What Happens in the Brain while Dreaming


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The brain does something special during sleep. Instead of simply switching off, it activates patterns of activity that can lead to dreaming. But what exactly is happening to the brain during a dream? It’s a question that has puzzled scientists for a long time, and new research is revealing some answers.

In the sleeping brain, there are two major states: rapid eye movement (REM) and non-REM. Non-REM is split into separate stages, each occupying a progressively deeper phase of sleep. The first and lightest stage of non-REM sleep features slow eye movements and brain waves at a relatively high frequency of around 4 to 7 Hz. On the other end of the spectrum, the deepest stage of non-REM sleep is commonly referred to as slow-wave sleep and features brain waves at lower frequencies (less than 4 Hz). During this deep stage, the brain is minimally reactive to external sounds, and people are less likely to wake up.

As its name suggests, one of the most distinctive features of REM sleep is that people exhibit quick and jerky eye movements. REM is not the only time people dream, but it does seem to be when people experience the longest, most vivid, and most bizarre dreams. In a bid to stop people from acting out these dreams while asleep, the brain inhibits motor systems to reduce muscle action or even paralyze muscles. People commonly experience this paralysis during nightmares when they try to run but their legs feel powerless.

In a 2017 study, researchers tested whether brain activity during REM and non-REM sleep would predict what happened in people’s dreams. The researchers woke people up at various intervals during the night and asked them to describe any dreams they were having. The researchers also measured the strength of people’s slow brain waves. For both REM and non-REM sleep, people were more likely to report having a dream when brain waves were weak. When they were stronger, the men and women in the study were more likely to report feeling unconscious with no dream experiences whatsoever.

Dreams can feature all kinds of physical feelings and sensations that occur in the real world. When people experience those sensations during REM sleep, the brain recruits some of the same networks that process the sensations while awake, creating vivid and realistic scenarios in the mind. Rather like lived experiences, the content of dreams depends on people’s personalities and interests. The similarities in brain activity during REM sleep and when people are awake may explain why some people are often confused enough during dreams to ask “Am I awake or am I dreaming?”

Researchers have questioned whether dreaming in the brain is more similar to perception (e.g. seeing an apple), or imagination (e.g. thinking of an apple). Some theories suggest that dreaming originates with the activation of low-level sensory areas of the brain, such as the area of the brain responsible for vision. Activity then propagates to other areas of the brain that contextualize the sensations by building a story around them. This is similar to what happens when an apple catches your eye, and you then consider whether to eat it. Other theories suggest that dreaming operates in the reverse direction, creating a story from memories, thoughts, and desires, and then playing out that story by adding the relevant sensations. This is similar to what happens when you imagine an apple, and then decide to go and find one to eat in the fruit bowl.

While the answer is still obscure, there are some hints that dreaming may have more in common with imagination than perception. When doctors electrically stimulate brain areas related to imagination, people receiving the stimulation often report mental experiences similar to dreams. Consistent with this, some typical features of dreams — such as the failure to recall the finer details of objects — also resemble the features of imagination. Imagination may therefore be the closer cousin of dreaming.

Why do people dream in the first place? The functions of sleep and dreaming are hotly debated, but a few theories hold sway. At least some of the content in dreams is believed to be related to experiences during the day. During one experiment in a 2010 study, people frequently reported dreams consisting of skiing-related imagery after playing a skiing arcade game. “I keep seeing all the places where I fall,” one person in the study said. Dreams don’t necessarily recreate a day’s events, but rather they combine elements from those events in unusual ways.

The experience of dreaming could be a crucial feature of a developing psyche, or it could be a mere epiphenomenon produced by the brain’s housekeeping systems during sleep.

Experiments with both animals and humans suggest that one crucial role of sleep is to allow the rehearsal of newly learned information. This rehearsal helps to stabilize memories overnight. As patterns in the brain are reactivated, relevant images (such as the skiing images in the experiment above) may enter the mind in the form of dreams. It could be that the sleeping brain acts as a testing ground for new memories, bouncing them against other existing pieces of information en route to organizing and consolidating the new material.

Another theory suggests that dreams act as a kind of virtual world: They are a simulation in which the brain can play out anxieties or decisions without the threat of real-life consequences. John Allan Hobson, a scientist who has contributed enormously to the science of sleep and dreaming, believes that this virtual world may even act as what he calls a “protoconsciousness”. Hobson suggests that before and just after birth, REM sleep may generate an imaginary sense of self that navigates an imaginary world and practices the basic functions of perception and emotion. This primal consciousness matures into full conscious awareness as children learn to interact with the real world around them. The idea that dreams serve as a building block for general consciousness may explain why REM is most prevalent during gestation and the first year of life.

The challenge of understanding dreams may be tied to one of the most enduring mysteries in neuroscience: What exactly is consciousness and how does it emerge? The experience of dreaming could be a crucial feature of a developing psyche, or it could be a mere epiphenomenon produced by the brain’s housekeeping systems during sleep. Either way, dreams are quintessential examples of the extravagant feats that the human mind is capable of.

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