The emotional blunting experienced by some antidepressant users may be due to the dampening effect the drugs have on their ability to learn from rewards, researchers at the University of Cambridge have found.

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According to the NHS, more than 8 million patients received antidepressant drugs last year. The most common of these were selective serotonin reuptake inhibitors (SSRIs). These drugs increase levels of serotonin – a messenger chemical that is thought to influence mood – emotions and sleep, in the brain by preventing it from being absorbed by nerve cells.

While SSRIs are effective in treating depression, 40 to 60 per cent of users report feelings of emotional blunting – an inability to experience pleasure and joy in the way they used to.

To investigate this effect, the team gathered 66 volunteers, 32 of whom were given a daily dose of escitalopram, an SSRI known to be one of the best-tolerated, while the other 34 were given a placebo. After 21 days, they had the volunteers complete a self-reported questionnaire to assess their mood and set them a series of tests designed to assess their cognitive functions.

They found no difference in the two groups’ scores in attention and memory tests, but the volunteers taking escitalopram performed worse in a test designed to assess their reinforcement learning. This is the ability to learn from the feedback we receive from our actions and environment.

In the test, the volunteers were asked to choose one of two options, A or B. If they chose A, then four out of five times they received a reward. If they chose B, they only received a reward one out of five times. They were not told this rule and had to learn it for themselves throughout the course of the experiment.

The team found that the volunteers taking escitalopram were less likely to use the positive and negative feedback to guide their learning. This suggests that the drug affected their ability to respond to the rewards.

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“Emotional blunting is a common side effect of SSRI antidepressants,” said co-researcher Prof Barbara Sahakian, from the Department of Psychiatry at the University of Cambridge.

“In a way, this may be in part how they work – they take away some of the emotional pain that people who experience depression feel, but, unfortunately, it seems that they also take away some of the enjoyment. From our study, we can now see that this is because they become less sensitive to rewards, which provide important feedback.”

The team now plan to carry out a follow-up study using neuroimaging techniques to further understand how escitalopram affects the brain during reward learning.

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Authors

Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.

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