Sleep laboratory, Central Institute of Mental Health, Mannheim
The body-mind interaction has fascinated mankind since the dawn of history. By reviewing studies conducted in the field of dream research it will be shown how these interactions can be investigated, e. g. the relationship of dreamt actions and EEG. The purpose of the present article is to emphasize how important the integration of psychological and physiological approaches will be in the future.
Keywords: Body-mind interaction, dream research, dreaming, REM sleep
The mind-body-interaction has fascinated mankind since the dawn of history. The subject of the present article is to review the contribution of dream research to this topic and to emphasize – as quintessence – the importance for future research in this area to integrate physiological and psychological dream models.
Since the discovery of REM sleep in 1953, the continuous measuring of physiological parameters such as EEG, EOG, ECG, EMG, respiration activity during sleep is a common standard technique not only used in research but in diagnosing sleep disorders (e. g., Kryger, Roth & Dement, 1994). The international accepted scoring system for sleep stages (Rechtschaffen & Kales, 1968) allows the comparison of research findings all over the world. Rechtschaffen and Kales (1968) classified polysomnographic recordings (EEG, EOG, EMG) into 6 stages: awake, stage NREM (1 to 4) and stage REM. REM sleep was the most interesting sleep stage for dream researchers since it was shown that the probability to recall a detailed visual dream after being awakened from REM was very high (Aserinsky & Kleitman, 1953). Since that time, the publications rate of papers on sleep and dream physiology increased; very similar to the rate after the publication of Freud’s "Traumdeutung" in 1900 (Nielsen & Germain, 1998). In both cases, the peak occurred about 15 years after the release of the work.
Although the findings of the first physiological dream studies supported the hypothesis that dreaming occurs during sleep, this is nevertheless not self-evident. Since dreams can only be reported as remembrance of a person who is awake, it is also possible that dreams are generated like a flash during the awakening process. Evidence that dreams can be interpreted as cognitive/emotional activity during REM sleep is provided in the section dream time and REM sleep duration. In a similar way, studies reported following sections such as dream content and eye movements, dream content and EEG, dream content and EMG, dream content and autonomic parameters support this viewpoint. In addition, this research investigated whether physiology and dreamt activity is related in the same way as they are in the waking state, e. g. whether eye movements during the dream are related to eye movements measured by the polysomnograph.
Dream time and REM sleep duration
The following dream example reported by the French sleep and dream researcher Alfred Maury (1861) raised the question whether the recalled dream is really a recollection of experiences made during sleep or is possibly generated like a flash during the process of awakening. Maury had a long and intense dream about the French revolution, saw the important politicians, murderers, riots etc. In the very last sequence he was led to the guillotine and in the moment the blade was hitting his neck, he awakened. Surprised, he recognized that a piece of his wooden bed top had fallen on his neck and had awakened him (he had slept in prone position). Because of the logical line of dream action, Maury (1861) hypothesized that the dream is generated backwards by the arousing stimulus. Accounts stemming from other domains such as near-death-experiences (e. g., reviewing very fast one’s whole life; Moody, 1989) support this view. Dement and Kleitman (1957) tested this hypothesis by awakening their subjects in random order after 5 minutes or after 15 minutes REM sleep. After having be woken up, the subjects should estimate the elapsed time interval. 83 % of 111 judgments were correct, i. e. the subjectively estimated time interval during dreaming correlated with the real duration of the REM sleep period prior to the awakening. These findings were replicated by Hobson and Stickgold (1995) using an ambulatory measurement unit ("nightcap") in the home setting. Jovanovic (1967) reported that awakenings conducted 20 seconds after the beginning of REM sleep yielded a report of a "dream start". In a similar way, dream reports of awakenings directly after gross body movements were mostly "dream ends" (Dement & Wolpert, 1958). These studies clearly indicate a relationship between dream time and duration of REM sleep and, thus, Maury’s hypothesis seems not to be correct. Moiseeva (1975), however, pointed out that the time interval is substantially underestimated if dreaming is very bizarre and intense. Research to test this hypothesis has not be carried out.
