Veneta Callpani| Department of Applied Psychology, New York University| Cognitive Neuroscience
We spend one third of our life sleeping and it’s only natural to think about the importance and the function that sleep has in our mental and physical wellbeing. It is obvious that sleep deprivation has many negative effects such as cognitive deficits: attention, working memory, ability to learn, also other symptoms such as lack of energy, compromised immune system, and emotional dysregulation (1). In terms of evolution, sleeping at night might reduce the time spent foraging, reproducing or monitoring the environment for any danger (1). However, it seems that sleep has evolved to have an essential function, alongside with dreaming. Dreams have been of interest since the dawn of time. In the Interpretation of dreams, Freud predicted that: “Deeper research will one day trace the path further and discover an organic basis for the mental event” (2). While the evolved biological function of sleep has advanced, no equivalent understanding of dreams has emerged (3). Some theories view dreams as epiphenomena, and many of the proposals for their biological function are contradicted by the phenomenology of dreams themselves (3). This review is an attempt to bring together the sleep and dream research in a parallel way and see a big picture of where we are so far in terms of underlying brain mechanisms for sleep and dreams, scientific methods of studying dreams, different theories, and how we can move forward into new areas of investigations into the enigmatic cognitive dimensions of sleep and dreams.
Methods of studying sleep and dreams: sleep stages
Polysomnography (PSG), which is considered “sleep study” (1), Electroencephalogram, fMRI, Electromyogram (EMG) Electrooculogram (EOG), Positron Emission Tomography (PET) (1,4,5) are methods used to study sleep and dreams. In the case of sleep disorders (and sleep disorders breathing) PSG is used in combination with electro-oculogram, chin and leg electromyogram, electrocardiogram, and air flow at the nose and mouth. (1). Before the advanced technology and neuroimaging studies, dreams were studied mostly through dream reporting. However, in order to better understand dreams and dream research, it is important to have some general background of sleep studies and stages. So far, scientists categorize sleep into two major components: Rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep (1). NREM sleep is further divided into: N1, N2, N3. In adults, approximately 5% of the total sleep time is Stage N1; 50% Stage N2; and 20% is Stage N3 sleep. The remaining 25% is REM stage sleep. (1). N1 is considered the stage of transition from wakefulness to sleep, and N3 is considered the “deep sleep” or slow wave sleep, and REM sleep is usually associated with dreaming, but the function of REM sleep is yet uncertain (1). However, there is a difference between NREM and REM phases of sleep in terms of somatic activity. During NREM the blood pressure, heart rate and respiratory rate are decreased while REM sleep is associated with an increase and irregularity of these functions, and a rise in global cerebral glucose metabolism (6). Neuroimaging studies have shown that the visual regions of the brain, the pontine reticular formation, limbic and paralimbic regions have an increased activity during REM sleep (6) and maybe this is one of the reasons scientists believed that dreams occur only during REM sleep. Another reason to believe that, was probably related to the fact that subjects woken from REM sleep reported that they were dreaming 70-95% of the cases, whereas only 5-10% of the cases after NREM sleep (5). Brain damage or brain lesions are another way to study sleep and dreams. These studies showed that, in fact, REMS can occur without dreaming, and dreaming can occur without REMS (5). Damage in or near the temporo-parieto-occipital junction was associated with a complete loss of dreaming. (5). Also, 80% incidence of nearly complete loss of dreaming was reported in the cases of prefrontal leucotomy. PET functional studies have identified areas that seem responsible for dream characteristics (5). Patients with lesions in the visual association cortex experience loss of visual imagery in dreams and when awake; but is preserved if the lesions are in the primary visual cortex (7).
Another interesting way and method developed to study the brain during dreaming, meditation and non-ordinary brain activities is through dissipative models. This model proposed to study the neuronal dynamics within mathematical frame of many-body physics (8)
Types of dreams
Knowing the types of dreams helps us categorize them and better understand their possible function and different hypotheses and theories. Prestson Ni (9) summarized 7 types of dreams in his review article: Current/recent events dreams are the most common dreams where events that occurred during the past 24 or 48 hours are reflected in the dream.
