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.[1]
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To Sleep, Perchance to Dream - Crash Course Psychology #9
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Wake up, I'm Speaking: The Neuroscience of Sleep and Dreaming
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Does Cognitive Neuroscience Research Validate Freud?
Transcription
Comedian Mike Birbiglia was having trouble with sleep. Though not with the actual sleeping part -- one night, while staying in a hotel, he dreamed that a guided missile was on its way to his bed, and in his dream, he jumped out the window to escape it. Unfortunately, he also did this not in his dream. From the second floor. And the window was not open. This little episode cost him 33 stitches and a trip to a sleep specialist. Mike now sleeps in zipped-up mummy bags for his own safety. The lesson here? Sleep is not some break time when your brain, or your body, just goes dormant. Far from it. In truth, sleep is just another state of consciousness. And only in the past few decades have we begun to really plumb its depths -- from why we sleep in the first place, to what goes on in our brains when we do, to what happens when we can’t sleep. And there is a lot that science has to say about your dreams! Talk about weird! It’s like Sigmund Freud meets Neil Gaiman. So, even though it may seem like you’re dead to the world, when you sleep, your perceptual window remains slightly open. And kinda like Mike Birbiglia’s hotel room window, a trip through it can make for a pretty wild ride. But for your safety and enjoyment, I’m here to guide you through this state of consciousness, where you’ll learn more than a few things about human mind, including your own. And here’s hoping you won’t need any stitches when we’re through. [INTRO] Technically speaking, sleep is a periodic, natural, reversible and near total loss of consciousness, meaning it’s different than hibernation, being in a coma, or in say, an anesthetic oblivion. Although we spend about a third of our lives sleeping, and we know that it’s essential to our health and survival, there still isn’t a scientific consensus for why we do it. Part of it probably has to do with simple recuperation, allowing our neurons and other cells to rest and repair themselves. Sleep also supports growth, because that’s when our pituitary glands release growth hormones, which is why babies sleep all the time. Plus, sleep has all kinds of benefits for mental function, like improving memory, giving our brains time to process the events of the day, and boosting our creativity. But even if we’re not quite sure of all the reasons why we sleep, technology has given us great insight into how we sleep. And for that we can thank little Armond Aserinsky. One night in early 1950s Chicago, eight-year-old Armond was tucked into his bed by his father. But this night, instead of getting a kiss on the forehead, little Armond got some electrodes taped to his face. Armond’s dad was Eugene Aserinsky, a grad student looking to test out a new electroencephalograph, or EEG machine, that measures the brain’s electrical activity. That night, as his son slept peacefully, he watched the machine go bonkers with brain wave patterns, and -- after making sure that his machine wasn’t somehow broken -- discovered that the brain doesn’t just "power down" during sleep, as most scientists thought. Instead, he had discovered the sleep stage we now call REM or rapid eye movement, a perplexing period when the sleeping brain is buzzing with activity, even though the body is in a deep slumber. Aserinsky and his colleague Nathaniel Kleitman went on to become pioneers of sleep research. Since then, sleep specialists armed with similar technology have shown that we experience four distinct stages of sleep, each defined by unique brainwave patterns. Say you’re just going to bed. All day your endocrine system has been releasing “awake” hormones like cortisol. But with nightfall comes the release of sleepy melatonin hormones from the pineal gland. Your brain is relaxed, but still awake, a level of activity that EEGs measure as alpha waves. You’re feeling sleepy, your breath slows, and suddenly you’re asleep. This exact moment is clearly evident on an EEG reading, as those alpha waves immediately transition to the irregular non-Rapid Eye Movement stage one (NREM-1) waves. It’s in this first stage of sleep you might experience hypnagogic sensations -- those brief moments when you feel like you’re falling, and your body jerks, startling you. As you relax more deeply, you move into NREM-2 stage sleep, as your brain starts exhibiting bursts of rapid brain wave activity called sleep spindles. You’re now definitely asleep, but you could still be easily awakened. NREM-3 comes with slow rolling delta waves. We now know that you can have brief and fragmentary dreams in the first three stages of sleep, but eventually you’ll get to the most important stage: full REM sleep, that famous stage of sugarplum slumber that makes eyeballs go nuts, grants vivid visual dreams, and provided the namesake for a certain famous rock