Sleep and Dreams

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Sleep and Dreams:

Sleep and Dreams AWAKE Group Presentation May 2, 2007 Victoria Zvonkina, M.D.

Dreams:

Dreams Dreams have fascinated people for centuries. The reasons why dreams occur are still obscure. When something is not clear, there is always room for speculation. Dream interpretation is still very popular. Internet has many dream interpretation websites ( example below).

Why We Dream:

Why We Dream The search for an understanding of dreams is thousands of years old. Yet to this day there are disagreements between researchers. Over the past 100 years, three theories dominated research: - Late 19 th century, Freud’s Interpretation of Dreams; - 1950s report of correlation of dreaming and newly discovered rapid eye movement (REM) sleep; - 1970s theory that dreaming was triggered by random neural activity in the brainstem. New approach to dreaming is to consider it as a memory processing mechanism.

Brain Activity During Sleep:

Brain Activity During Sleep To understand how dreams may be produced and their functions, it is important to understand how brain activity differs in wake and REM and non-REM sleep. A series of positron emission tomography (PET) studies showed that most brain regions are inactive during slow wave sleep, entry into REM leads to reactivation of some regions, along with deactivation of others. This pattern in REM suggests a shift away from conscious executive control and towards hallucinatory (activation of sensory association cortices) and emotional (activation of amygdala, anterior cingulate, and medial orbitofrontal cortex) processing. Maquet P, 2001; Hobson et al, 1998; Nofzinger EA et al, 1997.

Sleep and Memory Processing:

Sleep and Memory Processing Sleep has been shown to enhance prior learning of perceptual and motor skills, paired word associates, and emotionally charged episodic memories. In humans, positron emission tomography (PET) studies have shown that brain regions activated during learning a task were selectively reactivated during the next night’s REM sleep (Maquet P, et al, 2000.) In rats, similar results were shown as well (Louie K, Wilson MA, 2001.)

Memory Consolidation:

Memory Consolidation Not all sleep is equal in relation to learning and memory consolidation. For example, improvement on a motor skill task has been reported to correlate with amounts of late night light (stage 2) Non-REM sleep (Walker et al, 2002), but improvement on a visual perception task to correlate with both late-night REM sleep and early-night slow-wave Non-REM sleep in separate studies (Stickgold R, 2000; Karni A et al, 1994.)

Memory Consolidation – cont.:

Memory Consolidation – cont. If memory processing in sleep is differently activated during sleep stages, and dreaming at least parallels, and possibly contributes to these processes, then we would expect to see changes in the dream content during different sleep states . Simply put, this is exactly what is seen. In REM sleep, dreams are hallucinatory, emotional, narrative, and with frequent fictive movements. It is thought to facilitate consolidation of visual perceptual and emotional memories . Non-REM sleep is more thought-like and less hallucinatory, and is implicated in in simple memorization (word pair lists experiments).

Memory and REM sleep:

Memory and REM sleep One experiment focused on incorporation of waking events into the dream content (Fosse MJ, 2003.) Out of 299 dream reports, there were 364 waking dream elements. However, only 1-2% of these appeared as possible replay of waking events. Therefore, “we dream about what happened, but not what actually happened.” (Strickgold R, 2003.)

Incorporation of Waking Events Into Dreams:

Incorporation of Waking Events Into Dreams Another experiment involved subjects palying a game of Tetris 2-3 hrs for 2 or 3 days. Subjects: 12 subjects with no prior Tetris experience – “novices”; 10 with extensive Tetris experience – “experts”; and 5 subjects with dense amnesia with extensive medial temporal lobe damage from either anoxia or encephalitis – “amnesiacs.” On the evening of each day of game play subjects were awakened repeatedly during the first hour of night, and asked to recall thoughts, feelings, or images from sleep. Nine of the novices, and five of the experts (a total of 64%) reported visual images of the game at sleep onset. 60% of amnesiacs also reported images from the game, however they could not recall playing the game due to amnesia. The reports were similar between the groups, such as seeing little squares going down the screen, and occasionally rotating them; but none reported seeing the desk or the room. Thus the imagery had no characteristics of episodic memories, and it was also limited the aspects of the experience to which subjects paid most attention. Strickgold R, et al. Science 2000;290:350-353.

