Affective Neuroscience, Blog, Brain research, Cognitive Neuroscience, EEG research, Emotional Brain, Sleep research

Dreaming: A peculiar form of cognitive activity

Dreams are a complex phenomenon that most people experience during sleep. They are characterized by a series of thoughts, images, and emotions that are felt and recalled sometimes with high vividness. Such activity is associated with the so-called rapid eye movement (REM) sleep phase, which entails a separate phase from the no-REM (NREM) sleep cycle. Brainwave activity linked to REM sleep resembles that of the waking brain making it hard to distinguish between them. Recently, new research has found for the first time a novel brain pattern that allows predicting when someone is dreaming and even the content of dreams. In this post, you will get to know better what is going on in the brain when you dream and the purpose of dreaming.

What happens when we sleep?

When we sleep, the entire brain is active doing different work than that usually does during wakefulness. We call this activity dreams, and special type of cognitive activity characterized by experiencing vivid thoughts and “visualizations” during certain stages of the sleep cycle.

Most “realistic” or “lucid” dreams happen while we are experiencing the REM sleep stage (Nielsen, 2000; Voss et al., 2009). This stage usually begins about 90 minutes after falling asleep and can last up to an hour toward the end of sleep. Such activity is part of the normal sleep-wake cycle and is controlled by the reticular system circuits connecting the brain stem to the cortex through the thalamus.

During REM sleep, almost all muscle tone relaxes except the eye muscles and the diaphragm. This explains why sometimes people experience episodes known as sleep paralysis (Avidan et al., 2011), feeling unable to move while dreaming, for example, of a stressful situation (such as being chased).

Another type of dream activity also occurs during deep sleep or slow-wave stage (SWS). This type of NREM sleep appears much less realistic and not very well structured (Nielsen, 2000), which may explain why recall of that particular dream activity appears much fuzzier and apparently inaccessible.

Representation of the human sleep cycle. NREM and REM sleep cycle occurs every 90 min. During the first half of the night, prevail the SWS, while NREM and REM sleep dominate in the remaining half of the night (Van der Hem and Walker, 2009)

Why does the dream experience feels so real?

A striking aspect that characterizes peoples´ dreams is the specific content, which greatly varies from person to person. Whether is it that your dreams are of escaping monsters, traveling by train, or flying up through the clouds, the main responsible of specific narratives you see in your dreams is the visual cortex.

The emotional component also contributes to making you feel dreams with intense doses of vividness. As in waking, the limbic system in the mid-brain is responsible for the emotional aspect of dreams. The transient activation of the amygdala during REM sleep relates to the emotional content of dreams, partly contributing to the consolidation of emotional memories as well as for next‐day emotional regulation (Corsi-Cabrera, 2016).

Reasons why some people remember dreams and others forget them

However, not all people are capable of remembering their dreams so easily. Does this mean that they don´t dream at all? The accepted scientific answer is that all people do dream even if they don´t remember it. The question to be investigated is why don´t all remember them. There are some potential reasons that may explain why a person forgets or is unable to recall dreams easily:

  1. Neurotransmitter activity. In a published article in the journal Behavioral and Brain Sciences, researchers claimed that people forget their dreams due to changing levels of neurotransmitters acetylcholine and norepinephrine that occur during sleep (Becchetti and Amadeo, 2016)
  2. Brain structure. Specifically, differences has been shown in the density of white or grey matter of brain centers such as the Amygdala, Hippocampus, Medial Prefrontal cortex (MPFC) and Temporoparietal junction (TPJ). In 2018, a published study that involved 92 volunteers suggested that the participants who self-reported high dream recall had higher white matter density in their MPFCs than the group of the low dream recall (Vallat et al., 2018). The authors of another study demonstrated that increased activity in the TPJ and MPFC might promote intra-sleep wakefulness, facilitating the encoding of dreams in memory (Eichenlaub et al., 2014).
  3. Lack of sleep or sleep deprivation. Consistently don´t getting get enough rest will drop the proportions of REM sleep period, making harder to remember the content of dreams in the following day. A 2010 study investigated the effect of one night of sleep deprivation on dream recall. Results confirmed drastic decrease in dream recall (>75%) after 40h of prolonged wakefulness as compared to baseline (De Gennaro et al., 2010).
  4. A dream-prone personality. Although not drastically, personality traits have been proposed as a possible predictor factors of high dream recall (Schredl & Göritz, 2017). Scientific findings strengthen this association. Specifically, those people prone to imagination, creative thinking, and daydreaming are commonly much more likely than others to remember their dreams with vivid imagery (Schredl et al., 2003). At the same time, those who tend to be more focused on what is outside themselves often have difficulties to recall their dreams. For instance, art students tend to be greater dream recallers than engineering students (Schechter et al.,  1965).
  5. Other factors related to lifestyle, attitudes toward dreams and experiencing a trauma also could explain individual differences in dream recall. In general, current evidence supports a moderate correlation between these factors and dream recall. However, further research is needed to confirm such relationship more consistently (Armitage,1992; Punamäki, 2007; Schredl et al., 2002).