band. REM sleep is kinda paradoxical. Your motor cortex is jumping all over the place, but your brainstem is blocking those messages, leaving your muscles so relaxed that you’re basically paralyzed. Except for your eyes. That whole sleep cycle repeats itself every 90 minutes or so, transitioning back and forth between the stages of sleep. Obviously sleep is super important, and lack of sleep is terrible for your health, mental ability, and mood. In fact it’s a predictor for depression, and has been linked to things like weight gain, as your hunger-arousing and -suppressing hormones get out of whack. Sleep deprivation also causes immune system suppression, and slowed reaction time which is why you should not drive sleepy. Of course, a bad night’s sleep here and there is part of life, but there are a host of bona fide sleep disorders out there that can really make life pretty terrible, or in Mike Birbiglia’s case, land you in the emergency room. We’ve got insomnia, which is persistent problems of falling or staying asleep. And kind of its opposite, narcolepsy, whose sufferers sometimes experience brief, uncontrollable attacks of overwhelming sleepiness, called “sleep attacks.” This, as you can imagine, can get in the way of all sorts of things that you might enjoy doing, like driving, eating, pole-vaulting. Narcolepsy may have several different causes, including a deficiency in the neurotransmitter hypocretin, which helps keep you awake. But in more rare cases, brain trauma, infection, and disease may contribute to it as well. So, that’s rare, but you probably know someone with sleep apnea, the disorder that causes sleepers to temporarily stop breathing, until their decreased oxygen levels wake them up. Birbiglia, meanwhile, turned out to have a REM sleep behavior disorder, which we don’t fully understand yet, but appears to be associated with a dopamine deficiency. Then we’ve got night terrors, which are as terrible as they sound... spurring increased heart and breathing rates, screaming, and thrashing that’s seldom remembered upon waking. Night terrors are most common in children under seven, and may be spurred by stress, fatigue, sleep deprivation, and sleeping in unfamiliar surroundings. Much like sleepwalking and sleeptalking, night terrors occur during the NREM-3 stage of sleep, and are NOT the same as nightmares, which occur, like most dreaming, during REM sleep. But oh, in REM sleep, what dreams may come... There you are, running naked as your teeth fall out, being chased down the beach by a Matt Damon centaur. You wake up, feel around your mouth thinking what? What? What?! WHAT?! Welcome to your dreams, those vivid, emotional images racing through your sleeping brain, often providing a backdrop so bizarre that it may seem like David Lynch, Terry Gilliam, and Tim Burton are trying to out-weird each other in a film festival. A really, really long festival, considering the average person spends about six years of their lives dreaming. So yeah, sometimes you have really crazy dreams. But mostly, your average dream usually just sort of unpacks and reshuffles what you did that day. For example last night I dreamt about Tumblr, cause I spent a lot of time on Tumblr yesterday. If you played Tetris all afternoon, you might dream of blocks falling from the sky. If something traumatic happened to you, your brain might provide you with a nightmare to help extinguish your daytime fears - Thanks, brain! Then again...you might be unable to stop dreaming about the trauma, which we’ll look at in the future when we discuss post-traumatic stress disorder. Our two-track minds of course allow us to register more stimuli than we outwardly acknowledge during the day, and in that way, the sounds of car alarms or stinky dog farts that you might not even have noticed may get incorporated into your dream, too. And that’s all interesting and weird and sometimes a little gross, but what’s the real purpose of dreaming? Whyyy do we do this? Well, as you might have guessed, there’s more than one idea out there… The study of dreams is is a mix of neuroscience and psychology known as oneirology. Oneiros is the Greek for dream, and if you’re a Neil Gaiman fan you may recognize it as one of the Sandman’s many names. The one that comes with a toga and Orpheus’s head. But Sandman aside, if you want to talk dreams, we might want to start with our old friend Freud. In his landmark 1900 book The Interpretation of Dreams, Freud proposed that our dreams offer us wish-fulfillment. He thought a dream’s manifest content, the stuff you remember in the morning, was a sort of censored and symbolic version of whatever inner conflict was really going on in that dream’s unconscious, or latent, content. Not surprisingly, the wish-fulfillment theory lacks scientific chops and has for the most part fallen out of favor -- because, really, you can interpret a dream any way you want. Like, sometimes a cigar is just a cigar. Luckily we have some other theories to consider. The information processing theory proposes that our dreams help us sort out and process the day’s events and fix them into our memories. This may be particularly