Are animals capable of dreaming like humans? – 09/08/2023 – Science

Young jumping spiders hang by a thread overnight from a box in a laboratory. From time to time, its legs bend and its spinnerets (glands responsible for producing silk to make webs) contract.

The retinas of their eyes, visible through their translucent exoskeletons, move back and forth.

“What these spiders do seems to closely resemble REM sleep,” says Daniela Rößler (pronounced Rôsslar), a behavioral ecologist at the University of Konstanz in Germany.

During REM sleep (acronym in English for rapid eye movement), the eyes of a sleeping animal move unpredictably, among other characteristics.

In people, REM occurs when most dreams, especially the most vivid ones, occur. This leads to an intriguing question: If spiders have REM sleep, do dreams also play out in their poppy-seed-sized brains?

Rößler and his colleagues studied eye-moving spiders in 2022. Using cameras on 34 spiders, they found that the creatures engaged in brief REM-like behaviors every 17 minutes.

The eye rolling was something specific to these episodes: it didn’t happen at times of the night when the jumping spiders moved, stretched, readjusted their silk threads or cleaned themselves with the brush of a leg.

Although the spiders are immobile before these REM-like episodes, the team has yet to prove that they are asleep. But if that’s the case — and if what appears to be REM is really REM — dreaming is a real possibility, says Rößler.

She finds it easy to imagine that jumping spiders, as highly visual animals, might benefit from dreams as a way to process the information they’ve picked up during the day.

Rößler is not the only researcher thinking about these questions in animals far from us.

Today, scientists are finding signs of REM sleep in a wider range of animals than ever before: in spiders, lizards, cuttlefish (a type of marine mollusk) and zebrafish. The growing number has some researchers wondering whether dreaming, a state once thought to be limited to humans, is much more widespread than previously thought.

REM sleep and animals

REM sleep is generally marked by a set of characteristics in addition to rapid eye movements: temporary paralysis of skeletal muscles (those responsible for posture and movement), periodic body spasms and increased brain activity, breathing and heart rate.

Observed in sleeping babies in 1953, REM was soon identified in other mammals, such as cats, rats, horses, sheep, opossums and armadillos.

Events in the brain during REM have been well characterized, at least in humans.

During non-REM periods, also known as restful sleep, brain activity is synchronized. Neurons fire simultaneously and then go quiet, especially in the cerebral cortex, producing waves of activity known as slow waves.

During REM, on the other hand, the brain experiences bursts of electrical activity that resemble wakefulness.

Different types of sleep

Even among mammals, REM sleep doesn’t all look the same.

Marsupial mammals called echidnas exhibit characteristics of REM and non-REM sleep at the same time.

Reports on whales and dolphins suggest that they may not even have REM experience. Birds have REM sleep, which comes with twitching in the beak and wings and loss of tone in the muscles that support their heads.

Still, researchers are beginning to find similar sleep states across many branches of the animal tree of life.

In 2012, for example, researchers reported a sleep-like state in cuttlefish, as well as curious REM-like behavior during this supposed sleep state: periodically, the animals moved their eyes rapidly, twitched their tentacles, and changed the color of their bodies. bodies.

During a period of research at the Marine Biological Laboratory in Woods Hole, Massachusetts, behavioral biologist Teresa Iglesias investigated the phenomenon further, collecting terabytes of video from half a dozen cuttlefish.

All six showed episodes of REM-like activity that repeated approximately every 30 minutes: bursts of tentacle movements and eye movements, during which their skin put on a show, presenting a variety of colors and patterns.

The creatures emitted camouflage signals and attention-getting signals, both displayed during wakeful behaviors.

Because the cephalopod’s brain directly controls this skin pattern, “this suggests that brain activity is getting a little jittery,” says Iglesias, now at the Okinawa Institute of Science and Technology in Japan.

Since then, researchers have observed a similar state in octopuses.

If octopuses and cuttlefish dream, “it just breaks down the walls of what we think about humanity being so special,” says Iglesias.

Researchers also observed a REM-like stage in bearded dragons (also called pogonas, a type of lizard), recorded in their brains from electrodes.

They also reported at least two sleep states in zebrafish based on the fish’s brain signatures. In one of the states, neural activity was synchronized as it happens in the non-REM stage of mammals.