The purpose of dreaming

Much has been investigated about the role of sleep in regulating brain and body health. However, the functional role of dreams remains barely understood.

In 1977 two biologists, Hobson and McCarley, devised the activation-synthesis model contradicting Sigmund Freud’s (1900) theory of dreaming. Hobson´s theory suggest that dreaming is something random that occurs mainly for biological reasons without neccesarily entailing a crucial meaning.

A second widely supported theory is that dreams have benefits on store memories and learning consolidation. Research shows that sleeping immediately after learning new information may result in a better recall (Rasch & Born, 2013; Wamsley, 2014).

A third suggested reason explaining the purpose of dreams has to do with some sort of mental balance maintenance. We know from neuroimaging studies that brain regions involved in the encoding of emotional memories are highly activated during REM sleep (Phelps and LeDoux, 2005; Armony, 2013). These structures are directly responsible for the reprocessing and consolidation of emotions during REM sleep (van der Helm et al., 2011; Deliens et al., 2014). Through activating a traumatic event encoded in form of oniric memory, the attached emotion itself is no longer active. Such an extinction mechanism fulfills an important soothing role in helping relief our emotions, especially negative ones, which ultimately reduces anxiety (Scapelli et al., 2019).  

Although these theories have received acceptable scientific support, the results are not conclusive and researchers still debate about why we actually dream.

Electrophysiological patterns of dreaming

Brain correlates of dream experience has stimulated scientific interest for decades, becoming crucial also in the study memory processes and consciousness.

Disentangling the EEG pattern of dream activity encompasses the following “hot topics”:

  1. Dream recall

Some studies confirm a pivotal role of theta (5–7 Hz) and alpha (8–12 H) bands in dream recall which is consistent with the involvement of both brain rhythms in the episodic memory formation and retrieval (Esposito, Nielsen and Paquette, 2004; Klimesch, 1996; Scarpelli et al., 2015). In particular, scientist have shown that successful recall is consistent with frontal theta activity during REM sleep (Domhoff, 2003; Marzano et al., 2011).

Schematic representation of memory reactivation and consolidation during sleep (Adapted from Paller and Voss, 2005)

2. Dream occurrence

A subsequent focus is to detect when someone is dreaming with high level of accuracy. This is exactly what a group of scientists from the University of Wisconsin have investigated using EEG recordings during both, REM and no REM sleep (Siclari et al., 2017). Specifically, researchers compared the brainwaves differences between individuals who said they had experienced dreams while sleeping versus those who said they had not. As a novel finding, the study demonstrated that a decrease in low-frequency EEG power (1-4 Hz) in a posterior region of the brain (i.e., parieto-occipital) was systematically shown only when individuals reported experiencing a dream.  Interestingly, this brainwave pattern emerged regardless of the individuals´ ability to recall the dream and irrespective of the sleeping phase (REM or No-REM).

3. Dream content

Using the same EEG approach, researchers were also able to “decipher” the content of individuals´ dreams based on detected activity in other brain areas. For example, specific dream contents such as thoughts, speech or faces were associated in this case, with an increase of high-frequency EEG activity (>25Hz) over prefrontal / parietal cortex, which corresponds closely to those regions also engaged during waking processing of the same contents (Siclari et al., 2017; Scarpelli et al., 2017).

