The Function of Dreaming: Perspectives from Neuroscience

Abstract

Dreaming is a phenomenon that is shared by all of humanity and is fully understood by no one. Historically, the function of dreaming has been defined as philosophical or religious, although there is no scientific evidence to reinforce these beliefs. In the twentieth and twenty-first centuries, technological advances have given researchers the ability to measure the observable data produced by brain activity during sleep and dreaming. Due to these observable events, researchers have developed multiple, scientifically-based hypotheses to explain the function of dreaming. Three of these perspectives are explored and compared to discern further the currently understood function of dreaming. A discussion of the activation-synthesis hypothesis, neurocognitive hypothesis, and emotional regulation hypothesis are included. Also included are an explanation of the observable data and neurological implications of each study. Future implications of this research include possible dream intervention in the case of night terrors or post-traumatic stress disorder.

Keywords: dream, neuroscience, activation-synthesis, neurocognitive, emotional regulation

The Function of Dreaming: Perspectives from Neuroscience

Throughout history, people of all cultures and times have wondered what the function and purpose of dreaming is. In early history, many theories were based on philosophical and religious contexts, theorizing that the gods must be communicating to us through our dreams. Later in history, scientists like Freud asserted that dreaming must communicate the subconscious mind, our inner selves' desires. In the mid-1950s, researchers discovered the phenomenon of rapid eye movement (REM) during sleep by accident. When researchers awakened patients during REM sleep cycles, patients recalled vivid and bizarre dreams. When researchers awakened patients during non-rapid eye movement (N-REM) sleep cycles, patients only rarely remembered dreaming. A clear correlation was made between REM sleep and dreaming, which led to a flurry of excitement and subsequent hypotheses in the scientific community. Fast forward to the 21st century, and researchers seem to have even more questions than they do answers. Why do we dream? What is the function of dreaming? Does dreaming serve a physical purpose? Neurological? Psychological? Spiritual? Some combination of all of these? This empirical study will compare and contrast, analyze, and present commonalities amongst the top three dream function hypotheses to show that the function of dreaming is to process subconscious emotional information.

Activation-Synthesis Hypothesis

The first hypothesis is known as the activation-synthesis hypothesis. According to this hypothesis, dreaming represents the brain's attempt to process unrelated information by making sense of it in a dream format. This hypothesis asserts that the brain even connects unrelated physical posture into dreaming, which is believed to account for common dreams like flying or falling and being unable to move (Hobson & McCarley, 1977; Hobson, Pace-Schott, & Stickgold, 2000; McCarley & Hoffman, 1981). During dreaming, which usually occurs in REM sleep, the human brain is the most active than other sleep cycles, but the human body is the most relaxed. Therefore, the human mind sensing that the body is positioned either flat on its back or in the prone position attempts to make sense of these external stimuli by processing it into our dreams.

The activation-synthesis hypothesis provided the scientific community with new evidence for potential neural substrates of the dream process. The data from their research suggested that the process of dreaming depended on the physiology of the brain and could be quantified mathematically (Hobson & McCarley, 1977). Researchers found that forebrain activation, brain stem activity in the sensory-motor circuits, and shifts in sensory transmitter ratios could explain most of the characteristics of dreaming from the hypothesis's perspective. In other words, according to the nature of the data and observable brain activity, the brain is merely attempting to make sense of random information from external stimuli accrued throughout the day, and that is currently affecting the physical body of the dreamer (Hobson & McCarley, 1977).

The problem with this hypothesis is that it is almost impossible to test and prove because of dreams' nature. The collection of data from dreaming is self-reported, which nullifies the researcher's ability to fact check these memories. Another issue is that dreams are usually not remembered and if details do survive, they often fade quickly. The final issue is specific to this particular hypothesis, which points to a lack of consistency in physical stimuli integration and dreaming. Furthermore, dreaming has been proven to have no connection to current stimuli (Foulkes & Domhoff, 2014; Nir & Tononi, 2010). The inconsistency in the manifestation of characteristics that are supposedly hallmarks to this hypothesis has urged the scientific community to continue asking what the function of dreaming is.