In another state, the fish showed neural activity that resembles a waking state, as happens in REM. (The fish, however, did not show rapid eye movements.)

Observing multiple stages of sleep in an animal so evolutionarily distant from us, the authors suggested that different types of sleep arose hundreds of millions of years ago.

It is now known that flies can also switch between two or more sleep states. Roundworms seem to have only one sleep state.

Signs of wakefulness in sleep

Researchers consider the possibility that non-human animals dream during REM sleep because the creatures exhibit waking-like behaviors in this state — such as cephalopods changing color or spiders’ spinnerets quivering.

In pigeons, sleep scientist Gianina Ungurean of the Max Planck Institute for Biological Intelligence in Munich and the Medical University of Göttingen has observed, with colleagues, that pupils constrict during REM, just as they do during mating behavior.

This raises the question of whether the pigeons are dreaming or somehow reliving what happened during their waking courtship moments, she says.

REM sleep has also been linked to repeat experiences in some animals.

For example, when researchers observed the electrical brain activity of sleeping rats that had already run through a maze, they saw the firing of neurons that help with navigation and are linked to head direction, even though the rats’ heads were not moving.

They also observed activity in neurons associated with eye movement. The combination suggests that the mice may have had a dreamlike experience where they were scanning their surroundings, says Ungurean.

With all these signs, it’s fair to believe that the animals could be dreaming, says Ungurean.

“However, if we consider these reasons one by one, it turns out that none of them is enough”, ponders the researcher.

Function of REM is still uncertain

Brain activity associated with repetition, like that of maze-running rats, doesn’t just occur during REM or sleep, says Ungurean. It can also occur during planning or daydreaming.

And the link between REM and dreaming is not absolute: Humans also dream in non-REM periods, and when medications are used to suppress REM sleep, human study participants can still have long, bizarre dreams.

Ultimately, people know they’re dreaming because they can report it, says Ungurean.

“But animals can’t report, and that’s the biggest problem we have in establishing this in a purely scientific and robust way.”

There is still debate about what REM is for. “No one really knows what the function of sleep is – non-REM or REM,” says Paul Shaw, a neuroscientist at Washington University in St. Louis, United States.

One of the most accepted ideas is that REM helps the brain to form and reorganize memories.

Other theories are that REM supports brain development, helps develop the body’s movement systems, maintains circuits needed for waking activities so they don’t degrade during sleep, or increases brain temperature.

But if REM is present in distant species of the animal kingdom, this suggests that its role, whatever it may be, could be very important, says Iglesias.

Is it REM or not?

Not all scientists believe that researchers are observing REM.

They may simply be departing from preconceived notions that all animals have two sleep states and interpreting one of them as REM, says Jerome Siegel, a neuroscientist who studies sleep at the University of California, Los Angeles (UCLA).

Some of these animals – like spiders – may not even be sleeping, he argues.

“Animals can do things that look the same [aos humanos]but the physiology is not necessarily the same,” he says.

Researchers continue to search for clues.

Rößler’s team is trying to develop ways that allow them to image spiders’ brains – this could reveal activation in areas that are functionally analogous to those we use when we dream.

Iglesias and others implanted electrodes in the brains of cephalopods and captured their electrical activity during two sleep states — one that shows wake-like activity and another that is a quiet state, with neural signatures similar to those seen in mammals.

And Ungurean trained pigeons to sleep in an MRI machine and found that many of the areas of the brain that light up in human REM sleep are also activated in birds.

If cuttlefish, spiders and a wide variety of other creatures dream, it raises interesting questions about what they experience, says David M. Peña-Guzmán, a philosopher at San Francisco State University and author of the book When Animals Dream: The Hidden World of Animal Consciousness (When Animals Dream: The Hidden World of Animal Consciousness, in free translation).

Because dreams unfold from the observer’s perspective, dreaming animals would have the ability to see the world from their point of view, he says.

Dreaming would also suggest they have imaginative ability, he adds.

“We want to think that humans are the only ones who can make that break with the world [representada pela sonho]”, he says. “Maybe we have to think a little more about other animals.”

This article was originally published by Knowable Magazine and republished by BBC Future under a Creative Commons license