“Next we must inquire into the dream: first with what part of the soul it is apparent, i.e. whether this affection is the work of the thinking or the perceptual part.” – Aristoteles

Dreaming has fascinated mankind for centuries and has been subject of inquiry already by ancient philosophers. People have been especially interested in attributing meaning to the contents of dreams. The best-known example is probably the work by Freud, who, in his book “The Interpretation of Dreams”, suggests that dreams reflect at their core unconscious desires of the individual.

That being said, I will not be discussing theories trying to gauge the potential meaning or function of dreams here (psychoanalytic or otherwise). So, if you are reading this article hoping to find out what latent desires you are unconsciously repressing in your latest anxiety dream, I’m afraid you will not find a satisfying answer here.

Instead, I will approach the topic of dreaming from a neuroscientific perspective and will attempt to find answers to the questions: What is happening in the brain when we are dreaming? When do dreams occur during sleep? How are specific dream contents represented in the brain? And, what is changing in the brain when we are becoming aware that we are dreaming?

What eye movements during sleep have to do with dreaming

When you are falling asleep at night, your brain cycles through five stages of sleep, which are characterised by distinct Electroencephalographic (EEG) signatures (see the picture below to get an impression). Simply speaking, EEG records electrical activity of populations of neuros using a set of scalp electrodes. As you progress through stages one to four, the EEG is characterised by an increasing amount of irregular, slow frequency waves with large amplitudes. The fifth sleep stage, however, is qualitatively different in that it more closely resembles an awake, attentive state. The EEG signature of this sleep stage is characterised by high-frequency, low-amplitude waves. In this stage, another interesting observation can be made, namely that your eyes are rapidly moving back and forth. Hence, this sleep stage is also known as rapid-eye-movement (REM) sleep (distinct from non-rapid-eye-movement (NREM) sleep).

An early study suggests that these periods of REM sleep are closely associated to vivid dream experiences. The researchers found that in about 75% of cases, participants that were woken during periods of eye-movement reported having dreamed prior to waking, while in about 83% of awakenings during eye-inactivity, people reported not recalling any dream experiences.

A later neuroimaging study gave further insight into what might be happening in the brain during this potential “dream stage”. They injected participants with radioactive tracers to detect changes in cerebral metabolism during REM sleep. Most notably, they found increased activation in the amygdalae and several cortical areas known to be connected to this region, which they interpret as engagement of emotional memory processes during REM sleep.

Why REM sleep is not dream sleep

At this point, one could call it a day and conclude that dreaming is equivalent to REM sleep and thus the neural correlates of REM sleep are equivalent to the neural correlates of dreaming. While the association between REM sleep and dreaming has inspired much subsequent research, it seems that dreaming can also occur during NREM sleep.

In one exemplary study demonstrating the independence of dreaming and REM sleep, researchers collected dream reports every hour during the night and found that reports after the first sleep stage and REM sleep were very similar in terms of frequency, complexity and bizarreness. Furthermore, administration of clomipramine (an antidepressant known to significantly reduce or even abolish REM sleep physiology) only induced slight changes in NREM sleep dream recall characteristics. This suggests that dream recall during NREM sleep stages are not simply residuals of previous REM episodes.

Over the years, several hypotheses emerged trying to explain how dream experiences might be generated during NREM sleep. Some argue that different neural systems support dream generation in REM and NREM sleep, others argue in favour of a common neural mechanism, while yet others explain NREM dreams as the result of “covert REM sleep”.

A recent study sheds some light on the potential neural correlates of dreaming common to all sleep stages. The researchers found that independent of sleep stage (NREM or REM sleep), reports of dream experiences were consistently accompanied by a distinct pattern of change in EEG frequency bands over posterior cortical areas at the back of the head. This condition seems to be necessary for conscious experience during sleep, since the researchers could predict with an average accuracy of 87% whether a person just experienced a dream or not when awakening the participant at a certain point during the night according to this activation profile.

Decoding the contents of dreams

So far, we have only focussed on changes in brain activity while dreaming as compared to not dreaming. This simple dichotomy, however, neglects the rich diversity of contents of dream experiences. Therefore, one might ask the question, whether we can decode specific dream contents from patterns of brain activation (If you want to learn more about the science of mind reading, you should also check out this previous post).

Imagine that in your dream you are walking along a gravel path through a rose garden and at the end of the path you spot a house with an old woman sitting in a rocking chair on the porch. A team of researchers from Japan tried to decode the visual contents of such dream experiences.

First, they trained classification algorithms on response patterns in the visual cortex to the presentation of images of different categories (e.g. plants, buildings, female persons, etc.). They then applied these algorithms to functional MRI data acquired just before awakening from stage one sleep to predict reported dream contents. Using these decoders, the researchers could predict which one of two selected, mutually exclusive content categories was present in a given dream (e.g. asking whether the above dream experience contained plant versus food imagery) with a mean accuracy of 70.3%. They also obtained above chance-level prediction accuracy when trying to predict the presence or absence of each content category simultaneously. But don’t be alarmed. These read-outs are only based on category information and are not reliable enough to reconstruct specific dream scenarios. So, your dreams are save (for now).

These findings nevertheless imply that dream contents, like mental imagery, rely on similar neural substrates as actual visual perception. This conclusion was further supported by a follow-up study from the same lab, where the researchers demonstrated that features of dreamed objects are represented in the brain in a manner similar to waking perception.

Lucid dreaming: Awakening in your dream

When retelling last night’s dream to a close friend, chances are that you will start the conversation along the lines of: “I had the weirdest dream last night…”. Yet, at the time you were experiencing the dream, everything seemed perfectly normal and you did not question the strange circumstances of the situation. However, you may also have experienced at some point the sensation of realising that you are dreaming while you were still sound asleep. This strange phenomenon is called lucid dreaming and can actually be trained to some degree of success. But what is happening in the brain when you become aware that you’re dreaming?

An initial attempt of answering this question lead group of researchers to compare brain activity recorded with functional MRI during lucid to non-lucid REM sleep in one lucid dreamer (a heroic attempt given the scarcity of frequent lucid dreamers and the difficulty of falling asleep in the unusual scanner environment). Lucid dreaming can be identified in such studies because participants can indicate lucidity via eye-movement signals. They found that lucid dreaming was associated with activation in a fronto-parietal network of brain areas implicated in self-reflection and metacognition (thinking about your own mental processes), as well as with activation in higher visual areas.

In another pioneering study, researchers even tried to induce dream lucidity by applying alternating current between frontal and temporal parts of the head while participants were in REM sleep. They found that 25 and 40 Hz stimulation could induce certain aspects of lucidity as reported by participants following awakening.

Dream on

Hopefully, this brief excursion into the world of dream research could convince you that the scientific investigation of this topic is notoriously challenging yet it opens a unique window into human consciousness. Lucid dreaming in particular is a powerful tool in this endeavour since it allows researchers to study dreams as they are happening thus reducing reliance on retrospective reports. Still, it will take some time until we truly understand how the brain generates dreams. Until then we can only continue dreaming.