Dream Generation Research Articles

Dream mechanisms: Is REM sleep indispensable for dreaming?

This article aims to overview briefly major arguments about dream generation from the point of view of the pros and cons of the main thesis that rapid eye movement (REM) sleep is indispensable for dreaming. The methodological issues observed in empirical dream research are also discussed. The evidence, as seen in phenomenological corroboration and anatomical and neurocognitive findings specific to REM sleep, appears to support the contribution of REM mechanisms to dream experiences. As a representative model, Hobson’s “AIM model”, which proposes a three-dimensional state space in mind, is introduced. Evidence for the counter-argument is provided by the appearance of dreamlike mentation outside of REM sleep, psycholinguistic studies of report length, and neuropsychological corroboration between dream cessation and forebrain lesions. As representative models, Solms’ “forebrain dopaminergic mechanisms”, inductively developed by the body of neuropsychological findings, and “the functional state-shift hypothesis”, hypothesizing dream appearance depending on the proximity of electroencephalographic activities to wakefulness, are introduced. As an alternative between the two standpoints, the “covert-REM model” proposes REM mechanisms underlying dreamlike activities outside REM sleep in a covert way. This model has been elaborated through evidence such as the appearance of physiological activities specific to REM sleep preceding the REM period, and an increase in dream recall as its temporal proximity to peak REM activity. A possible contribution of arousal processes to mentation occurrences outside of REM sleep is also proposed, as seen in evidence such as the increase of mentation during nocturnal sleep, with frequent arousals and stimuli-induced mentation during NREM sleep. Other issues to be considered in dream mechanism studies are also discussed.

EEG activities during elicited sleep onset REM and NREM periods reflect different mechanisms of dream generation. Electroencephalograms. Rapid eye movement.

To be the first to compare EEG power spectra during sleep onset REM periods (SOREMP) and sleep onset NREM periods (NREMP) in normal individuals and relate this to dream appearance processes underlying these different types of sleep periods. Eight healthy undergraduates spent 7 consecutive nights in the sleep lab including 4 nights for SOREMP elicitation using the Sleep Interruption Technique. This enabled us to control preceding sleep processes between SOREMP and NREMP. EEG power spectra when participants did and did not report 'dreams' were compared between both types of sleep. Sleep stages, subjective measurements including dream property scores, sleepiness, mood, and tiredness after awakenings were also examined to determine their consistency with EEG findings. Increased alpha EEG activities (11.72-13.67 Hz) observed mainly in the central area were related to the absence of SOREMP dreams and appearance of NREMP dreams. Analyses of sleep stages combining two studies (16 participants) also supported the Fast Fourier Transform findings, showing that when dreams were reported there were decreased amounts of stage 2 and increased stage REM in SOREMP and increased stage W in NREMP. SOREMP dreams were more bizarre than NREMP dreams. Participants felt more tired after SOREMP with dreams than without dreams, while the opposite was observed after NREMP episodes. EEG power spectra patterns reflected different physiological mechanisms underlying generation of SOREMP and NREMP dreams. The same relationships were also reflected by sleep stage analyses as well as subjective measurements including dream properties and tiredness obtained after awakenings. This study not only supports the hypothesized relationships between REM mechanisms and REM dreams as well as arousal processes and NREM dreams, it also provides a new perspective to dream research due to its unique techniques to awaken participants and collect REM dreams during experimentally induced SOREMP.

Mental activity during sleep.

Since remote antiquity humankind has believed in the supernatural origin of dreaming. The scientific approach to mental activity during sleep dates back to the eighteenth century. One hundred years ago, psychoanalysis introduced dream analysis for diagnostic and therapeutic purposes. Only 50 years ago psychophysiology made it possible to collect mental sleep activity by means of experimental awakenings while monitoring physiological variables; this approach encouraged investigation into the relationship between the features of sleep activity and sleep state (REM versus NREM). Advances in neurophysiology, in neurochemistry, and recently in brain imaging techniques, have shed light on the roles played by the different cerebral structures in determining specific characteristics of mental activity during sleep. The development of cognitive psychology has enabled investigation of dream generation in terms of output from a complex multilevel system of information processing. In addition to sleep state, other factors, such as the time of the night and the sequence of the NREM-REM cycles, have been shown to influence the characteristics of mental activity. The usefulness of investigation of mental sleep activity as a clinical tool is controversial. The psychophysiological approach to mental sleep activity in clinical contexts has enabled the exploration of adaptation processes and contributed to neuropsychological studies on focal and systemic brain pathology.

