What do animals dream about while they sleep?

If you’ve ever watched a dog take a nap, you’ve probably wondered if animals can dream.

This is a complicated question. We still don’t know why we dream, or how those dreams can have any meaning. And it is more difficult to study the dreams of animals: dogs are quite incapable of telling us what makes them howl or run in their sleep.

Depending on how we choose to define them, animal dreams can have intriguing explanations.

“I think that dreams give us reason to apply some mental abilities to animals, such as emotions, memory and even imagination,” said David M. Peña-Guzmán, who -studying the philosophy of science. at San Francisco State University and more recently. wrote a book When Animals Dream: The Hidden World of Animal Consciousness.

We know that primates have emotions, but take spiders for example: according to a recent study, spiders experience periods of rapid retinal movement, similar to human rapid eye movement (REM ) and can produce visual dreams. It may seem strange to think that spiders dream, but apparently they do.

(Read: Do spiders dream in their sleep?)

“We see dreams as fantastic stories with crazy elements, which can seem real,” says Matthew Wilson, a neurobiologist at MIT. “But when we look at animals, we’re just trying to understand what happens during sleep that can influence learning, memory and behavior. »

CAT’S DREAM

Domestic cats were among the first animals to be the subject of dream research. Michel Jouvet, a pioneer in the study of sleep, discovered evidence for the existence of dreaming in cats in the 1960s by observing the behavior of cats while they were sleeping and then significantly changing it.

In humans, during REM, or REM, muscles are less active despite the intense mental activity that fuels our dreams. This state of atony ensures that the body does not interpret our dreams, even if they seem real. Jouvet found that, in cats, a structure in the brainstem called the pons seems to control REM sleep and produces partial paralysis.

(Read: This is what your dreams look like.)

By removing parts of the bridge, the scientist caused a significant change in behavior. While their brains are in REM sleep, cats begin to move as if awake, chasing, jumping, grooming and even violently defending themselves against unseen threats.

Jouvet called this period paradoxical sleep: the body does not move, but the mind remains fully active. This experiment provided a better understanding of what happens in the brains of cats while they sleep.

“You can easily interpret the behaviors of these cats as reproductions of their experiences when they were awake,” Peña-Guzmán said.

MICE RUNNING LABYRINTHS

One study showed that after walking a maze during the day, rats could repeat the same course while sleeping. When a rat is awake, its hippocampus, the part of the brain responsible for creating and storing memories, memorizes the neural patterns needed to navigate the maze. Later, during sleep, its brain repeats the same pattern, indicating that the rat remembers or relearns the course of the maze.

Made in 2001, this discovery was one of the first to suggest that animals can have complex dreams; and, according to Wilson, that’s just the beginning.

“We have done other studies that suggest that the resurgence of memories of past experiences during sleep can be similar to what we see as dreams. »

Studies of these mice’s brains show that when the memories of this maze returned during their sleep, the accompanying visuals also returned, meaning that the mice saw in their sleep what they saw in the maze. when they were awake. The same phenomenon was observed for auditory and emotional areas of the brain that reactivated when the rat recreated the maze course during REM sleep.

“There is a lot of evidence to suggest that a profound replication of the waking state occurs during sleep,” Wilson said. “I’m completely comfortable with the idea of ​​calling this phenomenon a ‘dream’. But if that’s what’s happening, what does it mean? That’s what’s interesting. »

(Read: How do animals sleep?)

THE DREAM SONGS OF THE MANDARIN DIAMONDS

Although known for their lyrical songs, zebra finches are not born singers. These little birds learn by listening, practicing, and perhaps dreaming.

In 2000, researchers discovered that when they sing, zebra finches’ forebrain neurons fire in a specific pattern that scientists can recreate note by note. When they sleep, their brain reproduces the same pattern; so they reproduce the song they heard and practiced that day, indicating that they remember it and practice it while they sleep.

According to the study’s authors, songbirds also sing in their dreams. So are they reliving their experiences of the day in their dreams? Or are these dreams more like algorithms operating on the unconscious bird? Scientists may be on the verge of finding out.

After more than twenty years of research, zebra finches are the first non-mammalian animals to show a sleep pattern similar to that of humans, including REM sleep. More recent work has shown that birds also move their vocal muscles in response to music in their brains, and can even be tricked into singing loudly by sounds played to them while they sleep.

When they sleep, zebra finches also generate variations of their songs: they collect sensory information during wakefulness, and make adaptive changes by improvising new versions to support the their studies in their dreams.

DO FISH DREAM LIKE HUMANS?

According to Philippe Mourrain, a neurobiologist at Stanford University, zebrafish also experience REM sleep. When they sleep, the muscle tone of these fish decreases, their heart beats become arrhythmic, but their brain activity remains similar to that of an awake fish. The notable difference between fish and humans is that fish do not move their eyes while sleeping; and, in the absence of eyelids, they do not close them either.

This finding suggests that REM sleep, the state in which most dreams occur, may have evolved at least 450 million years ago: before evolution gave rise to aquatic land animals. .

“Twenty years ago, I was told that fish don’t sleep,” says Mourrain. “Now we observe that these behavioral characteristics exist everywhere, from insects to spiders, to vertebrates. And during REM sleep, we lose control of our most important regulatory system. Evolution cannot preserve such a fragile state without its significance. »

But why are dreams important? If evolution has preserved REM sleep, can fish dream?

(Read: Our dreams, windows to our worries.)

Everything depends on the meaning we have of the dream. For Mourrain, the dream is the remaking of synapses or, in other words, a reset of neural connections that prepare our nervous system for the next day through processes such as memory consolidation and optimization . cognition.

“I would not be surprised to discover that animals dream like we do, and I think one day we will be able to demonstrate this scientifically,” said the neurobiologist.

“We do something during the day, and our brain replays it, integrates it, and mixes it with other experiences during sleep. We are not the only species capable of remembering and learning. »

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