Symbolic dreams help us process life events in a metaphorical way, and if properly interpreted can give us insights.
Fantasy/comfort dreams are aspiration, wish-fulfillments, and compensations of the struggles during waking life. It helps us find comfort if we are going through a difficult time.
Creative/ problem solving dreams help us solve problems that we have been thinking a lot during awaking time.
Nightmares can occur during a stressful situation in our life, and might also represent fears, anxieties or unresolved traumas.
“Supernatural” dreams are very rare but also the most memorable. Examples of these dreams include: premonition dreams in which the dreamer has a vision of the future event which later becomes true; telepathic dreams where the dreamer receives communication about a person or event occurring elsewhere and when the dreamer awakes, details of the dreams are verified to be true; shared dreams happen when different people have the same dream approximately at the same time, and visitation dreams in which a recently deceased family, fiend, partner, or pet appear. These dreams are often vivid, loving and life changing.
Lucid dreams are very interesting because it allows the dreamer to control their dreams. The dreamer is aware that is dreaming. This type of dream can have many potentials and benefits. A recent study (4) used lucid dreaming to communicate with the dreamer in real time. This study will be discussed further in the dream theories section.
Dream’s function hypothesis and theories
There are different types of dreams and each of them seems to have a different function and use. However, the question of dream evolution, the mechanism of how dreams are created, and scientific study is still very challenging despite the new technology and different methods used to study sleep and dreams. This might also be related to our limited knowledge, tools and scientific understanding of the complexity of our brain. So far, there are different hypotheses and theories to why the dream has evolved, and what function dreaming might play in our life both psychologically and physically.
Threat Simulation theory
The threat simulation theory postulates that dreaming may fulfill a neurobiological function by allowing an offline stimulation of threatening events, and this mechanism would promote adapted behavioral responses in real life situations (10). Continuing in the line of this theory, Stepernich et al. (10) conducted two studies to address two questions: do emotions in dreams (in this case fear) engage the same neural circuits as during wakefulness and is there a link between emotions experienced in dreams and brain responses to emotional stimuli during wakefulness. The aim of the first study was to find the neural correlates of fear in dreams. They woke the dreamers up with an intense alarm in different stages of sleep and got a report of the last thing they dreamed and if fear was involved. They used EEG to study the difference in signals between fear vs non-fear responses during dreaming. REM reports with presence of fear compared to those without fear were associated with decreased delta power in the bilateral insula and midcingulate cortex. The results of the first study indicated that the occurrence of frightening dreams coincided with increased activation of the insular cortex during both REM and NREM sleep, and midcingulate cortex during REM sleep. The aim of the second study was to establish whether neurophysiological responses to fear-eliciting emotions during wakefulness correlated with fear in dreams. Here fMRI was used and during the emotional tasks eye movements and pupil diameter were measured continuously. Taken together, these two studies showed that individuals who reported a high prevalence of fear related emotions in their dreams, had stronger fear inhibition during wakefulness (10).
Overfitted brain hypothesis
The hypothesis of overfitted brain (OBH) suggests that dreams have evolved to assist generalization (3). This generalization is assisted by stochastic corruptions of normal sensory input, which combats the highly biased nature of inputs during an animal’s daily learning that can lead to overfitting, a ubiquitous problem in artificial neural networks and machine learning in general (3). This hypothesis doesn’t contradict some hypothesis of dreams, it just adds new dimensions. As per OBH hypothesis, by providing departures away from the statistically biased input of an animal’s daily life, dreams can assist and therefore increase performance. (3)
Memory consolidation theory
This theory explains that dreams have evolved for memory consolidation (3). There is still debate about this theory, because a significant line of direct evidence for the consolidation theory comes in the form of “replay” of memories during sleep hypothesis. However, the increased firing in the hippocampus that counts as “replay”, occurs also during wakefulness (3).
