From the Book: NeuroLogic by Eliezer J. Sternberg. Copyright © 2015 by Eliezer J. Sternberg, published by arrangement with Pantheon Books, an imprint of The Knopf Doubleday Publishing Group, a division of Random House LLC.
In his famous 1944 painting Dream Caused by the Flight of a Bee Around a Pomegranate, the Spanish artist Salvador Dalí depicted a vision he imagined his wife dreamed moments before awakening from a nap. In the painting, Dalí reveals some insight into the true nature of dreams. He illustrates their vividness, their emotional intensity, and their propensity for being peculiar or fantastical. This particular painting has inspired a diverse set of interretations, a prominent one being that it depicts a scene of imminent rape, given the violent imagery and the possible use of the rifle as a phallic symbol. Others take a more straightforward approach, using the title of the painting as a guide.
"Dream caused by the Flight of a Bee around a Pomegranate a Second before Waking up" by Salvador Dali, 1944
If you look carefully, you'll notice that there is a second, small pomegranate toward the bottom of the painting, with a bee buzzing over it. Perhaps Dalí thought that the buzzing of a real bee around his wife's sleeping figure somehow entered her unconscious and guided the narrative of her dream. Her mind translated a sudden fear of being stung into violent imagery, with the stinger represented by the sharp rifle tip pressing into her arm. But how could such a simple stimulus, a buzzing sound, engender such an elaborate mental mirage?
Dalí painted what most of us already suspected: though they can often be bizarre, dreams may incorporate elements of our daily life and connect them in novel, occasionally nonsensical, or even metaphoric ways to create a narrative. The sleeping brain is a master storyteller, and that power emerges from its unique environment. When we sleep, our eyes are shut, sounds are muted. Essentially closed off from external sensory noise, the mind begins to fill with imagery generated from within.
However, when we dream, we are not completely cut off from the outside. Certain stimuli, like a buzzing insect, can make their way into our nocturnal imagery. External sensations infiltrate our dreams all the time. One of the most powerful effects of this kind occurs when you spray a sleeping person with water. Over 40 percent of the time, that stimulus becomes directly integrated into the person's dreams. Awakened subjects describe visions that incorporate things like a rainy day, being squirted by someone, or having to repair a leaky roof.
Still, most of what we dream is a quilt of our memories, thoughts, and emotions. Often our dreams are abstract reflections of things that occur to us in our daily lives, things we contemplate, worry about, and long for. Most dreams incorporate aspects that are familiar to the dreamer. In 2004, researchers in Belgium used positron emission tomography (PET scans, which track brain activity by following a radioactive tracer as it moves to active areas) to monitor the brain activity of subjects as they played a first-person shooter video game. The researchers noted the areas of the brain activated as the subjects moved through the streets of the virtual reality town. In the second part of the experiment, the research group had the participants go to sleep, but not before their scalps were outfitted with EEG (electroencephalogram) electrodes to monitor their brain waves overnight. The next morning, a comparison of the EEG recordings with the PET scans revealed that the same areas of the hippocampus that illuminated during the video game were again stirred as the subjects began to dream.
We know that vision is made possible by the visual pathway and that blindness can result from an interruption anywhere along that circuit. There's also a dream pathway in the brain. Like vision, dreams allow for the perception of imagery, even though the eyes are closed and effectively blinded to the outside world. The fact that we are still able to perceive this imagery implies that the dream circuit must be separate from the visual one. If it's true that the blind can see in their dreams, then the dream pathway must be the key. So, the obvious question is, what constitutes that circuit? How does the brain create our dreams?
As you enter REM sleep, while your eyes are shut, the dream system takes over the thalamus and the visual cortex. It controls your internal sensory switchboard, as well as your center of image processing, but it still has to get the images from somewhere.
Neurologists have discovered that, as we dream, the thalamus begins behaving differently: instead of responding to signals arriving from the eyes (of which there are none), the thalamus falls under the control of the brainstem, the central stalk that adjoins the brain to the spinal cord. A major function of the brainstem is to maintain REM sleep, when most dreaming occurs. Many neurologists believe that this link between the thalamus and the brainstem that occurs nightly during REM sleep is the basis of dream imagery.
By watching the brain waves that appear as we dream, neurologists have picked up on unique waves, called PGO (ponto-geniculo-occipital) waves. They have a recognizable shape and size. This same wave is seen in three locations in the brain as we dream: the pons (in the brainstem), the lateral geniculate nucleus (the visual part of the thalamus), and the occipital lobe (where the visual cortex lives). We can infer, therefore, that these regions are working together. Perhaps the brainstem, thalamus, and visual cortex form their own visual pathway, but the eyes are cut out of the process. The dream pathway is similar to the visual pathway except that the brainstem replaces the eyes as the source. The imagery is generated from within.
The renowned dream researcher John Allan Hobson, a psychiatrist at Harvard Medical School, has theorized that dreams occur due to random neuronal firings in the brainstem. From the brainstem, these haphazard signals go to the thalamus, which treats them as it would any visual signal. The thalamus is just a switchboard. It has no idea whether the signals it receives come from the eyes or the brainstem. It just routes them where they need to go: to the visual cortex.