Neurocognitive Hypothesis

The neurocognitive hypothesis approaches dreaming as information processing during circumstances that defy reality. During dreaming, the prefrontal cortex is relatively inactive, explaining that logic rarely comes into play during the dream experience. This hypothesis also draws attention to the fact that dreaming usually incorporates stimuli from recent memory and submits this information into a confusing world, explaining the oddity of dreaming (Solms, 1997, 2000). Many of the body's sensory functions are diminished or inactive during sleep; therefore, other areas are free to add input, which might not be processed in the same way otherwise. Finally, in the dream world, we lose all sense of planning as we experience the dream as it occurs (Hobson, 2009). This phenomenon is also due to the working memory's suppressed activity, located in the prefrontal cortex.

From the neurocognitive perspective, dreaming lacks a higher meaning; it is merely information processing under less restricted means than the waking mind provides. The weaknesses in this hypothesis are similar to the weaknesses discussed in the activation-synthesis hypothesis in that there is little to no way to test the data or predict its observable outcomes. Again, dreams are self-reported, which leads to a litany of difficulties with accuracy and verifiability. Interestingly enough, however, the neurocognitive hypothesis revealed exciting findings on which areas of the brain are highly active during dreaming. These are the hypothalamus, amygdala, and other primary processing centers for emotions and motivation (Gvilia, Turner, McGinty, & Szymusiak, 2006).

Emotional Regulation Hypothesis

Thanks to more recent advances in neuroimaging technology, researchers are able to determine the neural substrates involved in waking emotional responses. By using Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI), scientists can test and isolate regions of the brain that are used for emotional processes in awake subjects. By identifying these areas as the brain's emotional pathways, researchers can then hypothesize the connection between the emotional function of these areas in both waking and sleeping contexts. Although dream neuroactivity is not homogenous, there are similarities between specific brain regions in these two contexts (Maquet, 2000; Nir and Tononi, 2010). An exciting characteristic of brain activity during sleep is that the emotional memory encoding and consolidation regions are more active than others (Phelps and LeDoux, 2005; Armony, 2013). This leads scientists to consider an emotional function in dreaming.

Any layman will report that their dreams have similarities to their waking memories as many of the events throughout the subject’s day translate into the dream context. Dreaming is much more than just a replay of these events, however, as the dream world exists without logic, planning, or limitation (Hobson et al. (2014). Therefore, insisting the dreaming is merely a replay of the day’s events is reductive at best (Kirov, 2013; Hobson et al., 2014). Instead, dreaming allows a creative problem-solving state in which the dreamer can solve or process emotional stimuli in ways that the real world may not allow (Revonsuo, 2000, Revonsuo et al. 2015). Evidence that both the theta and gamma EEG activities are essential for dreaming and emotional processes (Klimesch et al., 1996; Marzano et al., 2011) coupled with the findings that neural regions for encoding and consolidation of emotions are highly active in both waking and individuals during REM sleep (Phelps and LeDoux, 2005; Armony, 2013), solidify the Emotional Regulation Hypothesis.

No hypothesis is without weakness, however. As with previous research on REM sleep and dreaming, subjects self-report; therefore, there is no way for researchers to verify information gathered from subjects or compare participants' experiences. Another weakness in this hypothesis is the lack of sufficient data in the common areas of waking emotional regulation and dreaming's neural activity. Technology is continually advancing, and with it, new research and hypotheses are tested, and historical research is invalidated. These are both weaknesses of this and every other current study.

Concluding Remarks

Each of these hypotheses presents a different perspective of the same objective – the function of dreaming. Rather than competing with one another, this writer views the hypotheses discussed as building blocks, one upon another, leading the scientific community closer to the truth of the sleeping mind's more significant function. The activation-synthesis hypothesis asserted that dreaming is the processing of internal and external stimuli. This hypothesis stops short of explaining as to why this occurs. Nonetheless, this theory helped the scientific world understand some neurological processes involved in dreaming. The neurocognitive hypothesis takes the previous hypothesis a step further, agreeing that one of the many functions of dreaming is to process information, but that is happening in a world with none of the barriers in reality due to some areas of the brain being less active. These hypotheses help explain the how of dreaming but do not dare to scratch the surface of why. Enter the emotional regulation hypothesis.
By combining the knowledge presented by previous research, expanding upon it, and connecting those findings with practical application, the emotional regulation hypothesis's originators offer both an explanation as to how dreaming occurs and why every person seems to share this sleeping experience. The function of dreaming is to process emotionally charged stimuli which humans are ill-equipped to evaluate within the confines of our physical and limited reality. Feelings are intangible, internal, limitless responses to stimuli. Therefore, it stands to reason that they are best processed in an intangible, internal, and limitless context like dreaming.

