Scholarly interest in the process and functions of dreaming has been present since Sigmund Freud's interpretations in the 1900s. The neurology of dreaming has remained misunderstood until recent distinctions, however. The information available via modern techniques of brain imaging has provided new bases for the study of the dreaming brain. The bounds that such technology has afforded has created an understanding of dreaming that seems ever-changing; even now questions still remain as to the function and content of dreams.
Preliminary observations into the neurology of dreaming were reported in 1951 by George Humphrey and Oliver Zangwill. Their report noted two cases of brain injury that resulted in the complete or almost complete cessation of dreaming. Both patients had undergone damage to posterior parietal regions, one of which involved predominately the left side of the parieto-occipital areas. Additional effects involved hemianopia, reduced visualization (in waking state), and disturbances in visual memory. Patients reported that their visual images were dim and hard to evoke. Although they reported only two cases, Humphrey and Zangwill offered preliminary ideas about neurological components of dreaming, specifically the association of forebrain areas and the link between visual imaging and the ability to dream.Humphrey, M. E., & Zangwill, O.L. (1951) Cessation of dreaming after brain injury. Journal of Neurology, Neurosurgery, and Psychiatry, 14, 322.
Methodological issues in scientific dream studies There are several difficulties encountered while studying subjective experiences like dreaming. Methodologies in dream studies are abound with conceptual complexities and limitations.
Reliance on verbal reports One significant shortcoming of dream studies is the necessary reliance on verbal reports. The dream event is reduced to a verbal report which is only an account of the subject's memory of the dream, not the subject's experience of the dream itself. These verbal reports are also at risk of being influenced by a number of factors. First, dreams involve multiple pseudo-sensory, emotional and motoric elements. The dream report is only narrative, which makes capturing the whole picture difficult. Verbal reports face other difficulties like forgetting. Dreams and reports of dreams are produced in distinct states of consciousness resulting in a delay between the dream event and its recall while awake. During this time lag forgetting may occur resulting in an incomplete report. Forgetting is proportional to the amount of time elapsed between the experience and its recall.Schwartz, S., & Maquet, P. (2002). Sleep imaging and the neuro-psychological assessment of dreams. Trends in Cognitive Sciences, 6(1), 23-30. Also, remembering is exposed to interference at the recall stage and some information is not accessible to recall. Reconstructing the dream from memory while awake might affect the accuracy of recall because the subject may report more information than actually experienced, and sequence of events may be reordered. Another issue is the difficulty of verbally describing mostly visual subjective experiences like those found in dreams (e.g. unreal objects, bizarre experiences, emotions). Furthermore, subjects may intentionally fail to report embarrassing, immoral, or private dream experiences for fear of judgement, which results in censored, incomplete reports.
The sleep laboratory environment The sleep laboratory environment is another major source of methodological issues. Sleep laboratories are an unnatural, awkward environment for sleeping. The subject may feel discomfort and anxiety, which may make sleep more difficult and of inferior quality. This is the well-known first night effect. Complete adaptation to the sleep laboratory may take four days or longer,Domhoff, B., & Kamiya, J. (1964). Problems in dream content study with objective indicators. Archives of General Psychiatry, 11(5) 519-532. which is longer than the duration of most laboratory studies. Also, the content of dreams at the laboratory has been observed to be different from dreams at home. Similarly, the laboratory environment may alter the content of dreams recalled from spontaneous awakenings at the end of a night's sleep, as indicated by high frequency of laboratory references in morning spontaneous awakenings in REM and NREM dream reports.
Statistical concerns
Statistical concerns in dream studies are another cause of methodological issues. Many investigators used small samples for sleep studies and statistical parametric mapping (a technique for examining differences in brain activity recorded during functional neuroimaging experiments).Braun, A.R., Thomas, J., Nancy, J., Gwadry, W. F., Carson, R. E., Varga, M., Baldwin, P., Belenky, G., & Herscovitch, P. (1998) Dissociated Pattern of Activity in Visual Cortices and Their Projections During Human Rapid Eye Movement Sleep. Science 279, 91 – 95. Results obtained from small samples must be interpreted with caution due to inherent statistical problems associated with small samples.
Technological limitations Technological limitations also pose methodological problems. Measures of global brain activity like electroencephalogram (EEG) voltage averaging or cerebral blood flow cannot identify small but influential neuronal populations like the locus coeruleus, the raphe nucleus and the pedunculopontine tegmental nucleus, which reveal mechanistic and functional details in dreaming.Hobson, J. A., Pace-Schott, E. F., & Stickgold, R. (2000). Dreaming and the brain: Toward a cognitive neuroscience of conscious states. Behavioral and Brain Sciences, 23(6), 793-842. Despite these shortcomings, it is widely agreed that clinical findings and data obtained from neuro-imaging are valid, affirming neuro-imaging as an essential tool in cognitive neuroscience.