Now imagine what the visual cortex has to deal with. It's 2:00 a.m., and a barrage of signals has just arrived from the thalamus. What's more, the signals are a mess of disorganization. After all, they were randomly generated by the brainstem. But the visual cortex doesn't know that. It assumes that whatever information it receives from the thalamus must have come from the eyes. How does the cortex respond? Just as it would do if we were awake, the visual cortex tries to make sense of the information. Using our store of knowledge and memory, it attempts to connect disparate signal fragments into a single narrative, creating a unified visual spectacle that we experience as dreaming.
The brain does its best to tell a story. The unconscious system in the brain has a knack for finding patterns, anticipating what comes next, and using contextual clues to fill in the gaps when encountering an incomplete picture. These might all come into play as the unconscious stitches together our nightly visions from the tattered signals it receives. The resulting patchwork of thoughts, memories, fears, and wishes can come together as an engrossing, occasionally metaphoric narrative. Usually, however, our dreams are pretty weird.
As weird as dreams can be, we never seem to notice their outlandishness while we're in them. Only when we wake up do we realize how bizarre the imagined scenario was. Why is that? In learning which parts of the brain are involved in dreaming, neuroscientists have also discovered which parts go dormant for the night. Most prominently, the prefrontal cortex, where higher-order decision making and self-reflecting thinking take place, is utterly quiet.
When we dream, we don't actively plan or strategize, nor do we reflect much on our thoughts. Those are abilities mediated by the prefrontal cortex, which is shut down during REM sleep. That's why we can't tell that we're dreaming. It's why dreams can get away with being so bizarre without our thinking, "Wait a minute . . . this doesn't make any sense." If you do happen to realize how weird the dream is, you're likely in the process of waking, and your prefrontal cortex is starting to fire up.
The prefrontal's inactivity may also be the explanation for why we don't feel as if we can control our imagined avatar and make decisions while dreaming. The dream is like a movie that's happening to us. We can't choose our own adventure; at least we usually can't. There is one major exception, known as lucid dreams, in which a person knows he is dreaming and can even willfully explore his imagined internal world.
How are lucid dreams possible? We just got finished saying that the prefrontal cortex is deactivated as we sleep, so how could someone actively control his dreams? In 2012, German sleep researchers asked that same question. They recruited some lucid dreamers to fall asleep in the company of their fMRI machine. As the participants entered REM sleep, the fMRI picked up an interesting pattern of activation. The BOLD signal, in addition to highlighting the regions normally active during dreams, appeared prominently in the prefrontal area. The prefrontal cortex was active. For reasons unknown, some people's prefrontal cortices resist the nightly shutdown that the rest of us experience.
Lucid dreamers have access to their capacities for self-reflection, self-control, and decision making, rendering each dream a thrilling exercise in virtual reality. What's more, lucid dreaming is a skill that you can acquire with training and has been used successfully as a treatment for nightmares. With practice, you can just politely ask the ghosts and ax murderers to get lost.
Most dreams are not simple replays of our daily lives; that accounts for just about 1 to 2 percent of dreams. The rest of the time, our unhinged thoughts and visualizations come together in new, often creative ways. Through dreams, the unconscious system in the brain provides us with an alternative way of associating concepts, closed off from all the distractions of waking life. Ideas can dance freely through the mind.
Perhaps that's why we think of great new ideas as we sleep. How often have you awoken from sleep only to frantically seek out a pen and paper to jot down a moment of inspiration? Studies demonstrate that if you give a math problem to two groups of people, one that tries to solve it right away and one that does so after sleeping through the night, the group that sleeps on it is much more likely to discover creative shortcuts.
What is it about the dreaming brain that makes our thoughts and experiences come together in unique ways? One explanation is that sleep protects us from external stimuli, preventing interference and allowing our imagination to flourish. Another possibility is that the prefrontal cortex is largely deactivated, so our more abstract, and even bizarre, thoughts can frolic without being subjected to our usual judgmental analytic rigor. There may be a third, more fundamental explanation for why dreams are so creative. Some neuroscientists theorize that during sleep the brain relaxes preformed synapses (the spaces between neurons through which they communicate), loosening connections between our memories and learned concepts. This is thought to enhance neuronal flexibility, allowing new pathways to form in the brain and fresh, creative ideas to burst forth. In fact, some studies show that the neurons that have worked together most extensively during the day are the most quiet as we sleep. The theory is that relaxing our synapses opens the door for dreaming. It creates the opportunity for novel connections between our thoughts to emerge, allowing the brain to spin its stories.
However they arise, our dreams are wildly different from our waking perceptions, and it's because we have two fundamentally different systems at work in the brain. On the one hand, there is the active, conscious system that we use when we're awake. On the other hand, there's the passive, inner world of dreaming that takes over when the conscious system shuts down. Lucid dreaming would represent some sort of in-between state that recruits brain regions from both systems. Dreams tend to happen as we sleep and end when we wake up; we generally don't dream and make conscious decisions simultaneously. When we awaken, we slip out of our internal fantasy as consciousness overtakes us. The conscious and unconscious systems take turns seizing and ceding control. However, as Dalí's painting and lucid dreams imply, that boundary between dream and reality can be a tenuous one.
By Eliezer Sternberg, from the book, Neurologic.