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Pavlov’s Rat: Scientists Demonstrate That Rats Have Imaginations

Experiments show that rats can picture something they’ve grown used to, even when it isn’t there.
​Image: Inge Habex

​Imaginative rats are a popular trope of fiction, from Pixar's culinary dreamer Remy to the wannabe dictator Brain from Animaniacs. But do real-life rats have any ability to imagine unseen possibilities in the world around them?

According to new research from behavioral neuroscientist Aaron Blaisdell, they absolutely do. Blaisdell presides over UCLA's Comparative Cognition Laboratory, and is particularly interested in the limits of reasoning and imagination in non-human animals.

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Blaisdell presented some of his latest findings in this field Sunday at the Cognitive Neuroscience Society conference in San Francisco. They are based on experiments that suggest rats are capable of intuiting the presence of a certain stimulus, even if it is not visible to them.

"The rats were trained to press a lever when lights were presented," Blaisdell told me. "But they had to pay attention to whether one or two lights were on."

One group of rats was rewarded for pressing the lever when only one light was on, but never when both lights were simultaneously shining. A second group were trained to expect the opposite scenario, and were only rewarded for pushing the lever when both lights were on, but never when only one was lit.

When the rats had been conditioned with these expectations, Blaisdell and his colleagues switched it up by covering one light and presenting the other.

"When one of the lights was covered on this test trial, the amount of times the rat pressed the lever was intermediate between the high amount they pressed on rewarded trials during training, and the low amount they pressed on non-rewarded trials during training." he said.

"This intermediate level of response suggests that rats believed the covered light might be on and thus responded with some uncertainty, but as if it might be on," Blaisdell continued.

The rats seemed to deduce the existence of something that wasn't immediately evident to them, simply because they had prior experience with it. This result suggests that rats have some form of counterfactual thinking, which is the ability to consider alternate versions of past life events, and their speculative effect on the present (for example, the statement: "if only I had invested in Google ten years ago, then I'd be rich today"). It is a sophisticated reasoning tool, and clearly extends to rats in some form.

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Indeed, one of Blaisdell's most compelling findings is that impairing the hippocampus portion of a rat's brain will completely change the outcome of this experiment. For context, the human hippocampus is deeply involved in counterfactual thinking.

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"When we temporarily inactivated the hippocampus, the rats no longer showed the intermediate level of lever pressing when tested with one light on and the other light covered," Blaisdell told me. "They treated these test trials as if it was simple a single light trial as in training."

The new research supports the idea that rats and humans share some level of reasoning ability, and that by extension, so do many other animals. Defining the precise limits of these similarities may lead to better treatment of certain neurological disorders, like schizophrenia, autism, and Alzheimer's disease. It will also provide insights into a wide variety of biological and psychological questions.

Along those lines, Blaisdell plans to delve much deeper into the mental mechanics that underpin reasoning in rats. For example, he would like to find out what is going on in a rat's imagination during the covered light trials. Is the animal compelled to push the lever out of expectation, or can it actually picture the other light when it is not there?

"Another interesting question is what is the role that such imagery plays in learning about events that have been experienced before but that are not currently present," Blaisdell said. "We only have direct access to an incredibly small portion of the known world at any given moment, but humans seem to be very good at entertaining the extended world that is perceptually missing."

"When we learn about new ideas, we often integrate them with our prior knowledge of the world," he said. "Do rats also do this, and is it by the same process as do humans?"

The only way to answer these tantalizing questions is to keep the lab tests rolling. It may turn out that the ability of imaginative gymnastics of real-world rats may not be too far off from their fictional brethren.

"What sets humans apart from the rest of the animal kingdom is our prodigious ability to reason," Blaisdell offered in a separate statement. "But what about human reasoning is truly a human-unique feature and what aspects are shared with our nonhuman relatives? This is the question that drives my passion for research on rational behavior in rats."