Dream content and eye movements
Early in 1892, Ladd has formulated the hypothesis that the eyes are moving during dreaming in a similar way as they do in the waking state. This hypothesis is known as "scanning-hypothesis". In order to test this hypothesis, two approaches were selected. First, global measures of visual dream activity were correlated with the number of eye movements occurring prior to the awakening. Second, it was attempted to relate the EOG pattern (polysomnographically recorded eye movements) to the dreamed gaze shifts. Dement and Wolpert (1958) scored 105 dream reports whether the last sequence was visual active (e. g., looking around) or visual passive (e. g. looking at a distant object). In a similar way, they classified the EOG recordings of the REM period prior to the awakenings. These independently made classifications were significantly correlated. Although in some studies this finding was replicated, others were not able to detect substantial relationships between visual dream content and global measures of eye movement activity (e. g. Firth & Oswald, 1975). Hence averaging over distinct time periods is necessary to derive global scores for visual dream activity and eye movements, correlations could be extinguished by this procedure.
In order to enable a direct matching of dream content and eye movements, Roffwarg et al. (1962) have chosen the following approach. Shortly after awakening the subjects were interrogated by an interviewer who had no access to the EOG recordings of the prior REM period, about the last 10 to 20 seconds of the dream experience. This interviewer who was familiar in relating particular actions to the corresponding eye movement patterns predicted the eye movements which he would expected to occur due to the elicited dream report. The following example gives an impression of this approach.
"The last thing I remember is looking down at a small piece of paper, held at about chest level, trying slowly and haltingly, dwelling on each word, to translate something that looked like 3 lines of French poetry. It took about 20 or 30 seconds to do it, probably. I don't remember if I looked up at any time from the paper. As I remember, essentially, I kept my eyes on the paper."
Interrogator’s prediction-"There should be relative REM quiescence with the exception of a few spaced leftward glances if the subject has finished a line of reading and return to the next line" (Roffwarg et al., 1962, p. 240).
The fact that the interviewer did not predict small eye movements (reading word by word) can be explained by the impossibility to detect such eye movements or the position of the eyeballs by the commonly used a. c. amplifiers. The corresponding EOG pattern showed three small but rapid eye movements to the left (22 seconds, 8 seconds and 0.5 seconds prior to the awakening). Overall, 80 % of the clear recalled dream actions corresponded with the EOG recording. Subsequent research confirmed small but substantial relationships (Herman et al., 1984).
Another interesting approach was adopted by Dement and Kleitman (1957). They have awakened their subjects after recording a distinct EOG pattern, e. g., solely horizontal or vertical eye movements or the lack of any eye movements. One subject who was awakened after vertical eye movements dreamed of standing at the bottom of a tall cliff and looking up at climbers at various levels. In a dream reported after a REM period with horizontal eye movements the subject was watching two peoples throwing tomatoes at each other. A quiet eye movement pattern was associated with driving a car (looking ahead) in the dream.
To summarize, a relationship between dream content and eye movements was demonstrable. It is, however, not clear how tight this relationship is, i. e. whether all eye movements are related directly to the dream gaze shifts. Additionally, methodological issues have to be kept in mind, e. g., the electric measurement of eye movements occurring during sleep, and that similar research areas, e. g. relating eye movements to waking imagery, encounter the same difficulties as found in dream research. This may be explained by the fact that the most eye movements are carried out without consciousness, e. g. looking at a small object, glances during a conversation, and, therefore, can not be exactly recalled by the subject.
Dream content and EEG
Several studies investigated the question whether corresponding brain areas are active during dreaming specific actions in the same way as they do in the waking state. Etevenon and Guillou (1986) found an activation of the left, central brain area accompanying two dreams with intense activity of the right hand. This finding supports a continuity between waking life and dreaming activity since this area correspond to an activation of the sensory motor projection of the right hand in the waking state. A recent study (Hong et al., 1996) investigated the relationship between talking and listening during the dream and the EEG. The advantage to utilize these activities in the context of measuring EEG is the relative good localization of the corresponding brain areas (Broca's area: talking and Wernicke's area: listening). The results confirmed the hypothesis that the lower the alpha power on the left sides of those areas corresponding to Broca's and Wernicke's area was, the more expressive or receptive language was present in the dream. The decrease of alpha power can be interpreted as an activation since alpha waves were found in relaxed wakefulness but not in an active waking state. In order to carry this research further it will be promising to draw upon modern neuroimaging techniques (Hobson et al., 1998) although due to the considerable expenditure very few subjects were studied during the dreaming state up to the present moment.
Dream content and EMG (Electromyogram)
During REM sleep the general muscle tonus is actively inhibited by the REM sleep generating area in the brain stem (e. g., Dement, 1974). This seems to serve a biological function in preventing the sleeping person to act out her or his dreams. However, in patients suffering from REM sleep behavior disorder, this atonia is partially not functioning (Schenck & Mahowald, 1996). The following case vignette illustrate this seldom occurring sleep disorder.