important when it comes to learning and remembering new information, and some studies show that people recall new tasks better after a good REM sleep full of dreams. But if brainwave readings show us anything, it’s that there’s a lot going on in your brain when you dream, and the physiological function theory suggests that dreaming may promote neural development and preserve neural pathways by providing the brain with stimulation. When our brains are stimulated, they expand their connections more. So, babies, for example, spend much of their sleep time dreaming, perhaps in part to help their brain circuitry develop more quickly. This is similar to the idea that dreams are part of our cognitive development. By this model, dreams draw on our knowledge and understanding of the world, mimicking reality, and engaging those same brain networks that light up when we daydream. And finally, there are theories that focus on the way REM sleep triggers neural activity, and the idea that dreams are just sort of accidental side-effects, the brain’s attempt to weave a story out of a bunch of random sights, emotions, and memories -- which is how in dreamland you might actually marry that Matt Damon centaur and give birth to a baby with banana fingers and a raccoon tail. For now scientists continue to debate the function of dreams, but one thing we know for sure is that REM sleep is vital, both biologically and psychologically. But, hey, you think your dreams are nut-bar? Next week, we’re looking at other altered states of consciousness, where you’ll learn what your brain really looks like on drugs, and whether you can actually hypnotize someone to do your evil bidding… or just act like a chicken. For now, if you’ve stayed awake during this episode, you learned about the four stages of sleep -- NREM 1, 2, 3 and REM itself -- as well as some major theories for the psychological purpose of dreaming, including information processing, physiological function, cognitive development, and neural activity models. Thanks for watching, especially to all of our Subbable subscribers, who make this whole channel possible. If you’d like to sponsor an episode of Crash Course, get a special Laptop Decal, or even be animated into an upcoming episode, just go to Subbable.com/crashcourse. This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins, the script supervisor is Michael Aranda, who’s also our sound designer, and the graphics team is Thought Café.
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.[2] Also, remembering is exposed to interference at the recall stage and some information is not accessible to recall.[2] 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.[2] 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,[3] 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.[3] 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.[3]
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).[4] 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.[5] 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).[6]
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.[citation needed]
It has been recognized that following REM awakenings dream reports are obtained substantially more frequently than after NREM awakenings.[7] 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.[8] Reports from REM awakenings tend to be longer, more multimodal perceptually, have intensified emotionality, and are less reminiscent of waking life than NREM awakenings.[5] 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.[5]
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.[9] 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.[9] In dreams there is a reduction or absence of self-reflection or other forms of meta-cognition relative to during waking life.[5] Dreams are also characterized by a lack of "orientational stability; persons, times, and places are fused, plastic, incongruous and discontinuous".[9] In addition, dreams form a single narrative to explain and integrate all dream elements.[9] Lastly, NREM reports contain thought-like mentation and depictions of current concerns more frequently than REM reports.[5]
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.[10] 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;[6] 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.[6] Furthermore, these NREM dreams seemed to cluster around specific sleep stages (stage 1 and late stages).[10] 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.[6] 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.[10] Supporting evidence did come from the flip-side of clinico-anatomical correlations however. In a compilation of all reported cases of dream cessation (111 cases in all) damage was located in an entirely different area of the brain than the brain stem.[10] Furthermore, REM sleep was maintained. Remember that the pons is crucial for REM. Loss of dreaming only occurred when higher parts of the cerebral hemispheres were damaged. REM sleep is controlled by cholinergic activation in the pons. It is now believed that dreaming may be a dopaminergic process that occurs in limbic and frontal areas of the brain.
