In order to gain insight into the phenomenology and neural basis of dreams, it is useful to consider both similarities and differences between waking consciousness and dreaming consciousness, and to relate these differences to changes in brain activity and organization[11]. Perhaps the most striking feature of conscious experiences in sleep is how altogether similar the inner world of dreams is to the real world of wakefulness. Indeed, at times the dreamer may be uncertain whether he is awake or asleep. Certainly, dreams are not created in a vacuum but closely reflect the organization and functions of our brain.
In most dreams, perceptual modalities and submodalities that dominate in wakefulness are heavily represented. Dreams are highly visual, in full color, rich in shapes, full of movement, and incorporate typical wakefulness categories such as people, faces, places, objects, and animals[3]. Dreams also contain sounds (including speech and conversation), and more rarely tactile percepts, smells and tastes, as well as pleasure and pain[4, 12–14]. Experiences in typical dreams have a clear sensory character (i.e. they are seen, heard, and felt) and are not mere thoughts or abstractions.
These phenomenological similarities are reflected in neurophysiological similarities between waking and dreaming. For historical and methodological reasons, most electroencephalogram (EEG) and neuroimaging studies have contrasted brain activity during quiet wakefulness with that observed during REM sleep, when subjects are most likely to report dreams[15–20]. At least superficially, the EEG looks remarkably similar in active waking and REM sleep. Positron emission tomography (PET) studies have shown that global brain metabolism is comparable between wakefulness and REM sleep[11, 20]. Such studies have also revealed a strong activation of high-order occipito-temporal visual cortex in REM sleep, consistent with the vivid visual imagery during dreams (Fig. 1)[16, 17, 19].
There is also remarkable consistency between a subject s cognitive and neural organization in dreaming and waking[13, 14]. For instance, children studies demonstrate that dream features show a gradual development that parallels their cognitive development when awake[21] (Box 2). Patients with brain lesions that impair their waking cognition show corresponding deficits in dreams. For example, subjects with impaired face perception also do not dream of faces[22, 23] (Box 3).
...
Despite these remarkable similarities, what makes dream consciousness so fascinating are the ways in which it differs from our waking experience. Some of these phenomenological differences are accompanied by consistent neurophysiological differences.
Reduced voluntary control and volition. We are generally surprised on awakening from a dream (“it was only a dream”) mainly because we didn’t consciously will that we would dream it. In fact, during dreaming there is a prominent reduction of voluntary control of action and thought. We cannot pursue goals, and have no control over the dream’s content. The fact that we are so surprised, excited and even skeptical about lucid dreaming – possibly a way to control some dreams[25] - illustrates how dreams normally lack voluntary control[9]. Interestingly, recent evidence points to the role of the right inferior parietal cortex (Brodmann’s Area 40) in waking volition[26, 27], an area that is deactivated during REM sleep[15, 16] (Fig. 1).
Reduced self-awareness and altered reflective thought. Our dreaming consciousness consists of a single “track”: we are not contextually aware of where we are (in bed) or of what we are doing (sleeping, dreaming). There is a strong tendency for a distinct narrative of thoughts and images to persist without disruption (“single-mindedness”[28]). Indeed, reports of mental activity in REM sleep are longer than reports obtained from awake subjects[28]. Dreaming is almost always delusional since events and characters are taken for real. Reflective thought is altered in that holding contradictory beliefs is common, and a dreamer easily accepts impossible events such as flying, inconsistent scene switches, sudden transformations and impossible objects[29] such as a pink elephant. There is often uncertainty about space, time, and personal identities[30]. For example, a character may have the name, clothes and hairstyle of a male friend, but have mother’s face. Reduced self-monitoring in dreams may be related to the deactivation of brain regions such as posterior cingulate cortex, inferior parietal cortex, orbitofrontal cortex, and dorsolateral prefrontal cortex[15, 16] (Fig 1). Indeed, deactivation of prefrontal cortex has been shown to accompany reduced self-awareness during highly engaging sensory perception in wakefulness[31]. However, some dreams may have conserved reflective thought processes such as thoughtful puzzlement about impossible events[32], contemplating alternatives in decision-making[32], reflecting during social interactions[32], and “theory of mind”[33], demonstrating that individual dreams can differ from each other substantially.
