May 28, 2019
Written by: Carolyn Keating
You have a huge exam at school that you’re nervous about. But oh no, you’ve overslept and now you’re going to be late! You rush to school but when you get there you realize…you forgot your pants? And your teeth are falling out? And now you’re flying? What’s going on? Although they can seem like real life, these mundane or bizarre dreams we experience are produced entirely by our own minds without any input from our environment. But how does the brain produce these remarkable visions?
To start, we should define what exactly a dream is anyways. Dreams are a series of images, ideas, emotions, and sensations experienced during sleep that follow a story-like structure. Even though dreams are something experienced by everyone, studying dreams is difficult for scientists. What a person experienced while dreaming can only be known through what the dreamer reports once they wake up, instead of being available for scientists to observe directly and independently. What’s more, researchers normally try to figure out how things work by changing one thing at a time and noting the effects; but it’s difficult to reliably manipulate the content of dreams for experiments. For these reasons, figuring out what’s going on in the brain during specific individual dreams is not usually possible, so most researchers study the neuroscience behind general properties of dreams1.
Despite being difficult to study, scientists have made progress. For a long time, people believed that dreams only and always occurred during the rapid eye movement (REM) phase of sleep, a period characterized by increased brain activity and eye movements2. But occasionally people don’t report dreams after waking from REM sleep, and further study showed that dreams can also happen during non-REM (NREM) sleep, a deeper phase of sleep when brain activity is lower3. So if sleep stages can’t be used to determine if someone will dream, what can?
To find out, researchers from the University of Wisconsin – Madison brought subjects into the lab for a sleep study4. Subjects were awakened periodically throughout the night and asked to report if they had been experiencing anything (dreaming), experiencing something but could not remember the content, or not experiencing anything. If subjects reported a dream they were asked to describe the last thing they remembered about it and to rate it on a scale ranging from exclusively thought-like (thinking or reasoning, with no sensory content) to exclusively perceptual (vivid sensory content, without thinking or reasoning). They also had to estimate the how much time passed during the dream and report whether it contained specific content like faces, a spatial setting, movement, or speech.
While the subjects slept and were awakened, their brain activity was recorded with electroencephalography (EEG). The recorded waveforms reflect cortical activity, and different waveforms are thought to reflect different activity states in the brain. For instance, slow frequencies between 1 and 4 Hz during sleep are associated with low neuronal activity and loss of consciousness. Researchers examined these low frequencies during NREM and REM sleep, and found that reports of dreaming were associated with less low-frequency activity in the back of the brain during either stage of sleep, specifically parts of the occipital and parietal lobes (Figure 1). Since the occipital lobe is involved with processing visual information, it was no surprise that this region is also involved with dreams, which are almost always visual. Reports of dreaming without remembering the content of the dream also were associated with decreased low-frequency activity in this posterior region, suggesting that less low-frequency activity in this area is important for dreaming, regardless of whether or not the content of the dream can be remembered and regardless of the stage of sleep.
Next the scientists looked at high-frequency (20-50 Hz) EEG activity, thought to reflect high rates of neuronal firing. In REM and NREM sleep, dreams were associated with increased high-frequency power in the same parieto-occipital region that showed reduced low-frequency power, plus a few other neighboring areas like parts of the lateral frontal cortex and the temporal lobe (Figure 1). Dreams that could be remembered had increased high-frequency activity in medial and lateral frontoparietal areas compared to dreams that couldn’t be remembered, suggesting that these regions might be involved in remembering dreams.
Could the precise locations of this increased high-frequency activity be associated with the specific content of the dreams? To answer this question, scientists monitored EEG activity during REM sleep of subjects who had been trained in dream reporting. These subjects were asked to rate their dreams along a scale of perception (sensations) versus thought. When dreams were more thought-like, high-frequency activity was increased in frontal brain regions, but when dreams were more sensory, it was increased in parietal, occipital and temporal areas. Interestingly, this matches the activity patterns that occur when we’re awake: frontal brain regions mediate thought-like experiences, while posterior regions are involved in perceptual aspects of the experience.
But there’s more to dreams than just thinking versus sensory. What about other, more specific aspects of dream content? Incredibly, increases in high frequency EEG activity in particular locations may reflect what a person is dreaming about! Dreams containing faces, compared to those without, were associated with increased high-frequency activity in a region that closely matched the fusiform face area, which as the name suggests, is a key region involved in facial processing. Dreams with a definite spatial setting (whether indoors or outdoors) were associated with increased high-frequency activity in the right posterior parietal cortex, an area involved in spatial perception and visuospatial attention, and in which lesions can cause spatial neglect. Dreams in which subjects reported a sense of moving were associated with increased high-frequency activity in a region surrounding the right superior temporal sulcus, an area involved in the perception of biological motion and in viewing body movements. And finally, dreams containing speech were associated with increased high-frequency activity over a region corresponding to Wernicke’s area, an important language center in the brain. So it appears that if a brain region involved with a specific function during consciousness is active during sleep, the features it helps process will be present in the dream.
This study identified many brain regions that underlie dreaming in general as well as some specific content in dreams, but there is more to learn. Are different brain areas active when people experience lucid dreaming? Why do we often forget our dreams after waking? Why do we dream in the first place? Future studies will hopefully be able to uncover some answers. Until then, sweet dreams.
- Nir, Y. & Tononi, G. Dreaming and the brain: from phenomenology to neurophysiology. Trends Cogn. Sci. 14, 88–100 (2010).
- Aserinsky, E. & Kleitman, N. Regularly occurring periods of eye motility, and concomitant phenomena, during sleep. Science 118, 273–274 (1953).
- Stickgold, R., Malia, A., Fosse, R., Propper, R. & Hobson, J. A. Brain-mind states: I. Longitudinal field study of sleep/wake factors influencing mentation report length. Sleep 24, 171–179 (2001).
- Siclari, F. et al. The neural correlates of dreaming. Nat. Neurosci. 20, 872–878 (2017).
Cover image from Pixabay, https://pixabay.com/illustrations/narrative-history-dream-tell-794978/
Figure 1 created with Motifolio Neuroscience Toolkit