Depression is complex and influenced by many factors, but the way depressed people think is a likely contributor to the disorder. Depression is often associated with cognitive biases, including paying more attention to negative than positive events and recalling them more easily. People with depression also tend to ruminate over perceived failures and criticism, and they are extra sensitive to negative feedback.
Analogous cognitive biases can be found in animals. Now, in a new study, researchers have demonstrated for the first time a link between pessimism and sensitivity to performance feedback in rats. It’s the latest finding to show parallels between human depression and rat pessimism – an important result that lends further legitimacy to using animal research to shed light on human psychological problems.
You might wonder how on earth it is possible to measure pessimism in rats. One way to do this was shown in a 2004 study in which animals were trained to press a lever to receive a food reward in response to hearing one tone, and to press a different lever to avoid a mild electric shock upon hearing a different tone. Then the scientists presented the animals with intermediate tones, in between the ones that signaled either food or shock. Which lever the rats pressed in response to these ambiguous cues was considered an indicator of whether the animals expected a positive or negative event. In other words, their behaviour revealed their relative optimism or pessimism.
In the new research, Rafal Rygula and Piotr Popik of the Polish Academy of Sciences used the same paradigm to compare the reaction of rats displaying optimistic and pessimistic traits to positive and negative feedback. First, they divided rats into two groups based on how they performed in the ambiguous-cue interpretation test. Some rats tended to interpret ambiguous cues as signaling a reward, indicating a positive cognitive bias, while others were more likely to interpret them as signaling punishment, indicating a bias toward more pessimistic judgments.
Then the optimistic and pessimistic rats were trained and tested in a probabilistic reversal-learning (PRL) task, which essentially involves using negative or positive feedback to teach the animals to change or maintain a response that they’ve learned previously. Rygula and Popik determined how likely each rat was to switch its response after receiving negative feedback and to maintain its response following positive feedback.
The researchers found that the two groups did not differ in their responses to positive feedback, but that pessimistic rats were more sensitive to negative feedback than optimistic rats. That is, the pessimistic rats were much quicker to drop a previously learned response once it started to be met with negative feedback – you could see this as akin to a depressed human giving up more quickly in response to criticism.
This new finding builds on earlier research by Rygula and his colleagues, in which they demonstrated that the trait of pessimism can also influence rats’ motivation levels (the optimistic rats were more motivated than pessimistic rats to obtain a sip of sugary water), and their vulnerability to “stress-induced anhedonia” – after being restrained, which they find stressful, pessimistic rats showed a longer-lasting lost appetite for sugary water. This might represent a reduction in their ability to experience pleasure that is analogous to human anhedonia, which is another important symptom of depression.
This new study on sensitivity to negative feedback in pessimistic rats, in combination with Rygula’s two previous studies, supports the claim that rats that tend to be pessimistic are also more likely to demonstrate a variety of behavioral and cognitive processes that are linked with increased vulnerability to depression.
It’s hard to know how similar pessimistic rats are to depressed people, but studies like these certainly provide intriguing commonalities. Scientists use animals such as rats as models for human disorders like depression, and use such models to test new therapies and drugs. It seems that rats can display the same negative cognitive biases as people, tending to make negative judgments about events and interpreting ambiguous cues unfavorably. And these biases, in turn, affect both rats’ and humans’ sensitivity to negative feedback.
Post written by Mary Bates (@mebwriter) for the BPS Research Digest. Mary is a freelance science writer specialising in the brains and behaviour of humans and other animals. She has been published in National Geographic News, National Geographic's Weird & Wild blog, New Scientist, the Society for Neuroscience's BrainFacts website, plus many other outlets. She earned her PhD from Brown University, where she researched bat echolocation and bullfrog chorusing. You can follow her on Twitter and Facebook and see all of her work at her website.
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