You can't see your sleeping pet's brain waves, but its behavior can tell you when Fido or Fluffy might be dreaming. If you watch closely, you'll see that as your cat falls asleep, her breathing becomes slow and regular and her body still. She has entered the first stage of sleep, called slow-wave sleep. After about 15 minutes you'll notice a change in her breathing. Her paws and whiskers twitch(抽动) and she flicks an ear. Fluffy has entered REM(Rapid Eye Movement), or dreaming stage of sleep. Although she twitches and makes little grunting noises, messages from her brain to the large muscles in her legs are blocked, so she can’t run about. She is in a state of “sleep paralysis”.

Back in 1963, Michel Jouvet, a French scientist who was studying sleep in cats, interrupted their sleep paralysis. Even though they were completely asleep, the dreaming cats began to chase balls that Jouvet couldn't see and arched their backs at invisible enemies. He figured he was watching them act out their dreams!

What were they dreaming about? Mostly, the dreaming cats seemed to be practising important cat skills: stalking, pouncing and fighting.

In another study, neuroscientist Matt Wilson recorded rats' brain waves while they learned mazes(迷宫). One day, he left the brain-wave-recording machine on while the rats fell asleep. The pattern of brain waves in the sleeping rats matched the pattern from the maze so closely that Wilson could figure out exactly which part of the maze each rat was dreaming about!

Many researchers now think that in both people and animals, one purpose of dreams is to practise important skills and figure out recent learning. This may explain why so many people dream about fighting and escaping, skills that were probably important to our ancestors, and why dreaming affects our ability to learn.

Do all animals dream? From looking at the brain waves of sleeping animals, scientists think that all mammals dream, but reptiles, fish, and invertebrates(无脊椎动物) don't.(They're not sure about birds.)

Scientists once thought bigger brains made smarter animals. But birds fly in the face of that logic: with a brain smaller than a walnut, they can develop complicated tools and remember where they hid food. Now research published in Current Biology suggests birds can pull this off because their brain neurons (神经元) use less energy than those of mammals (哺乳动物), letting their bodies support a higher proportion of these cells.

A 2016 study showed that bird brains are denser (密度大的) than those of many other animals. For example, a parrot’s 20-gram brain holds as many neurons as a squirrel monkey’s 30-gram brain.

In the new research, when compared against the neuronal energy budget of mice, humans and other mammals, a pigeon neuron used three times less energy than the average animal’s neuron―a “really surprising” result, says the bird scientist Kaya von Eugen of Ruhr University Bochum in Germany. Although bird neurons are likely smaller than a typical mammal’s, she adds, the difference in energy use “is so big that this cannot be the only explanation. “Perhaps, she suggests, bird brains are organized so that neurons can more easily exchange signals, or maybe birds’ warmer body temperatures let neurons function faster. The author guesses that complex mental needs such as song and flight could have pushed the evolution of more efficient brain cells.

The finding is “pretty remarkable,” says Vanderbilt University scientist Suzana Herculano-Houzel, who worked on the 2016 study but was not involved in the new research. Based on the density difference between mammal and bird brains, she says, the energy difference is “exactly the math you’d expect.” Birds may have evolved this feature simply to work with their limited energy supply, rather than to consider advanced processing needs.

A new study, led by Huijeong Jeong and Vijay Namboodiri of the University of California, San Francisco, has turned the world of neuroscience (神经科学) on its head.

It proposes a model of associative learning which suggests that researchers have got things backwards. Their suggestion, moreover, is supported by a series of experiments. The old model looks forward, associating cause with effect. The new one does the opposite. It associates effect with cause. They think that when an animal receives a reward (or punishment), it looks back through its memory to work out what might have caused this event. Looking at things this way deals with two things that have always made the old model hard to understand. Making predictions based on every single possible cue (暗示) would be somewhere between difficult and impossible. It is far simpler, when a meaningful event happens, to look backwards through other potentially meaningful events for a cause.

In practice, however, it is hard to distinguish experimentally between the two models. And that is especially true if you do not even bother to look—which, until now, people have not.

Dr. Jeong and Dr. Namboodiri have done so. They conducted 11 experiments that were designed specifically for the purpose. During these they measured, in real time, the amount of dopamine (多巴胺) being released by the nucleus accumbens, a region of the brain in which dopamine is involved in learning and addiction. All of the experiments came down in favor of the new model.

“The study is thought-inspiring and represents a stimulating new direction,” says Ilana Witten, a neuroscientist at Princeton University uninvolved with the paper.

More experiments will be needed to confirm the new findings. But if confirmation comes, it will have influences beyond neuroscience. Dr. Namboodiri thinks so, and is exploring the possibilities. Evolution has had hundreds of millions of years to better the process of learning.

So learning from nature is rarely a bad idea.