Memory and Dreams:

Memory and Dreams These examples illustrate that memory processing indeed occurs during sleep. They also show that different types of memories are processed differently by the brain. Different stages of sleep serve different functions in memory processing.

Dream Content From an Evolutionary Standpoint:

Dream Content From an Evolutionary Standpoint

Dream Content From an Evolutionary Standpoint:

Dream Content From an Evolutionary Standpoint There is no unified explanation, what role do dreams play, and due to what evolutionary pressures they have evolved. It seems maladaptive to lose responsiveness to the surroundings and experience hallucinations instead of real events. Sleep however, is an easily reversible state, and different stimuli can be sensed strongly enough to trigger an awakening.

Sleep In a Novel Environment:

Sleep In a Novel Environment One evolutionary adaptation is that animals sleep in safe sites where they can be less vulnerable to attack by predators. Therefore, many mammals spend a lot of energy and time seeking and protecting sleep sites (Hobson, 1989.) A novel environment can present unknown dangers, and individuals will often not have a such a good night sleep as in a familiar environment. Sleeplessness and fitful sleep that are often experienced in a novel environment function to provide an opportunity to learn to identify external stimuli, which on subsequent nights may be ignored (Symons, 1993.) Sleep in a novel environment will contain a higher number of nocturnal awakenings and a shorter latency of REM sleep (Gundel, et al, 1993.)

“Vigilance” Mechanism:

“Vigilance” Mechanism Vigilance hypothesis (Symons et al, 1993) does not give any explanation to the function of dreams, but it states that natural selection has disfavored those hallucinations that compromise external vigilance. Since the sleeper is unable to see or move, vision and movement do not play a role in providing accurate information about external world. These are non-monitored modalities. Therefore, vision and movement hallucinations can exist during sleep without compromising vigilance.

“Vigilance” Mechanism – cont.:

“Vigilance” Mechanism – cont. Modalities such as touch, smell, or sound can compromise vigilance by interfering with signals coming from the external environment. These modalities are monitored by the sleeper.

Vision in Dreams:

Vision in Dreams Dreams during REM sleep are predominantly visual (Hobson, 1988.) Some detection of external visual stimulation still occurs. Gross changes in illumination will cause awakening, because they can either signify a potential danger, or a suitably timed bright light will be interpreted as a circadian cue, such as light of dawn (Deacon & Arendt, 1994.) External light flashes may be incorporated into dreams (Dement & Wolpert, 1958.)

Movement in Dreams:

Movement in Dreams We are generally immobile during sleep, so dreaming of movement (similar to dreaming of vision) does not compromise vigilance. Immobility during sleep is an adaptive mechanism, protecting us from ourselves. People with narcolepsy often have injuries due to sleep walking (Bear et al, 1996.) Frequent vestibular activation during REM sleep may explain dreams of flying, floating, and falling. Dreams of falling, however, may trigger awakening due to the vigilance theory, because falling can occur during sleep unlike other kinds of motion (Symons, 1993.)

Kinesthetic Sense in Dreams:

Kinesthetic Sense in Dreams Kinesthetic sense, or a sense of detecting the actual body position, is important to monitor during sleep. Therefore, there are virtually no dreams with kinesthetic sensation in them (Hobson, 1988.) Rechtschaffen, one of the fathers of sleep medicine, (1973) writes: “yet it is true that someone might sleep through a very exciting dream, whereas we would probably not have very much difficulty in awakening him by lifting his arm.”

Sense of Touch in Dreams:

Sense of Touch in Dreams Sensation of touch is not very often present in dreams in order for us to be able to recognize a real stimulus. Symons (1993) reports frequencies of touch sensations in dreams from 1% to 9%. According to the vigilance hypothesis natural selection could not favor vivid dreams of tactile sensations, because inability to detect a real tactile sensation could compromise survival. We have all heard, when a person is wondering whether they are sleeping they may pinch themselves.

Sense of Smell in Dreams:

Sense of Smell in Dreams Dreams of smell, or olfaction would also not be favored by evolution because they would compromise vigilance to real smells. Olfactory sensations were reported only in 1% of dream narratives (Symons, 1993.) The speculation is that a smell signifying a known danger should awaken the sleeper. However, studies of neutral smells (i.e., peppermint) on sleep show a low percent of awakening (Badia et al, 1990.)