(REM sleep behavior disorder; man, 67 years old): "I was a halfback playing football, and after the quarterback received the ball from the center he lateraled it sideways to me and I'm supposed to go around end and cut back over tackle and - this is very vivid - as I cut back over tackle there is this big 280-pound tackle waiting -, so I, according to football rules, was to give him a shoulder and bounce him out of the way, supposedly, and when I came to I was standing in front of our dresser and I had knocked lamps, mirrors, and everything off the dresser, hit my head against the wall and my knee against the dresser. (Schenck et al., 1986, p. 294)"
This account clearly demonstrates the necessity of treating the disorder. However, it should not mixed up with sleepwalking which occurs during NREM sleep. Specific experimental set brain lesions in the brain stem of cats can be followed by complete absence of muscle atonia during REM sleep (Jouvet, 1979). These animals showed aggressive behavior, prey catching behavior, hygienic behavior etc. during REM sleep, i. e., it can be assumed that they were acting out their "dreams".
Despite the general muscle atonia, electrical impulses are measurable in the limb muscles causing at maximum small twitches (Dement, 1974). Wolpert (1960) have shown that dreams including a lot of dreamed body movements are associated with an elevated EMG activity in arms and legs. The studies of Grossman et al. (1972) and Gardner et al. (1975) replicated this finding and even were be able to differentiate between dreamed arm and dreamed leg activity by analyzing the limb EMGs. McGuigan and Tanner (1971) have measured chin and lip EMG during REM sleep and demonstrated that dreams with talking were accompanied by elevations of EMG activity in chin and lip muscles. This finding was confirmed by Shimizu and Inoue (1986). Based on research studies investigating the expression of emotions in the waking state, Gerne and Strauch (1985) recorded EMG potentials of the corrugator (brow lowerer, negative emotions) and of the zygomatic (lip corner puller, positive emotions). The following dream was reported after zygomatic EMG activity.
"I was sitting in class, at school. The woman teacher, up front, was saying that she was waiting for a patient who had been admitted because of an accident. And all of us were laughing and finding it quite amusing. The teacher was terribly embarrassed, as it hadn't worked out, but she laughed right along with us. (Strauch & Meier, 1996; p. 144)"
Whereas the study of Gerne and Strauch (1985) obtained statistically significant relationships, a replication study (Hofer, 1987) failed to do so.
To summarize, a correspondence between EMG activity and dreamed action or dreamed emotions had been demonstrated. Future studies have to pay attention to problems regarding measurement techniques (e. g. face EMG; Hofer, 1987) and, for example, for the possibility of interindividual different physiological patterns of expressing emotions (see next paragraph).
Dream content and autonomic parameters
The studies subsequently reviewed investigated the relationship between dream content and autonomic parameters such as respiratory rate, heart rate, skin resistance or sexual excitation. Psychophysiological studies of nightmares (e. g. Fisher et al., 1970) have shown that severe anxiety dreams are accompanied by elevations in heart rates and respiratory rates. Several correlational investigations (Fahrion, 1967; Engel, 1972; Stegie, 1973; Hauri & Van de Castle, 1973) have studied the hypothesized association between dreaming of emotional material and physiological arousal. Overall, the results are inhomogeneous for which Stegie et al. (1975) gave responsibility to two factors: First, the night time effect was not controlled in all studies, i. e., since physiological arousal as well as dream intensity tend to increase during the course of the night, artificial correlations will be computed in the case all awakenings across the night will be included. Second, Stegie et al. (1975) pointed out that psychophysiological research has been demonstrated what is known as "individual response specificity" in the waking state, i. e., different persons will show different physiological arousal patterns in response to the same emotional stimulus. If data of several persons were included into the correlational analysis the specificity will markedly reduce the correlation coefficient. In order to avoid this problem, Stegie et al. (1975) elicited dream series (14 to 28 dreams) of six subjects. The results, indeed, confirm their hypothesis since correlation patterns differed considerably from subject to subject. On has, however, to consider that quite a lot of correlation coefficients have been computed in that study and only 4 of 74 coefficients reached significance (p < .05) so that this just may be explained by chance.
Hobson et al. (1965) have reported that in dreams of healthy persons after awakenings following short apneas, specific respiratory themes, e. g. being choked, occurred more often. On the contrary, dreams with respiratory content were seldom found in patients with sleep apnea who experience long apneas often with a considerable reduction of blood oxygen saturation (Schredl, 1998a). The following dream example was reported by a patient with sleep apnea.
