In a recent class I'm taking, a student expressed interest in dream research in apes. The instructor cautioned not to assume that the animal was necessarily dreaming, even if they're making noises and twitching in REM sleep much the way humans do. He said we don't have good evidence that non-human animals dream.

Most people have probably seen their dog lying on the floor, paws twitching or moving, whimpering or growling while they sleep. If this phenomenon is noted in a laboratory setting under controlled conditions, is it evidence that dogs dream? This gets to one of the fundamental issues of cognitive science, which is that an individual's cognitive state cannot be directly observed. As scientists, we have to make inferences based on what evidence we do have.

This isn't just true of dreams, but of any cognitive phenomenon. How do we know that rats have memories? We cannot directly observe their memories, but we can run them through a maze and then record their performance on repeat runs. If the rat runs the maze faster each time, that's evidence that it is encoding features of the maze in memory.

Back to dreams, what kinds of evidence with regard to dreaming in animals have been analyzed in a scientific way? Last year I read David Linden's excellent book, The Accidental Mind. The book's primary goal is to explain how an understanding of human cognition must be based on an understanding of human brains, which are a product of the messy, indirect engineering of evolution. But it's a great introduction for the interested layman in how neurons work, as well as having great chapters on love and sex, emotion, and sleeping and dreaming.

In that chapter on dreaming, Linden notes some of the evidence for dreaming in animals. He mentions the work of French scientists Jouvet and Mouret, who in the 1960's demonstrated that lesions to the pontine tegmentum (a part of the brain thought to inhibit motor activity during REM sleep) can artificially produce what's called REM Behavior Disorder, or RBD, in rats and other mammals. People who have RBD basically act out their dreams, sometimes posing a danger to themselves or others. When animals such as rats or cats are operated on to induce RBD, they often run, jump, and carry out other actions while in REM sleep. Again, we don't have privileged access to their subjective mental state, so we don't really know what they're experiencing when they're carrying out actions while in REM, but this is highly suggestive that they are dreaming.

Another suggestive bit of evidence that Linden mentions is work done by Louie and Wilson from MIT. They placed arrays of electrodes in the place cells of rats' hippocampus. Place cells are implicated, as the name implies, in representations of space and location. They recorded activity from many cells as the rats ran a maze in a waking state. Then they kept on recording those cells after the rat had fallen asleep. While in REM sleep, they found statistically-significant similarities in neural activity in those same cells. In short, the part of the rat's brain used for navigation was lighting up in almost the same way while it was in REM sleep as when it was running the maze, suggesting that the rats may have been dreaming about the event.

So I would disagree that there is no evidence that animals dream. The sort of research mentioned here constitutes such evidence. We can never know exactly what another being is experiencing (this is true of humans, too). The best we can do is look at the evidence and try to make sound inferences.

Can anyone think of any experiment or observation not mentioned here that would provide very strong evidence that non-human animals dream?