Audition in Dreams:

Audition in Dreams Sleepers should also be vigilant to external noises. The data on this subject is contradictory. External noises are being well detected by sleepers (Degaute et al, 1992; Arkin, 1981.) However, 63% of dream narratives contain auditory sensations (Symons, 1993.) The theory is, that auditory dreams relate primarily to speech, but not to other types of auditory signals that could compromise survival.

In Summary:

In Summary Sleep research so far has not disproved the vigilance hypothesis. It explains from an evolutionary standpoint why we experience only certain sensation in dreams, and not others. However, this hypothesis does not explain why we dream in a general sense.

Conclusions:

Conclusions The leading scientific theory is that sleep has a large function in memory processing. It appears from different research studies that different stages of sleep have different functions with respect to memory processing. Future research is needed to better identify these functions. It also remains to be understood, whether dreams have a specific function, or are simply a byproduct of memory processing in sleep.

References:

References Arkin AM. Sleep Talking: Psychology and Physiology. 1981, Hillsdale, NJ: Erlbaum. Badia P, Wesensten N, Lammers W, Culpepper J, Harsh J. Responsiveness to olfactory stimuli presented in sleep. Physiology and Behavior 1990; 48:87-90. Bear MF, connors BW, Paradiso MA. Neuroscience: Exploring the Brain. 1996, Williams & Wilkins. Deacon SJ, Arendt J. Phase-shifts in melatonin, 6-sulphatoxymelatonin and alertness rhythms after treatment with moderately bright light at night. Clinical Endocrinology 1994; 40:413-420. Degaute JP, Borne P, Kerkhofs M, Dramaix M, Linowski P. Does non-invasive ambulatory blood pressure monitoring disturb sleep? Journal of Hypertension 1992; 10:879-885. Dement WC, Kleitman N. Cyclic variations in EEG during sleep and their relations to eye movements, body motility, and dreaming. Electroencephalogr Clin Neurophysiol 1957; 9:673. Dement W, Wolpert E. The relation of eye movements, body motility, and external stimuli to dream content. Journal of Experimental Psychology 1958; 55:543-553. Fosse MJ, et al: Dreaming and episodic memory: A functional dissociation? J Cogn Neurosci 2003; 15:1-10. Freud S: The Interpretation of Dreams. 1955, Basic Books. Gundel A, Nalishiti V, Reucher E, Vejvoda M, Zulley J. Sleep and circadian rhythm during a short space mission. Clinical Investigator 1993; 71:718-724. Hobson JA. The Dreaming Brain. 1988, Basic Books. Hobson JA. Sleep. 1989, Scientific American Library. Hobson JA, McCarley RW. The brain as a dream-state generator: an activation-synthesis hypothesis of a dream process. Am J Psychiatry 1977; 134:1335-1348.

References – cont.:

References – cont. Hobson JA, Stickgold R, Pace-Schott EF. The neuropsychology of REM sleep dreaming. Neuroreport 1998; 9:R1-R14. Karni A, et al. Dependence on REM sleep of overnight improvement of a perceptual skill. Science 1994; 265:679-682. Louie K, Wilson MA. Temporally structured replay of awake hyppocampal ensemble memories during sleep. Neuron 2001; 29:145-156. Maquet P, et al. Experience-dependent changes in cerebral activation during human REM sleep. Nat Neurosci 2000; 3:831-836. Maquet P. the role of sleep in learning and memory. Science 2001; 294:1048-1052. Nofzinger EA, et al: Forebrain activity in REM sleep: an FDG PET study. Brain Res 1997; 770:192-201. Rechtschaffen A. The psychophysiology of mental activity during sleep. The Psychophysiology Of Thinking. 1973, Academic Press. Strickgold R: Memory, cognition, and dreams. Sleep and Plasticity. Oxford University Press, 2003, pp 17-40. Strickgold R, et al. Replaying the game: hypnagogic images in normals and amnesiacs. Science 2000;290:350-353. Strickgold R, et al. Visual discrimination task improvement: a multi-step process occuring during sleep. J Cogn Neurosci 2000;12:246-254. Symons D. The stuff dreams aren’t made of: why wake-state and dream state sensory experiences differ. Cognition 1993; 47:181-217. Walker M, et al. Practice with sleep makes perfect: sleep dependent motor skill learning. Neuron 2002; 35:205-211.

Thank You:

Thank You