Like islands jutting out of a smooth ocean surface, dreams puncture our sleep with disjointed episodes of consciousness. How states of awareness emerge from a sleeping brain has long baffled scientists and philosophers alike.
For decades, scientists have associated dreaming with rapid eye movement (REM) sleep, a sleep stage in which the resting brain paradoxically generates high-frequency brain waves that closely resemble those of when we’re awake.
Yet dreaming isn’t exclusive to REM sleep. A series of oddball reports also found signs of dreaming during non-REM deep sleep, when the brain is dominated by slow-wave activity—the opposite of an alert, active, conscious brain.
By closely monitoring the brain waves of sleeping volunteers, a team of scientists at the University of Wisconsin pinpointed a local “hot spot” in the brain that fires up when we dream, regardless of whether a person is in non-REM or REM sleep.
“You can really identify a signature of the dreaming brain,” says study author Dr. Francesca Siclari.
What’s more, using an algorithm developed based on their observations, the team could accurately predict whether a person is dreaming with nearly 90 percent accuracy, and—here’s the crazy part—roughly parse out the content of those dreams.
“[What we find is that] maybe the dreaming brain and the waking brain are much more similar than one imagined,” says Siclari.
The study not only opens the door to modulating dreams for PTSD therapy, but may also help researchers better tackle the perpetual mystery of consciousness.
The anatomy of sleep
During a full night’s sleep we cycle through different sleep stages characterized by distinctive brain activity patterns. Scientists often use EEG to precisely capture each sleep stage, which involves placing 256 electrodes against a person’s scalp to monitor the number and size of brainwaves at different frequencies.
When we doze off for the night, our brains generate low-frequency activity that sweeps across the entire surface. These waves signal that the neurons are in their “down state” and unable to communicate between brain regions—that’s why low-frequency activity is often linked to the loss of consciousness.
These slow oscillations of non-REM sleep eventually transform into high-frequency activity, signaling the entry into REM sleep. This is the sleep stage traditionally associated with vivid dreaming—the connection is so deeply etched into sleep research that reports of dreamless REM sleep or dreams during non-REM sleep were largely ignored as oddities.
These strange cases tell us that our current understanding of the neurobiology of sleep is incomplete, and that’s what we tackled in this study, explain the authors.
To reconcile these paradoxical results, Siclari and team monitored the brain activity of 32 volunteers with EEG and woke them up during the night at random intervals. The team then asked the sleepy participants whether they were dreaming, and if so, what were the contents of the dream. In all, this happened over 200 times throughout the night.
Rather than seeing a global shift in activity that correlates to dreaming, the team surprisingly uncovered a brain region at the back of the head—the posterior “hot zone”—that dynamically shifted its activity based on the occurrence of dreams.
Dreams were associated with a decrease in low-frequency waves in the hot zone, along with an increase in high-frequency waves that reflect high rates of neuronal firing and brain activity—a sort of local awakening, irrespective of the sleep stage or overall brain activity.
“It only seems to need a very circumscribed, a very restricted activation of the brain to generate conscious experiences,” says Siclari. “Until now we thought that large regions of the brain needed to be active to generate conscious experiences.”
That the hot zone leaped to action during dreams makes sense, explain the authors. Previous work showed stimulating these brain regions with an electrode can induce feelings of being “in a parallel world.” The hot zone also contains areas that integrate sensory information to build a virtual model of the world around us. This type of simulation lays the groundwork of our many dream worlds, and the hot zone seems to be extremely suited for the job, say the authors.
If an active hot zone is, in fact, a “dreaming signature,” its activity should be able to predict whether a person is dreaming at any time. The authors crafted an algorithm based on their findings and tested its accuracy on a separate group of people.
“We woke them up whenever the algorithm alerted us that they were dreaming, a total of 84 times,” the researchers say.
Overall, the algorithm rocked its predictions with roughly 90 percent accuracy—it even nailed cases where the participants couldn’t remember the content of their dreams but knew that they were dreaming.
Since the hot zone contains areas that process visual information, the researchers wondered if they could get a glimpse into the content of the participants’ dreams simply by reading EEG recordings.
Dreams can be purely perceptual with unfolding narratives, or they can be more abstract and “thought-like,” the team explains. Faces, places, movement and speech are all common components of dreams and processed by easily identifiable regions in the hot zone, so the team decided to focus on those aspects.
Remarkably, volunteers that reported talking in their dreams showed activity in their language-related regions; those who dreamed of people had their facial recognition centers activate.
“This suggests that dreams recruit the same brain regions as experiences in wakefulness for specific contents,” says Siclari, adding that previous studies were only able to show this in the “twilight zone,” the transition between sleep and wakefulness.
Finally, the team asked what happens when we know we were dreaming, but can’t remember the specific details. As it happens, this frustrating state has its own EEG signature: remembering the details of a dream was associated with a spike in high-frequency activity in the frontal regions of the brain.
This raises some interesting questions, such as whether the frontal lobes are important for lucid dreaming, a meta-state in which people recognize that they’re dreaming and can alter the contents of the dream, says the team.
The team can’t yet explain what is activating the hot zone during dreams, but the answers may reveal whether dreaming has a biological purpose, such as processing memories into larger concepts of the world.
Mapping out activity patterns in the dreaming brain could also lead to ways to directly manipulate our dreams using non-invasive procedures such as transcranial direct-current stimulation. Inducing a dreamless state could help people with insomnia, and disrupting a fearful dream by suppressing dreaming may potentially allow patients with PTSD a good night’s sleep.
Dr. Giulo Tononi, the lead author of this study, believes that the study’s implications go far beyond sleep.
“[W]e were able to compare what changes in the brain when we are conscious, that is, when we are dreaming, compared to when we are unconscious, during the same behavioral state of sleep,” he says.
During sleep, people are cut off from the environment. Therefore, researchers could hone in on brain regions that truly support consciousness while avoiding confounding factors that reflect other changes brought about by coma, anesthesia or environmental stimuli.
“This study suggests that dreaming may constitute a valuable model for the study of consciousness,” says Tononi.
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