Does your dog dream? Yes? How do you know? Do you notice that your dog whimpers or barks while it is sleeping? Or maybe you notice its legs twitching as if chasing an imaginary cat in its dream.
If you’re of a certain age, I’m talking to you, Generation X; you may remember the scene in Cinderella where Bruno, the dog, sleeps on a rug on the kitchen floor. While Bruno sleeps, he growls, and his legs move like he is running or chasing something in his dream. Cinderella wakes Bruno up and asks him if he is chasing Lucifer, the cat, to which Bruno nods enthusiastically. Before we ponder why anyone would name their cat Lucifer, let’s focus on Bruno’s response. Can we take Bruno’s word for it that dogs dream? If it’s good enough for Cinderella, then surely it’s good enough for us. Well, maybe when I was a kid watching this before the era of mobile phones and social media, but now, as a scientist, I want more robust evidence.
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REM and dreaming
Let’s start by looking at what science says about dreaming. We now know that dreaming occurs during the stage of sleep known as REM, which stands for Rapid Eye Movement. We also know that dogs spend considerable time in REM sleep, just like us. They even appear to spend more time in REM sleep than their human owners. Dogs spend about 36% of their sleep in REM, while humans spend between 20 and 25% of it in REM. But here’s the thing: during REM sleep, the voluntary muscles are paralysed, except the eye muscles, and it’s for a good reason, too. If our muscles weren’t paralysed, we’d act out our dreams. So, to prevent us from acting out our night-time delusions, such as the ability to fly, our muscles become paralysed to prevent us from scaling tall buildings and testing whether our dreams can defy gravity. Going back to Bruno, it begs the question, why was he moving whilst dreaming? If we know dreaming occurs in REM sleep, shouldn’t his muscles have been paralysed?
REM behaviour disorders – in cats
This leads us nicely to a sleeping disorder called REM behaviour disorder, or RBD for short. This is where the mechanism of paralysis during REM goes on the blink. In Bruno’s case, we can now diagnose that he’s probably suffering from some form of RBD. This rare sleep disorder is thought to affect less than 1% of the population. It typically appears in middle age or later. Researchers first discovered the origins of this disorder in animals and then speculated whether the same could occur in humans.
In the 1950s, Michel Jouvet, a scientist at the University of Lyon investigating sleep patterns in felines, embarked on a series of experiments to unravel the mechanisms underlying REM sleep regulation by the brain. Through his research, Jouvet observed that lesions in a specific region of the brain known as the brainstem, responsible for relaying all neural signals between the brain and the body, led to peculiar behaviours in cats during REM sleep, such as hissing, walking, fighting, or exhibiting hunting-like behaviours. Intriguingly, despite these actions, the cats displayed typical brain activity associated with REM sleep; however, the anticipated muscle paralysis typically observed during this sleep stage was absent due to brainstem damage.
These feline behaviours mirrored those of Bruno in Cinderella, yet it’s unlikely Cinderella was aware of any brainstem damage. After all, the film Cinderella premiered in 1950, a full fifteen years before Jouvet’s groundbreaking findings were published.
REM behaviour disorders in humans
After scientists observed similar phenomena in cats, dogs (and even cartoon characters), they shifted their focus to humans. Before the 1960s, numerous case studies documented such behaviours in humans. However, it wasn’t until 1968 that this condition was formally recognised and classified as a disorder known as REM sleep behaviour disorder. This development represented a leap forward in our understanding of dreaming across different species.
REM behaviour disorder versus sleepwalking
RBD shares similarities with night terrors or sleepwalking in some respects. However, unlike night terrors or sleepwalking, the eyes remain closed during RBD, and movements tend to appear aimless, often involving flailing or striking out. There’s minimal interaction with the external environment, and individuals typically remain in bed. Speech during RBD is usually incomplete and may manifest as shouting or cursing, lacking coherence. Contrasting RBD with sleepwalking reveals distinct differences. In sleepwalking, individuals commonly leave the bed with open eyes, engaging in tasks such as handling objects, driving cars and even, alarmingly, potential acts of violence such as murder. Speech during sleepwalking, though nonsensical, is typically comprehensible.
But, how do we KNOW dogs dream?
Now, we understand that dogs dedicate a considerable portion of their sleep to REM, but how can we be sure that they dream? To uncover how scientists determined that animals indeed experience dreaming, let’s look at another researcher’s work on rats, chocolate, and mazes. Once again, like many other classic scientific discoveries, we will learn about serendipity’s role in discovering that dreaming occurs in animals.
We start by asking a rat
Matt Wilson was a thirty-year-old postgraduate at the University of Arizona when he noticed strange things occurring in the brainwaves of rats while they slept. He implanted microelectrodes in a part of the rat’s brain called the hippocampi and set them loose in a maze strewn with delicious chocolate treats. His experiment aimed to learn how the rodents’ place cells fired as they ran through the labyrinth, searching for the chocolate goodies.
