The term âdreamingâ or âdreamâ is widely used in everyday language with different connotations, for example, âdreamerâ, âdreamyâ, âdream holidayâ, or âdreaming of being wealthyâ, so it makes sense to start with a clear definition of the term in the field of dream researchâeven though the particulars of this definition are debated. These issues related to defining dreaming are discussed in this chapter.
Definition
A dream or a dream report is the recollection of subjective experiences that occurred during sleep after waking up.
Following this definition, dreaming denotes the subjective experiences occurring during sleep. Although this definition sounds very simple, a closer look shows that the matter is more complicated. First, how can we be sure that the remembered dream upon awakening is really a recollection of experiences during sleep? Second, does it make sense to define dreams according to qualities of perception and action, for example, differentiate between thought-like (sleep mentation), minimal dreaming (a single image/scene), and action-packed dreams? Third, are there non-conscious cognitive processes during sleep that can never be remembered? Fourth, how can nocturnal dreams be differentiated from daydreams, subjective experiences while sleepwalking, and other phenomena?
Before looking more deeply into the problem regarding an exact definition of dreaming, it should be made clear that it is important to differentiate between the physiological level (sleep) and the psychological level (dreaming); some authors used dream sleep and rapid eye movement (REM) sleep interchangeably, for example, William Dement (1960) chose the title âThe effect of dream deprivationâ for his study which aimed at deprivation of REM sleep and not dream deprivation. Keeping these two levels apart is crucial for theories about dream function(s) compared to functions of REM sleep (see Chap. 9). It is also important regarding a possible taxonomy of dreams as subjective experiences in contrast to the classification of brain states with physiological measures like electroencephalogram (EEG) or imaging (wakefulness, REM sleep, NREM sleep, slow wave sleep, etc.). For example, are REM dreams qualitatively different from non-rapid eye movement (NREM) dreams or is this idea of classification mixing up the criteria for REM sleep and NREM sleep (American Academy of Sleep Medicine 2007) with a classification of subjective experiences that might be affected by the underlying brain state but also show a considerable overlap in phenomenological characteristics if obtained from different brain states, for example, daydreaming and experiences under the influence of hallucinogenic drugs, useful?
The phenomenology of dreamingâif recalledâhas been described by many authors (for a review see: Windt 2015). Typically the dream experiences are perceived as real as waking-life experiences while dreaming (with the exception of lucid dreaming; although even in these dreams the experiences are real [sometimes more intense and detailed] despite the knowledge that it is a dream) and several analogues have been suggested over the years, for example, dreaming has similarities to viewing a film (Cook 2011), dreaming as a form of virtual reality (Hobson and Schredl 2011), dreaming as an organic psychosis with hallucinations and delusions (Hobson and Voss 2011), dreams as immersive spatiotemporal hallucinations (Windt 2015), or dreams as a world simulation (Revonsuo et al. 2015). The question is whether these descriptions are helpful in understanding dreaming. The analogy between film viewing and dreaming seems difficult, for example, there is still a discussion ongoing whether dreams can be black-and-white like films in the black-and-white area (König et al. 2017). Hallucinations are defined in a psychiatric context as perception-like experiences that occur without an external stimulus (American Psychiatric Association 2013). The term hallucination might be an analogue for the visual, auditory, and other sensory impressions while dreaming but is not very adequate to describe the experienced emotions and thoughts of the dreamer. The psychiatric definition also includes that hallucinations must occur in the context of a clear sensorium, that is, the waking state (American Psychiatric Association 2013), for example, hearing voices that are perceived as distinct from the individualâs own thoughts. While dreaming, this differentiation between real perceptions and âhallucinationsâ does not make sense, so one might debate the comparison of dreaming with hallucinations occurring in the waking state. Similar delusions which are defined as fixed beliefs that are not amendable to change in light of conflicting evidence (American Psychiatric Association 2013) seem not to be an adequate analogue for the dreaming stateâexperiencing a real world without the knowledge that the physical body is sound asleep in the bed. Also, a comparison being in a virtual reality might be misleading as this activity occurs during waking and does not share the criterion that the brain is asleep. Nevertheless, even though the experiences might be quite similar on a phenomenological level, it seems difficult to use these analogues to define dreaming. Regarding the world simulation view, Tart (1987) pointed out that dreaming and waking are based on the same world simulation process; the only difference is that in waking consciousness has constantly to adapt to the input via the senses and the primary sensory cortices. In his view dreaming is richer and more varied because of the lack of external constraints.
In addition to these philosophical considerations, the simple question âHow do we know that a dream or dream report is a recollection of dreaming?â is of utmost importance to empirical dream research. As mentioned in the definition above, the subjective experience can only be reported by the dreamer himself/herself if dream recall is successful. What kind of evidence would support the assumption that subjective experiencing is present during sleep? First, the event-related potential paradigm provided data that the sleeping brain is processing incoming stimuli and is also capable of simple cognitive operations like differentiating between different stimuli, for example, odd-ball paradigm (Ibanez et al. 2009). Furthermore, external stimuli can be incorporated into the dream (see Sect. 4.â6). Within these studies the stimuli is typically presented during REM sleep with an intensity that does not awake the sleeping person, thenâafter 30 or 60 secondsâthe experimenter awakens the person who did not know at this point whether a stimulus was presented or not (if an appropriate control condition with no stimulus or a sham stimulation is part of the study). That is, if the stimulus is incorporated and the dream includes some action between the incorporation and the awakening, it is very likely that the dream reflects the experiences that occurred during sleep. Similarly, the correlations between physiological parameters measured during sleep and the dream report obtained after awakening do not support the idea that dream reports are generated during the awakening process (see Chap. 5). If, for example, muscle twitches during sleep correlate with dream actions or heart rate increases minutes before being woken up by a nightmare, these reported experiences must have occurred during sleep. Sleep talking has also been related to dream content (Arkin et al. 1970) and patients with REM sleep behavior disorder act out their dreams, observed movements are in sync with the remembered actions after waking up (Valli et al. 2012). Similar, lucid dream research is providing evidence for the notion that dreaming is taking place during sleep, proficient lucid dreamers can perform eye movements within the lucid dream that can be measured via the electrooculogram and even communicate with the experimenter via Morse code; the experimenter uses two different tones and the lucid dreamer can react with different eye movement patterns (Appel and Pipa 2017). Taking together these lines of research, the evidence is clearly in favor of the assumption that dreams are subjective experiences during sleep and that the dream report is a recollection of these experiences. Whether the dream report is an accurate account of the dream experiences is an important topic discussed in Chap. 3 within the context of dream content analysis. If the dreamer, for example, focuses on reporting only the dream action but not the accompanying thoughts and emotions, findings based on such reports might yield a biased picture of what dreaming is like. Therefore, researchers (Kahan 2012; Nielsen 2000a) strongly recommend to train participants regarding their skills of dream reporting.
The question whether all remembered sleep experiences reported upon awakening should be considered as dreams had a strong impact on the REM-NREM debate that started after the discovery of REM sleep in 1953 (Montangero 2018). In the first sleep laboratory studies (e.g., Aserinsky and Kleitman 1953) the participants were asked about dreams after they were awakened, resulting in high recall rates after REM awakenings and very low recall rates after NREM awakenings. Foulkes (1962) changed the question to âWhat was going through your mind be...