- author, Carolyn Wilk
- stock, Navabel magazine
Small jumping spiders hang by their webs at night inside a laboratory box. Occasionally, their legs buckle and their spines quiver. Their retinas, visible through their transparent exoskeleton, move back and forth.
“What’s happening to these spiders looks very similar to what’s called rapid eye movement sleep,” says Dr Daniela Rössler, a behavioral ecologist at the University of Konstanz in Germany. During this phase of sleep, the sleeping animal’s eyes move unexpectedly.
For humans, rapid eye movement sleep is the stage in which they dream most of their dreams, especially dreams that contain details that resemble reality. This leads us to a perplexing question: If spiders experience REM sleep during sleep, does that mean dreams unfold in their poppy seed-sized brains?
Roesler and his colleagues conducted a 2022 study on spinning web spiders, where they placed surveillance cameras over 34 spiders and found that they went through REM-like sleep stages every 17 minutes. Nocturnal eye movement behavior was restricted to these bouts, when jumping spiders were moving, stretching, adjusting their silk threads, or grooming themselves with one of their legs.
Although the spiders were sitting during the period that leads to REM-like seizures, the team was still unable to prove that they were actually sleeping. But if it turns out to be happening during his sleep, Rössler says, dreams become a distinct possibility if REM becomes one. She adds that it’s easy for her to imagine that jumping spiders, animals that rely heavily on their sense of sight, might benefit from dreams as a way to absorb information they receive during the day.
Roesler is not the only researcher trying to answer these questions regarding animals far down the evolutionary chain from humans. Other scientists have found evidence that a wide range of animals, including spiders, lizards and zebrafish, undergo REM sleep. It made researchers wonder if dreams, previously thought to be unique to humans, are more common than they thought.
REM sleep is characterized by several characteristics in addition to REM: temporary paralysis of skeletal muscles, frequent twitching of body parts, and increased brain activity, respiratory rates, and heart rate. This type of sleep was first observed in newborns in 1953, and has also been observed in other animals such as cats, mice, horses, sheep, opossums and armadillos.
Brain activity during this stage of sleep has unique characteristics, at least in humans. In states where rapid eye movements do not occur, known as quiet sleep states, brain activity is synchronized. Neurons fire their signals simultaneously and then stop, especially in the cerebral cortex, known as slow waves. On the other hand, during REM sleep, bursts of electrical activity occur in the brain that occur during wakefulness.
We know that humans dream because they can talk about those dreams, but animals lack the ability to do so
Even in mammals, REM stages are not uniform. Hedgehogs, a marsupial mammal, show some features of simultaneous REM sleep and restful sleep. Studies on whales and dolphins indicate that they may not go into REM sleep, while birds go through those stages, which are characterized by beak twitching and muscle relaxation.
Researchers are beginning to find similar sleep stages in many animal species.
For example, in 2012, researchers observed a sleep-like state in cuttlefish and behavior similar to REM sleep: from time to time, the animals’ eyes would move rapidly, their arms would twitch and their body color would change. During a fellowship at the Woods Hole Marine Biological Laboratory in Massachusetts, behavioral biologist Teresa Iglesias studied the phenomenon and photographed six squid over several hours.
All six fish behaved in a manner similar to what occurs during eye movement sleep every 30 minutes. These creatures emit signals to hide and attract attention, both of which are part of their behavior when awake. Because the brains of cephalopods directly control changing skin patterns and colors, this “suggests that brain activity is somewhat out of control” during sleep, says Iglesias, who now works at the Okinawa Institute of Science and Technology in Japan.
Researchers later observed a similar condition in octopuses. If the octopus and squid dream, “it undermines our belief that humans are very unique creatures,” says Iglesias.
The researchers also monitored a phase similar to rapid eye movement by recording signals emitted by electrodes in the bearded dragons’ brains. They also indicated that zebrafish have at least two sleep stages based on their brain fingerprints. In one of these two states, brain activity occurs synchronously in mammals during restful sleep. In another phase, the fish showed neural activity similar to that occurring during wakefulness and REM sleep (but the fish did not show rapid eye movements).