What are place cells?
Place cells are neurons that become active or fire when an animal reaches a particular location. These cells are vital for animals, including humans, to learn and navigate new surroundings. For instance, as a rat crosses a maze, a place cell fires each time it encounters a new intersection, creating a mental map of its surroundings. Consequently, when the rat revisits the same location, its brain recalls the cognitive map formed during the initial exploration, enabling easier and faster navigation through the maze…and to the chocolate treats.
Can you listen to thoughts?
In addition to using the electrodes to produce images of the rat’s brain waves, Wilson also connected the rodents to an audio monitor, which basically allowed him to listen to the rat’s brain. This let Wilson determine what the rat was doing by listening to the audio monitor. He soon learned he could see if the rat was running or resting simply by listening to the different rhythms produced by the sounds of neurons firing in different parts of the brain. Basically, Wilson had connected his audio-monitoring equipment to allow him to hear the rat’s thoughts. So, if you want a pop culture reference, think of this machine as Professor X’s first iteration of Cerebro.
Serendipity strikes again
It was then that the hand of fate allowed Wilson to make a major discovery with regard to whether dreaming occurs in rats. After completing a round of foraging the maze for chocolate treats, Wilson returned the rat to its cage, where it promptly fell asleep. Although the rat’s job was done for the day, Wilson still had to process the day’s data, and so while the rat slept, he busied himself with his work. Luckily for science, Wilson had forgotten to disconnect the audio monitor, so it startled him when it crackled back to life.
When he heard the sounds from the audio monitor, he immediately recognised the familiar rhythms of the rat scurrying around the maze. Initially, a sense of horror gripped him, fearing that the rat had escaped its cage and was roaming freely in the lab. However, his apprehension shifted to amazement when he discovered the rat still sound asleep in its cage. It suddenly dawned on him that the sounds emanating from the audio monitor indicated the rat was dreaming. There was no mistaking it: the firing pattern of the animal’s place cells mirrored those heard as it navigated the maze while awake. The rat’s neurons appeared to be replaying the same task it had performed during wakefulness.
How do rats dream?
Wilson shifted his research focus from studying daytime cognition in rats to investigating their nocturnal brain activity instead. He honed his skills in tracking the precise cellular firing patterns of rats to the extent that he could identify which part of the maze they were mentally replaying during sleep. He discovered that the frequency of activation of associated place cells during sleep correlated with how frequently the animal had visited a particular area during the day. Additionally, Wilson observed that the repetition of daytime activity at the cellular level extended beyond the hippocampus, a seahorse-shaped region deep within the brain known for housing many place cells and playing a crucial role in spatial learning. He noted that sensory areas of the brain, such as the visual cortex, also exhibited activity during sleep, suggesting that rats might experience visual imagery during their slumber.
The objective study of dreams
Through his publications on the nocturnal rodent brain, Wilson revitalised the study of dreams. His research introduced an objective method for recording and analysing dreams that didn’t rely on the subjective reporting of human subjects. Unlike humans, rats can’t fabricate their dreams, making their brain activity a reliable model for studying dreams in a controlled setting. The controlled daytime environment of rats allows for comparisons between their daytime and their nocturnal neuronal activity, facilitating research with relative ease.
So, DO dogs dream?
When considering whether dogs dream, it’s essential to explore research findings on animal dreaming, including those involving rats. These studies have revealed fascinating insights into the neural activity associated with dreaming in animals. For instance, researchers like Matt Wilson have shown that rats exhibit neural firing patterns during sleep that mimic their daytime experiences, suggesting that they may indeed dream. Wilson’s work not only shed light on the nocturnal brain activity of rodents but also rejuvenated the study of dreams by providing an objective method for investigation. By using animal models like rats, researchers can bypass the limitations of relying on subjective human reporting of dreams and instead analyse neural activity in a controlled environment. This approach offers valuable insights into the nature of dreaming across species and enhances our understanding of the mind’s activities during sleep
So, if rats can dream about their daytime activity in their hunt for chocolate treats, then surely dogs like Bruno can dream of chasing cats like Lucifer….which leads to the question posed earlier: who would name their cat Lucifer?
References:
- Louie, K. and Wilson, M.A., 2001. Temporally structured replay of awake hippocampal ensemble activity during rapid eye movement sleep. Neuron, 29(1), pp.145-156.
- Hobson, J.A., 1965. The effects of chronic brainstem lesions on cortical and muscular activity during sleep and waking in the cat. Electroencephalography and Clinical Neurophysiology, 19(1), pp.41-62.
- Robb, A. (2018) Why we dream: The Science, Creativity and Transformative Power of Dreams. Pan Macmillan.
- Leschziner, G. (2020) The secret world of sleep: The Search for Salvation During the Night.
- Walker, M. (2017) Why we sleep: Unlocking the Power of Sleep and Dreams. Simon and Schuster.