Plailly et al. (11) presented an overview of an experiment to further establish the possible link of dreaming and memory consolidation. In this experiment, thirty-two high dream recallers freely explored new visuo-olfactory episodes for 3 consecutive days. The first three sessions were used for encoding, and the retrieval occurred on the fourth session to assess participants memory of the episodes perceived during encoding. All participants had to wear a wrist actimeter to assess sleep parameters, and dream reports were taken. Participants who experienced learning-related and/or experiment related dreams had significantly better visuo-spatial memory of episodes, in comparison with other participants. The results support the hypothesis that the learning phase is loosely incorporated into dreams and that this incorporation is associated with sleep related memory consolidation (11)
Another recent study showed a study method that allowed for a two-way communication between the experimenters and the dreamers (participants). Konkoly et al (4) conducted experiments with 36 individuals in 4 different places: USA, Germany, France and Netherlands. All participants had some prior experience with lucid dreaming, and one of them (French) had narcolepsy and therefore had a lot more experience and abilities with lucid dreaming. Participants were trained to give signals when lucid dreaming (left-right eye movement: LR) and also were trained to answer some questions (math questions or YES and No answers) by either eye movement signals or facial muscle movements. However, they didn’t know which question would be presented in their sleep during the experiment. When conducting the experiment, the information was transmitted to the dreamer in the form of spoken words, flashing lights, or beeping tones. Electroencephalogram (EEG) electrooculogram (EOG) and electromyogram (EMG) were used to measure sleep states and wave patterns, and the dreamers’ signals. Dream reports were obtained after each correct answer. In total, 18.4% of participants answered correctly the questions during sleep. These results document examples of sleep learning. Participants were able to remember pre-sleep instructions on how to respond and apply it in a novel way during sleep. This new method of studying dreams opens up new doors for new experiments and a possible deeper level of understanding of sleep and dreaming.
Reverse learning hypothesis
This hypothesis was proposed by Notably et al. pointing out that the purpose of dreaming is to remove the “undesirable” connections and help the brain “unlearn” (3). This hypothesis has been ignored largely, however recent computational modeling in spiking neural networks have shown that “reverse learning” in the form of reverse leaning rules can indeed be helpful (3)
Another interesting hypothesis was proposed by Dr. David Mauroce, professor of ocular physiology in the Department of Ophthalmology at Columbia-Presbyterian Medical Center (12). He started to wonder about the function of REM sleep after learning about a young man who was in an accident and suffered from immobilization of eyes. This leads the cornea to become laced with blood vessels, (normally cornea has no blood vessels) presumably to supply the cornea with oxygen (12). He knew that when eyes are closed during non-REM sleep, oxygen can reach the cornea from the iris only by diffusion across the stagnant aqueous humor. Using a mathematical model, he established that oxygen supplied under those conditions would be insufficient. Therefore, he proposed that humans experience REM sleep to supply much needed oxygen to the cornea of the eye.
Despite all hypothesis and theories, one thing we know for sure is that sleep is very important for our survival, and we have come to realize that so are dreams. While there is no final answer about the function of either one, it is important to point out that studying only REM sleep to know more about dreams, or studying only the physiological processes, are not enough. Understanding the physiological function of dreams can be insightful and play an important role, however, the psychological component and the meaning of dreams is also important. Humans are creatures of meaning, and as seen in this review, the content and the meaning of dreams can be very revealing and have a huge impact on our mental wellbeing. As everything else, there are pros and cons, and there is always room for misusing scientific knowledge, and maybe this can be an area of future exploration. How do we use the power of dreams; what is the limit, and where should we draw a line? Will there come a time where creating a protocol of lucid dreaming might become necessary as a safety precaution? If dreams (and sleep) can be used as a powerful tool to ‘predict’ the future, to communicate with a deceased person, or communicate with other people via dream sharing; to what extent can we use this without damaging our body? To what extent controlling our dreams won’t turn against us, and to what extent this can interrupt the natural functioning of dreams.
As seen from this review, dreams can have multiple functions, and dreaming can occur during all stages of sleep. As an analogy; same as neurotransmitters can have a different function when in a different part of the brain, dreams can have different functions (or take on different forms) depending on the stage of sleep. Also, depending on people’s interest, dreams can be used for many different things, and one of them can be as a therapeutic tool.
Understanding the importance of sleep/ dreams, might also help our society become more aware of the role sleep plays in our life and the cost of sleep deprivation on our body/mind but also in the society as a whole.
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