prefrontal cortex DLPFC and the orbitofrontal cortex, but also in regions including the posterior cingulate gyrus, the precuneus, and the inferior parietal cortex. These areas are typically involved in cognitive control, metacognition, and ego functions e.
Hypoactivation of the prefrontal cortex may also be a contributing factor for dream amnesia Fox et al. A recent study by Horikawa et al. Brain activity underlying these hypnagogic hallucinations may differ from that underlying dreams occurring during REM sleep though Underwood Utilizing high-density EEG recordings and performing serial awakenings, Siclari et al.
Schematic brain images showing increased and decreased activity of different brain areas during non-rapid eye movement NREM sleep. Periods of lucid dreaming show increased activity in areas such as the DLPFC, bilateral frontopolar prefrontal cortex, and parietal areas including the precuneus, the inferior parietal lobules, and the supramarginal gyrus Fig.
It has been suggested that the fronto-parietal activity during lucid dreaming corresponds to the reinstantiation of reflective capabilities Dresler et al. In fact, research on the neural correlates of lucid dreaming might be key in understanding the neural substrates of secondary consciousness Hobson and Voss ; Dresler et al.
So far, the study of lucid dreaming is limited to periods of REM sleep because the classical method to investigate lucid dreaming, the eye-signalling technique, is not applicable to NREM sleep Dresler et al.
Furthermore, research on lucid dreaming mostly pertains to case studies, and conclusions about the neural correlates of lucid dreaming should be regarded as preliminary. Schematic brain images showing increased and decreased activity of different brain areas during lucid dreaming. The heterogeneous brain activity during sleep may explain some of the typical features of, and variation across, dreams during different stages of sleep Desseilles et al.
During REM sleep dreaming, the dreamer thinks that he or she is awake, which is a delusion that might be due to persistent inactivation of frontal and parietal circuits necessary for waking memory, self-reflective awareness, and insight. The global deactivation of the pontine tegmentum, the amygdala, the anterior commissure, the parietal operculum, deep frontal white matter, and the mid-line thalamus during NREM sleep compared with waking, and their subsequent reactivation during REM sleep might explain why some aspects of consciousness during REM sleep might be more readily available than during NREM sleep Maquet et al.
However, preliminary findings suggest that localized activity changes in the parieto-occipital region, irrespective of global activity in the rest of the cortex, may constitute a marker of conscious experience during sleep Siclari et al. The neural correlates of lucid dreaming largely overlap with brain areas that are involved in self-reflective thought and subserve volitional capabilities Voss et al.
According to the principle of perceptual equivalence Finke , there are common neural substrates for perception and imagery.
Some of the visual experiences during the sleep onset phase are represented by brain activity patterns that are similar to the ones observed during stimulus perception Horikawa et al. In line with this, patients with extrastriate occipito-temporal lesions report cessation of visual dream imagery Solms Several other behaviours share similar neural correlates during wakefulness and dreaming Erlacher and Schredl ; Siclari et al.
For instance, dream hand movements activate the same areas of the motor cortex that are active when actually executing hand movements during wakefulness Dresler et al.
Since a range of subcortical and neocortical structures that are active during waking are also active during REM sleep, but inactive during NREM sleep, this might explain why the phenomenological experience of dreaming during REM sleep is much more diverse than during NREM sleep Hobson In what follows we will highlight some important shortcomings that have emerged from reviewing the current literature on the neural correlates of dreaming and we will suggest potential ways to address these in future research.
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