Shedding old assumptions and consolidating what we know: Toward an attention-based model of dreaming

Most current theoretical models of dreaming are built around an assumption that dream reports collected on awakening provide unbiased sampling of previous cognitive activity during sleep. However, such data are retrospective, requiring the recall of previous mental events from sleep on awakening. Thus, it is possible that dreaming occurs throughout sleep and differences in subsequent dream reports are owing to systematic differences in our ability to recall mentation on awakening. For this reason, it cannot be concluded with certainty that sleep cognition is more predominant or in any way different during REM compared to NREM sleep. It is our contention that REM sleep and ponto-geniculo-occipital (PGO) waves do not necessarily represent “pseudosensory” stimulation of the cortex in the generation of dreams, but might rather represent enhanced arousal of attention mechanisms during sleep, which results in the subsequent recall of attended mentation on awakening.[Hobson et al.; Nielsen; Revonsuo; Solms; Vertes & Eastman]

The “problem” of dreaming in NREM sleep continues to challenge reductionist (two generator) models of dream generation

The “problem” of dreaming in NREM sleep continues to challenge models that propose a causal relationship between REM mechanisms and the psychological features of dreaming. I suggest that, ultimately, efforts to identify correspondences among multiple levels of analysis will be more productive for dream theory than attempts to reduce dreaming to any one level of analysis.[Hobson et al. ; Nielsen]

Needed: A new theory

Dream content is more coherent, consistent over time, and continuous with waking emotional concerns than most brainstem-driven theories of dreaming allow, but dreaming probably has no adaptive function. A new neurocognitive perspective focusing on the forebrain system of dream generation should begin with the findings on dream content in adults and the developmental nature of dreaming in children.[Hobson et al.; Nielsen; Revonsuo; Solms; Vertes & Eastman]

REM sleep dream mentation in right hemispherectomized patients

Investigations of dream mentation in brain damaged patients have shed some light on the controversial issue of cerebral lateralization of dreaming. To examine further the relationships between brain function and dreaming, we studied REM sleep dream recall and content in four patients having undergone right functional or anatomical hemispherectomy and eight matched control subjects. Patients were found to have the capacity to report dreams to much the same extent as control subjects. Further, the patients' dream content was overall similar to that of the control subjects. The results provide strong evidence that dreaming is not a right-hemisphere function, and that the left hemisphere may be more critical for the generation of dreams. In addition, some characteristics of hemispherectomized patients' dream content (characters, smells) are consistent with the possibility that a history of epilepsy may influence REM sleep imagery over the long term.

Mind and consciousness during sleep

The epistemological-methodological constraints for an approach to dream as an object of scientific-experimental investigation are pointed out, along with the suggestion of a new memory paradigm, that utilizes the linguistic convention (‘word play’) proposed by Wittgenstein. A model of dream generation is proposed, in which mnemonic materials activated from long-term memory stores enter the dream production system (bottom-up process); such materials are elaborated and interpreted by a top-down process. A circular feedback activity, operating between these two components, is able to activate additional mnemonic materials. Some experimental results, concerning the incidence of dream recall in different stages of sleep, show evidence of the continuity of dreaming activity throughout the period of sleep. The relationships between dream and consciousness/awareness are then considered. Experimental data suggest that consciousness, interpreted as awareness (phenomenal awareness; meta-awareness; self awareness), can be present along the whole sleep/dream process, even if many modifications of meta-awareness and of self-awareness are possible. Some hypotheses on the relationship between sleep and dreaming are formulated.

Bizarreness effect in dream recall.

This study aimed to ascertain a) whether morning reports of dream experience more frequently reproduce bizarre contents of night reports than nonbizarre ones and b) whether this effect depends on the rarity of bizarre contents in the dream or on their richer encoding in memory. Ten subjects were awakened in rapid eye movement (REM) sleep three times per night for 4 nonconsecutive nights and asked to report their previous dream experiences. In the morning they were asked to re-report those dreams. Two separate pairs of judges scored the reports: the former identified the parts in each report with bizarre events, characters or feelings and the latter parsed each report into content units using transformational grammar criteria. By combining the data of the two analyses, content units were classified as bizarre or nonbizarre and, according to whether present in both the night and corresponding morning reports, as semantically equivalent or nonequivalent. The proportion of bizarre contents common to night and morning reports was about twice that of nonbizarre contents and was positively correlated to the quantity of bizarre contents present in the night report. These findings support the view that bizarreness enhances recall of dream contents and that this memory advantage is determined by a richer encoding at the moment of dream generation. Such a view would seem to explain why dreams in everyday life, which are typically remembered after a rather long interval, appear more markedly bizarre than those recalled in the sleep laboratory.

Cognitive Aspects of Mental Activity during Sleep

Upon nighttime experimental awakening of 27 subjects in four sleep conditions (sleep onset early; sleep onset late; Stage 2; and rapid eye movement, REM, sleep), 108 dream reports and their association reports were collected. Dream reports were analyzed for length (temporal units) and content categories (continuity; implausibility; presence of the dreamer [i.e., "the self"], a setting, characters). Associations were classified as episodic, abstract self-referred, and semantic memories. The two sets of results tend to show a basic homogeneity among mentation reports in the four sleep conditions considered. These findings are interpreted as supporting the hypothesis that the same cognitive mechanisms operate, at different levels of engagement, in dream generation rather than the hypothesis of multiple dream-generation systems dependent upon the physiological characteristics of the various sleep stages.

The laterality of dreaming

The cortical locus of dream generation could be lateralized to the right or left hemisphere, or be bilaterally represented with either equal or unequal contributions from each hemisphere. In this paper we review the neurological literature for cases of loss or alteration of dream report after brain damage. The distribution of lesion sites is used to test the various hypotheses concerning the laterality of dreaming. The hypothesis receiving best support is that dreaming is lateralized to the left hemisphere in individuals with typical neurologic organization.