Many figured that Rover was romping somewhere in dreamland, and scientists say they were right: Pets do dream while sleeping.
As dogs and cats doze, images of past events replay in their minds much the same way humans recall experiences while dreaming, said Matthew Wilson of MIT's Picower Institute for Learning and Memory in Cambridge, Mass. That's because the hippocampus, a part of the brain involved in memory, is basically wired the same way in virtually all vertebrates and mammals, he said.
"If you compared a hippocampus in a rat to a dog; in a cat to a human, they contain all of the same pieces," said Wilson, an associate professor of brain and cognitive sciences.
Like people, pets go through multiple stages of sleep, from periods of slow wave sleep to REM (rapid eye movement), where most dreaming occurs.
"From the minute your head hits the pillow and you're out, the dreaming process begins," he said.
Non-REM dreams consist of quick snapshots of things usually done that day. During the deeper sleep state of REM, dreams last much longer and tap into a vast pool of past experiences drawn from weeks, months, even years in the past.REM occurs approximately every 90 minutes in people, and every 25 minutes in cats.
In dogs, research shows the frequency and length of dreams is linked to their physical size, said psychologist Stanley Coren, author of several books, including "How Dogs Think: Understanding the Canine Mind."
For example, he said, mastiffs and Great Danes might dream every 45 minutes for about five minutes, compared to their smaller canine cousins that enter a dream state every 10 minutes with episodes lasting less than 60 seconds.
Owners can tell if their dozing dog or feline is dreaming by looking for these clues: whisker twitching, paw tremors, irregular breathing and — in dogs — occasional high-pitched yips.
But what do our pets dream about? Researchers believe they know the answer. Older studies, done decades ago in cats, involved temporarily releasing the suppression of motor activity that happens during REM sleep so they'd act out their dreams.
What researchers witnessed is sleepwalking cats doing things they'd normally do while awake — walking, swatting their forepaws, even pouncing on imaginary prey.
Similar research showed the same held true for dogs.
"Pointers point at dream birds, and Dobermans growl at dream burglers," Coren said.
Those experiments were not a demonstration of actual dreaming, said MIT's Wilson, but do suggest that in REM sleep the brain is functioning the same way it behaves during normal wakefulness. As early as 2001, he decided to find out if animals did in fact dream by eavesdropping on the sleeping brain.
Wilson used electrodes to record the brain activity of rats as they ran a circular track and later as they slept. He discovered, by examining more than 40 REM episodes recorded while the rats slept, that the sleeping rodents often appeared to replay images of navigating the track in real time. About 50% of the episodes repeated the unique signature of brain activity created as the animal ran. In fact, because records of the neural signals in both the sleep and waking states were so similar, Wilson said he could reconstruct where the dreaming rats were on the track and whether they were standing still or running.
This human-like ability to dream about actual experiences almost certainly applies to pets, he said.
"My guess is — unless there is something special about rats and humans — that cats and dogs are doing exactly the same thing," he said.
In the scientific community, animals are often thought of as reflex machines, operating by instinct alone. But this view is slowly starting to change, noted Wilson, as new information about dreaming in animals is unearthed.
Coren, the psychologist, agreed. He said that one of his heroes, Charles Darwin, "basically claimed if you can prove that an animal dreams, then, in effect, you can prove that's consciousness. Because after all, what is a dream other than a conscious image?"
Wilson's current work goes beyond analyzing dream content and relates to what's going on inside the brain during wakefulness. Using lab-built devices with an array of electrodes, he's found that rats appear to replay memories while doing normal, everyday activities like nibbling on food or sitting quietly. In other words, he said, they're thinking about the past, and possibly contemplating the future.
"The idea that rats may actually be thinking — just as humans think when they're sitting, appearing not to be doing anything — suggests the full range of cognitive abilities that we have," he said.
Wilson believes his work extends beyond using animal models to explore human memory and cognition. "It really is using animal models to study animal cognition," he said. "Understanding the differences will give us a better understanding of where we stand in the hierarchy of organisms on the planet."