Lesion and activation interpretations Brain-damaged patients offer valuable but rare information about human brain mechanisms. Eugene Aserinsky and Nathaniel Kleitman observed REM sleep and concluded that it was the physiological manifestation of dreaming. This was assumed to be a breakthrough in the understanding of such an elusive process as dreaming. Indeed, 95% of subjects awakened during REM reported that they had been dreaming whereas only about 5-10% reported dreams after being awakened during non-REM sleep (NREM).Solms, M. (2005). The interpretation of dreams and the neurosciences. Neuroscience and Freud's Dream Theory.
REM and NREM dream reports compared
There are several important differences between REM and NREM dream reports. There is disagreement amongst experts about the existence of qualitative differences, but there is a general consensus that there are quantitative differences.
It has been recognized that following REM awakenings dream reports are obtained substantially more frequently than after NREM awakenings.Stoyva, J.M. (1965). Posthypnotically suggested dreams and the sleep cycle. Archives of General Psychiatry, 12(3), 287-294. Subjects dream reports are related to the length of REM sleep. Word count and subjectively estimated dream duration increase as length of preceding REM sleep increases, revealing a positive relationship.(Dement, W., & Kleitman, N. (1957). The relation of eye movements during sleep to dream activity: An objective method for the study of dreaming. Journal of Experimental Psychology, 53(5), 339-346.) Reports from REM awakenings tend to be longer, more multimodal perceptually, have intensified emotionality, and are less reminiscent of waking life than NREM awakenings. Judges are able to differentiate unaltered REM and NREM dream reports, while some subjects are able to discern whether they themselves had been awakened from REM or NREM.
The characteristics of REM sleep consistently contain a similar set of features. While dreaming, people regularly falsely believe that they are awake unless they implement lucidity. Dreams contain multimodal pseudo-perceptions; sometimes any or all sensory modalities are present, but most often visual and motoric.Hobson, J.A. 1988. The dreaming brain: How the brain creates both the sense and the nonsense of dreams. Basic Books, NY. Dream imagery can change quickly and is regularly of a bizarre nature, but reports also contain many images and events that are a part of day-to-day life. In dreams there is a reduction or absence of self-reflection or other forms of meta-cognition relative to during waking life. Dreams are also characterized by a lack of "orientational stability; persons, times, and places are fused, plastic, incongruous and discontinuous". In addition, dreams form a single narrative to explain and integrate all dream elements. Lastly, NREM reports contain thought-like mentation and depictions of current concerns more frequently than REM reports.
Neuroanatomy of dreaming
REM sleep and dreaming Aserinsky and Kleitman's discovery prompted further research into the brain mechanism involved in REM sleep (and by their assumption, dreaming). It was found that REM is generated by a small region of cells located in the brain stem called the pons (it sits slightly above the spinal cord at the nape of the neck). The pons releases acetylcholine which travels to parts of the forebrain. Cholinergic activation of these higher areas was thought to result in the meaningless images that make up our dreams. This process is switched off by noradrenaline and serotonin which are also released by the brain stem.
The formation of the Activation-Synthesis Model put forth by Allan Hobson and McCarley in 1975 rested largely on these discoveries. Their model posits that dreams are actively generated by the brain stem and then passively synthesized by the forebrain. That is, the cholinergic activation that occurs in any forebrain areas (via transmission from brain stem) results in attempts by the brain's cognitive areas to enforce sense or structure onto meaningless activation. Solms, M. (2000). Dreaming and REM sleep are controlled by different brain mechanisms.Behavioral and Brain Sciences, 23(6), 843–850. https://doi.org/10.1017/s0140525x00003988 Cerebral areas were not thought to play any sort of causal role because REM sleep occurs as long as the pons is intact, even if higher areas are disconnected or removed; an inference based on the assumption that REM sleep is dreaming.
A shift to NREM Although this assumption has remained a predominant view, disputing evidence has been present since the 60s. Foulkes for example reported that complex mentation is indeed possible during NREM. Previously participants reported dreams mainly after being awakened from REM, however upon awakening during NREM Foulkes asked subjects about what had just been passing through their heads, rather than whether or not they were dreaming. As many as 50% of subjects reported some form of complex mentation. Furthermore, these NREM dreams seemed to cluster around specific sleep stages (stage 1 and late stages).Solms, M. (2000). Dreaming and REM sleep are controlled by different brain mechanisms. Behavioral and Brain Sciences, 23, 843-850. This offered evidence that dreaming was not restricted nor caused by mechanisms controlling REM sleep, and that perhaps there are entirely different brain areas associated with dreaming.
An investigation of the differential brain structures can be conducted by clinico-anatomical correlations. Here, the mechanisms associated with REM sleep are removed to observe whether there is a cessation in dreaming as well, then the areas thought to be associated with dreaming are removed to see if REM sleep is also made impossible. These studies, with the exception of natural accidents, are conducted with animals. A main problem with obliterating REM sleep is that the associated area, the brain stem, is responsible for consciousness. Lesions large enough to stop REM completely can also render the subject unconscious.