Dopaminergic activation
Two main frontal areas have been implicated in the dream process. The first involves the deep white matter of the frontal lobes (just above the eyes). The main systems at work here involve the mesolimbic and mesocortical dopaminergic pathways. There are connecting fibres that run between frontal and limbic structures. A dopaminergic pathway runs from the ventral tegmental area, ascends through the lateral hypothalamus, various basal forebrain areas (nucleus basalis, stria terminalis, shell of nucleus accumbens) and terminates in the amygdala, anterior cingulate gyrus and frontal cortex. Damage to the dopaminergic pathway results in a loss of dreaming. Furthermore, chemical stimulation of the pathway (with L-DOPA for example) increases the frequency and vividness of dreams without affecting REM sleep.[10] The mesolimbic and mesocortical pathways are considered the seeking areas or the motivational command centers of the brain. Damage not only results in the loss of dreams but also of motivated behaviour.[6] Transection or inhibition of the dopamine pathway also reduces some positive symptoms of schizophrenia, many of which have been likened to dream-like states. Drugs that block the system have anti-psychotic effects but also reduce excessive and vivid dreaming.[10] Further evidence that dreaming can occur independently of REM sleep is found in the occurrence of nocturnal seizures during NREM that often present themselves as nightmares. Activation here is seen in the temporal lobe, again a forebrain area.[6][10]
The evidence of the involvement of mesolimbic and mesocortical dopaminergic pathways suggests that dreaming occurs when a motivational component is activated. Only when this pathway is removed do dreams cease to occur. This system can be activated by mechanisms of REM sleep but can also occur independently during NREM stages of sleep.
Perceptual processing
Another area thought to be involved in the generation of dreams is the Parieto-Occipito-Temporal junction (PTO).[10] This is an area of grey cortex towards the back of the brain involved in the highest levels of perceptual processing. It is here that perceptions are converted into abstract thoughts and memories.[6] The PTO is also vital for mental imagery.[10] Damage specifically to this area results in complete loss of dreaming, however damage to lower levels of perceptual processing merely results in reduced aspects of dream imagery. This is the basis for the suggestion that dreaming involves a reversed sequence of perceptual events. Instead of bottom-up it is top-down (higher levels activating lower levels instead lower to higher). Activation of the motivational mechanisms in the brain would normally be directed toward goal-oriented actions. However, during sleep access to the motor system is blocked (by inactivation of the dorsolateral frontal convexity). As a result, activation moves backwards toward the perceptual areas. This is why the dreamer doesn't engage in motivated behaviours but imagines them. Furthermore, there is inactivation of the reflective system in the limbic brain which leads the dreamer to mistake the dream for reality. Damage to this area also results in the inability to distinguish dreams from reality during waking state.
Notes
- ^ Humphrey, M. E., & Zangwill, O.L. (1951) Cessation of dreaming after brain injury. Journal of Neurology, Neurosurgery, and Psychiatry, 14, 322.
- ^ a b c Schwartz, S., & Maquet, P. (2002). Sleep imaging and the neuro-psychological assessment of dreams. Trends in Cognitive Sciences, 6(1), 23-30.
- ^ a b c Domhoff, B., & Kamiya, J. (1964). Problems in dream content study with objective indicators. Archives of General Psychiatry, 11(5) 519-532.
- ^ 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.
- ^ a b c d e 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.
- ^ a b c d e f g Solms, M. (2005). The interpretation of dreams and the neurosciences. Neuroscience and Freud's Dream Theory.
- ^ Stoyva, J.M. (1965). Posthypnotically suggested dreams and the sleep cycle. Archives of General Psychiatry, 12(3), 287-294.
- ^ (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.)
- ^ a b c d Hobson, J.A. 1988. The dreaming brain: How the brain creates both the sense and the nonsense of dreams. Basic Books, NY.
- ^ a b c d e f g h i Solms, M. (2000). Dreaming and REM sleep are controlled by different brain mechanisms. Behavioral and Brain Sciences, 23, 843-850.
This page was last edited on 12 May 2024, at 20:20