Althought findings described above provide important insights about dreaming, these evidence should be taken with caution because the study does not demonstrate a causal relationship between specific EEG marker with the experience of dreaming. Reliable ways of investigating the dreaming brain may include higher consistency between methods and neuroimaging techniques. An integrative approach will allow scientists to overcome the challenges posed by dreams as an object of study.

Conclusions

Dreaming is a mental phenomenon, not directly observable with the naked eye. Thanks to modern neurotechnology underlying brain mechanisms characterizing sleep and the possible purpose of dreaming are today a bit more accessible.  The latest contributions of dream research based on the study of brainwave patterns shows an important upgrade in the current landscape.

Although sleep is a fascinating field of research, still further studies are needed to identify neural markers of dreams reliably and thus, reinforcing the current evidence of the role of brainwaves in predicting peoples´ dreams. One current challenge in the field is to determine the influence of dreams on emotional processing and cognition as well as their relationship to consciousness.

References

Armitage, R. (1992). Gender differences and the effect of stress on dream recall: A 30-day diary report. Dreaming, 2(3), 137–141. https://doi.org/10.1037/h0094354

Armony J. L. (2013). Current emotion research in behavioral neuroscience: the role(s) of the amygdala. Emot. Rev. 7, 280–293. 10.1177/1754073912457208  https://doi.org/10.1177/1754073912457208

Avidan, Alon Y.; Zee, Phyllis C. (2011). Handbook of Sleep Medicine (2 ed.). Lippincott Williams & Wilkins. p. Chapter 5. ISBN 9781451153859.

Becchetti, A., & Amadeo, A. (2016). Why we forget our dreams: Acetylcholine and norepinephrine in wakefulness and REM sleep. Behavioral and Brain Sciences, 39, E202. doi:10.1017/S0140525X15001739

De Gennaro L, Marzano C, Moroni F, Curcio G, Ferrara M, Cipolli C. Recovery sleep after sleep deprivation almost completely abolishes dream recall. Behav Brain Res. 2010 Jan 20;206(2):293-8. doi: 10.1016/j.bbr.2009.09.030. Epub 2009 Sep 27. PMID: 19788898.

Deliens G., Gilson M., Peigneux P. (2014). Sleep and the processing of emotions. P. Exp. Brain Res. 232:1403. 10.1007/s00221-014-3832-1 [PubMed

Domhoff GW. The scientific study of dreams: Neural networks, cognitive development, and content analysis. Washington, APA Press, 2003.

Eichenlaub, JB., Nicolas, A., Daltrozzo, J. et al. Resting Brain Activity Varies with Dream Recall Frequency Between Subjects. Neuropsychopharmacol 39, 1594–1602 (2014). https://doi.org/10.1038/npp.2014.6

Esposito MJ, Nielsen TA, Paquette T. Reduced alpha power associated with the recall of mentation from Stage 2 and Stage REM sleep. Psychophysiology 2004;41:288-97. [PubMed]

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Schredl, M., & Göritz, A. S. (2017). Dream recall frequency, attitude toward dreams, and the Big Five personality factors. Dreaming, 27(1), 49–58. https://doi.org/10.1037/drm0000046

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Siclari, F., Baird, B., Perogamvros, L. et al. The neural correlates of dreaming. Nat Neurosci 20, 872–878 (2017). https://doi.org/10.1038/nn.4545

Vallat, R., Eichenlaub, J. B., Nicolas, A., & Ruby, P. (2018). Dream Recall Frequency Is Associated With Medial Prefrontal Cortex White-Matter Density. Frontiers in psychology9, 1856. https://doi.org/10.3389/fpsyg.2018.01856

van der Helm E., Yao J., Dutt S., Rao V., Saletin J. M., Walker M. P. (2011). REM sleep depotentiates amygdala activity to previous emotional experiences. Curr. Biol. 21, 2029–2032. 10.1016/j.cub.2011.10.052  [PubMed]

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