This taught the animals that the light could cause either of two effects: one useful (food) and one not (noise).
Once the rats were accustomed to this, Blaisdell divided the rodents into two groups.
Levers were added to the cages of both groups. Group A, as we'll call them, got levers that produced the tone—but no flash—when pushed. Group B got levers that didn't do anything at all.
The Group A rats, with working levers, quickly discovered that pushing them produced the tone but no light.
Group A quickly realized that the tone was separate from the flash of light—that the light was not the cause of the tone or the cause of the sweet liquid being released.
Group B, with nonworking levers, also heard the tone when their Group A pushed their levers. But Group B got excited and hurried to check for sweets.
That's because Group B had know way to know that Group A's levers were the cause of the tone. Group B, it seems, figured they must have missed the flash and therefore might be about to miss out on food.
The experiment is "a bit convoluted," said Howard Eichenbaum, a psychology professor at Boston University in Massachusetts.
"But the gist is that rats can distinguish cause from coincidence," he said.
Blaisdell, the study leader, gives the rats even more credit.
He says that the animals were distinguishing the effects of their own actions from those of outside events.
In other words, the rats realized that they had caused the tone and that it therefore had nothing to do with the light and the possibility of food.
However you interpret it, the animals were demonstrating a fairly sophisticated form of reasoning.
Blurry Lines
The experiment blurs another long-presumed distinction between humans and other animals.
"Our ability to reason causally might not be unique," Blaisdell said. "You can't just draw a line around the human species and say, They reason, and other animals don't."
Still, Boston University's Eichenbaum says, that doesn't mean rats are as smart as humans.
What it does mean, he says, is that rats demonstrate some of the elements of higher-order human thought processes.
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