Emotionality. Some dreams are characterized by a high degree of emotional involvement, including joy, surprise, anger, fear, and anxiety[34–36]. Interestingly, sadness, guilt, and depressed affect are rare[11], possibly due to reduced self-reflection. Some claim that fear and anxiety are enhanced in dreams to a degree rare in waking life[37], in line with Freud’s suggestion that dream narratives originate in perceived threats or conflicts[5]. Whether or not this interpretation has merits, REM sleep is in fact associated with a marked activation of limbic and paralimbic structures such as the amygdala, the anterior cingulate cortex, and the insula[15, 17, 19] (Fig. 1). However, emotions are feeble in other dreams, and are absent altogether in 25–30% of REM sleep reports[34–36], including in situations where emotions would likely be present in waking[34], once again highlighting the variability in dream phenomenology.
Altered mnemonic processes. Memory is drastically altered for the dream and within the dream. Unless the dreamer wakes up, most dreams are forever lost. Upon awakening, memory for the dream often vanishes rapidly unless written down or recorded, even for intense emotional dreams. It is not clear why this is the case since from a neuroimaging perspective, limbic circuits in the medial temporal lobe that are implicated in memory processes, are highly active during REM sleep[15–18] (Fig. 1). Perhaps the hypoactivity of prefrontal cortex, also implicated in mnemonic processes, plays an important role in dream amnesia. Contemporary theories of dreaming (Table 1) offer different accounts of dream amnesia. For example, according to psychodynamic models, dream amnesia is due to processes of active repression[5]. According to Hobson s Activation-Input-Mode [AIM] model, dream amnesia is related to a state-change involving inactivity of monoaminergic systems (“aminergic de-modulation”) and deactivation of dorsolateral prefrontal cortex[11]. The neurocognitive model claims that dreams are usually forgotten because they are internal narratives; unless internal experiences are tied to external cues such as times and places they are bound to be forgotten[13].
...
In summary, dream consciousness is remarkably similar to waking consciousness, though there are several intriguing differences. These include reduced attention and voluntary control, lack in self-awareness, altered reflective thought, occasional hyperemotionality, and impaired memory. Traditionally, dream phenomenology has often been compared to madness or psychosis[3, 11, 47], but in fact the hallucinations, disorientation, and subsequent amnesia of some bizarre dreams may be more akin to the acute confusional state – also known as delirium - which occurs after withdrawal from alcohol and drugs[48]. However, most dreams are less bizarre, perhaps more similar to mind wandering or stimulus independent thoughts[14, 49, 50]. Waking thoughts jump around and drift into bizarre daydreaming, rumination, and worrying far more than stereotypes of rational linear thinking suggest[51]. Importantly, individual dreams are highly variable in their phenomenology, and only some conform to the typical monolithic template that is often portrayed. Thus, just like diverse waking experiences, “Not all dreams are created equal”, and future studies should consider different kinds of dreams and their neural correlates separately.
What mechanisms are responsible for regional differences in brain activity between waking and REM sleep, and thus presumably for some of the cognitive differences between waking and dreaming? Single-unit physiology indicates that generally, cortical activity in REM sleep reaches similar levels as found in active wake (Fig. 2), but variability between brain areas remains poorly explored. Regional differences may likely stem from changes in the activity of neuromodulatory systems (Fig. 2). During REM sleep, acetylcholine is alone in maintaining brain activation, whereas monoaminergic systems are silent, an observation that could explain many features of dreams[11]. For example, consistent with imaging results, cholinergic innervation is stronger in limbic and paralimbic areas than in dorsolateral prefrontal cortex[52], which may explain why limbic regions are highly active in REM sleep while dorsolateral prefrontal cortex is deactivated (Fig. 1). Dopaminergic modulation may also play a role[23], since dreaming is decreased by prefrontal leucotomies that cut dopaminergic fibers[53] and is increased by dopaminergic agonists[23] (Table 1 and Fig. 2).
...
On the whole, relating typical dreams to the neurophysiology of REM sleep has proven to be a useful starting point for revealing the neural basis of dreaming. However, dream consciousness can not be reduced to brain activity in REM sleep. Indeed, some fundamental questions concerning the relationship between the brain and dreaming linger on. We shall discuss three in turn: i) what determines the level of consciousness during sleep; ii) why the dreamer is disconnected from the environment; and iii) whether dreams are more akin to perception or to imagination.