Cuttlefish sometimes change colors during sleep, indicating that they are reacting to events in their dreams
By observing multiple stages of sleep in humans’ most distant relative from an evolutionary perspective, the researchers indicated that different types of sleep began hundreds of millions of years ago. It is now known that flies move between two or more stages of sleep. For nematodes, they go through a phase of dormancy.
Researchers are studying the possibility that animals dream during REM sleep, because during that stage, organisms exhibit behaviors similar to their waking behavior—such as changing skin patterns in cuttlefish or vibrating spider webs. Gianina Ankorian, a sleep scientist at the Max Planck Institute for Biological Intelligence and Göttingen School of Medicine in Munich, and her colleagues observed that the pigeons’ pupils constricted during the rapid eye movement phase, similar to what happens during these birds’ playful behavior. . This raises the question of whether the pigeons are dreaming or reliving the experience while awake, Ankorian says.
Rapid eye movement sleep has been linked to repetitive play situations in some animals. For example, when researchers examined the electrical activity in the brains of sleeping rats that ran through a maze while awake, they observed neurons firing signals that helped them locate locations. They also detected the activity of neurons related to eye movement. Both of these things suggest that the rats may have had a dream-like experience during which they scanned their surroundings, Ungorian said.
Taking this evidence into account, he adds, we can assume that animals can dream. “But if we look at these reasons individually, it is clear that none of them are sufficient.” Replay-related brain activity in mice doesn’t just occur during REM sleep, the researcher says. This happens during planning or daydreaming. The relationship between REM sleep and dreaming is not firmly established: Humans also dream during restful sleep, and when drugs were used to block REM sleep, this did not prevent participants from having long, strange dreams.
Ultimately, Ungorian adds, people know they’re dreaming because they can talk about their dreams, “but animals can’t, and that’s the biggest problem when we try to verify this in a precise scientific way.”
There is still controversy about the benefits of REM sleep. “No one knows for sure the function of sleep — REM or restful sleep,” says Professor Paul Shaw, a professor of neurology at Washington University in St. Louis. One of the most widely accepted theories is that REM sleep helps the brain form and reorganize memories, while other theories suggest that this stage of sleep helps the brain and body’s motor systems develop, and maintains the electrical circuits necessary for waking activities. Worse during sleep.
Pigeons sleeping in an MRI machine showed brain activity similar to that of humans during sleep.
Iglesias says that if species distantly related to humans in the animal kingdom undergo REM sleep, their role, whatever it is, may be more important.
But not all scientists believe what the researchers observed is rapid eye movement sleep. Jerome Siegel, a neuroscientist who studies sleep at the University of California, Los Angeles, says it’s possible the researchers were simply fulfilling a hypothesis that explains that all animals go through two sleep stages and interpret one of those stages as REM sleep. He adds that some animals, such as spiders, may not sleep: “Animals may do things that seem similar, but the physiology is not necessarily similar.”
Researchers continue to search for clues to this mystery. Roessler’s team is trying to develop dyes that can help them take pictures of spiders’ brains, perhaps showing activity in areas similar to those we use when we dream. Iglesias and others implanted electrodes into the brains of cephalopods and recorded electrical activity during two sleep states — one active and one quiet — and whose neural signatures were similar to those seen in mammals. Dr. Ungurian trained pigeons to sleep in an MRI machine and found that many areas of the human brain that light up during rapid eye movement sleep are also active in birds.
A philosopher at San Francisco State University and author of Animals Dream: The Hidden World of Animal Emotions, Dr. David M. Pena-Guzman says that when squid, spiders and other creatures dream, it grows. Important questions about what exactly is happening to her. He adds that since dreams occur from the perspective of the person who sees them, dreaming animals must have the ability to see the world from their perspective.
Guzmán says that dreaming indicates that they have the ability to imagine: “We like to believe that only humans can disconnect from the world for a while… But we have to think a little more about other animals. .”
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