"During the dream I felt tied up or chained. I saw thick ropes around my arms and legs and was not able to move. I experience the fear of suffocation without being able to cope with the situation. Powerlessness and also resignation came up. (patient with sleep apnea, male, 39 years, respiratory disturbance index (RDI): 68.1 apneas per hour, maximal drop of blood oxygen saturation: 43 %; Schredl, 1998a, p. 295)"
For another physiological variable, the penile erections which occurs regularly during REM sleep in males, the relationship to dream content was investigated. Fisher, Gross and Zuch (1965) hypothesized an interaction between physiological and psychological factors. Since penile erections occur very often during REM sleep, they assumed an underlying physiological control system. Psychological factors (e. g. erotic dream content or dreamed anxiety) can modulate the degree of the erection. 8 out of 30 dream reports elicited after REM periods with marked erections were characterized by erotic themes whereas this topic was not present in any dream report elicited after a REM period without erection (Fisher, 1966). In the latter often anxiety were found. The phenomenon of "wet dreams", however, seems not to be related directly to dream content. Fisher (1966) who measured one nocturnal ejaculation of a subject in the sleep laboratory reported that his dream was not explicitly sexually toned: the dreamer held the hand of his girl friend. Similarly, LaBerge (1985) reported than none of his dreamed ejaculation in a lucid dream was accompanied by a real ejaculation.
First, the studies reviewed in the present article clearly demonstrate that dreaming can be seen as cognitive/emotional activity which occurs during sleep and that a dream is not generated during the awakening process. Second, evidence was brought together showing an interaction of mind and body during REM sleep. However, there seems not to exist a one-to-one correlation between dream content and physiological parameters. Therefore, a dualistic viewpoint (c. f., Fiss, 1979) of dreaming and REM sleep seems to be in line with the research hitherto done, i.e., neither physiological models nor psychological models alone are sufficient for interpreting the findings in this research area. This dualism should also kept in mind when the function of dreaming/REM sleep is taken into consideration. Since dreaming as mental activity which can be recalled after awakening is only a small part of the brain’s activity during REM sleep (similar to the relation of waking consciousness and activity of the whole brain), the function of dreaming can not be equated with the function of REM sleep, e. g. memory consolidation (Smith, 1995). The beginning of renewed growth in publication rates of psychological oriented papers and the relative decline of articles on sleep physiology since the early 1980s, may reflect the field's shift from pure physiological approaches to models including cognitive aspects as well (Nielsen & Germain, 1998).
Despite the clear evidence of a relationship between physiological processes and dream content, a lot of questions remained unresolved. First, how can it be explained that dreaming also occurs during NREM sleep. Although Antrobus (1983) has shown that these reports were more thoughtlike and contained less often images, the variability is considerable so that some NREM reports can not be distinguished from REM reports. The question arising is how is it possible that visual imagery is present and eye movements are absent. Unfortunately, research in the mind-body interaction during NREM sleep is scarce and should be done in the future.
A second question concerns the possibility how one level is altered as a result of changes in the other level. The research in patients with sleep apnea tested the hypothesis that physiological processes directly influence dream content which was shown to occur very infrequently. The findings of a recent study (Schredl, 1999b), however, can be interpreted in a way that the frequent arousal at the end of sleep apneas interfere with the dreaming process, i. e. reduce bizarreness substantially. In a similar way, patients with narcolepsy which is a disorder of REM sleep regulation show deviations in dream content in comparison to healthy controls (Schredl, 1998b). Their dreams were more bizarre and more negatively toned; that can be seen as a reflection of the overshooting regulations system. For future studies it will be fruitful to focus on persons with extraordinary sleep patterns or to choose an experimental approach, i. e., to manipulate distinct physiological processes (respiration, heart rate, endocrinology parameters) and measure the effect of these procedures on dream content.
On the other hand, research in lucid dreaming has shown that specific dream actions can be measured at the physiological level. The most often replicated finding is that dreamt eye movements (e. g. three large eye movements right-left-right) can be clearly seen in the EOG recording (Hearne, 1978; LaBerge, 1980). In quite similar studies, LaBerge (1985) has shown that sexual excitation and respiration rate can also be influenced by lucid dreamers. Although research has demonstrated that lucid dreams occur during REM sleep, it will be very interesting to test whether manipulation of ordinary, non-lucid dreams affect physiological processes in a similar way. For that purpose, one can utilize procedures such as presleep suggestions or films which have to been shown to be effective in altering dream content (Schredl, 1999a).
To summarize, it seems very important for dream research in the future to integrate psychological and physiological approaches in order to shed more light on the mind-body interaction in general and during REM sleep in particular.
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