As scientists discover more about the process and function of dreaming, treatments, and interventions are being developed for those that struggle with sleep disorders. By studying frequency bands created during dreaming, researchers can understand more about processing emotional and traumatic memories like those that struggle with Post Traumatic Stress Disorder (PTSD). Understanding more about dreaming may help scientists develop a way to influence dream content for those who suffer from nightmare disorders like night terrors. Techniques like Transcranial Alternating Current Stimulation (tACS) (Voss et al., 2014) may be enhanced by further research, especially of dream enacting behaviors (DEB) (Rivera-García et al., 2018), which may offer a more quantifiable and verifiable way to measure the effects and function of dreaming. Through future research, dreaming may be used to defuse emotional trauma, resolve traumatic memories, prepare an individual for future challenges and teach an individual how to cope with life's most profound and sometimes most debilitating events.

References

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

Foulkes, D., & Domhoff, G. W. (2014). Bottom-up or top-down in dream neuroscience? A top- down critique of two bottom-up studies. Consciousness and Cognition, 27, 168–171.

Gvilia, I., Turner, A., McGinty, D., & Szymusiak, R. (2006). Preoptic area neurons and the homeostatic regulation of rapid eye movement sleep. Journal of Neuroscience, 26, 3037– 3044.

Hobson, J. A., & McCarley, R. W. (1977). The brain as a dream state generator: An activation- synthesis hypothesis of the dream process. American Journal of Psychiatry, 134, 1335– 1348.

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, 793–1121. Hobson, J. A. (2009). REM sleep and dreaming: Towards a theory of protoconsciousness. Nature

Reviews Neuroscience, 10, 803–813.
Hobson, J. A., Hong, C. C. H., and Friston, K. J. (2014). Virtual reality and consciousness

inference in dreaming. Frontiers in Psychology. 5, 1133.
Klimesch, W. (1996). Memory processes, brain oscillations and EEG synchronization. Int. J.

Psychophysiol. 24, 61–100. doi: 10.1016/S0167-8760(96)00057-8
Marzano, C., Ferrara, M., Mauro, F., Moroni, F., Gorgoni, M., Tempesta, D., et al. (2011).

Recalling and forgetting dreams: theta and alpha oscillations during sleep predict subsequent dream recall. J. Neurosci. 31, 6674–6683. doi: 10.1523/JNEUROSCI.0412- 11.2011

Maquet, P. (2000). Functional neuroimaging of normal human sleep by positron emission tomography. J. Sleep Res. 9, 207–232. doi: 10.1046/j.1365-2869.2000.00214.x

McCarley, R. W., & Hoffman, E. (1981). REM sleep, dreams, and the activation-synthesis hypothesis. American Journal of Psychiatry, 138, 904–912.

Nir, Y., & Tononi, G. (2010). Dreaming and the brain: From phenomenology to neuro- physiology. Trends in Cognitive Sciences, 14, 88–100.

Phelps, E. A., and LeDoux, J. E. (2005). Contributions of the amygdala to emotion processing: from animal models to human behavior. Neuron 48, 175–187. doi: 10.1016/j.neuron.2005.09.025

Revonsuo, A. (2000). The reinterpretation of dreams: An evolutionary hypothesis of the function of dreaming. Behav. Brain Scie. 23, 877–901. doi: 10.1017/S0140525X00004015 Revonsuo, A., Tuominen, J., and Valli, K. (2015). “The Avatars in the Machine - Dreaming as a Simulation of Social Reality,” in Open MIND, eds. T. Metzinger and J. M. Windt

(Frankfurt am Main: MIND Group).
Rivera-García, A. P., López Ruiz, I. E., Ramírez-Salado, I., González Olvera, J., Guerrero, F. A.,

and Jiménez-Anguiano, A. (2018). Emotional facial expressions during REM sleep

dreams. J. Sleep Res. 28:e12716. doi: 10.1111/jsr.12716
Solms, M. (1997). The neuropsychology of dreams. Mahwah, NJ: Erlbaum.
Solms, M. (2000). Dreaming and REM sleep are controlled by different brain mechanisms.

Behavioral and Brain Sciences, 23, 843–850.
Voss, U., Holzmann, R., Hobson, A., Paulus, W., Koppehele-Gossel, J., Klimke, A., et al.

(2014). Induction of self-awareness in dreams through frontal low current stimulation of gamma activity. Nat. Neurosci. 17, 810–812. doi: 10.1038/nn.3719