Commonly-prescribed antidepressants can cause patients to become emotionally numb by affecting a key cognitive function that allows people to learn from their actions, a new study suggests.
A class of antidepressants, known as selective serotonin reuptake inhibitors (SSRIs), is widely used to treat patients with major depressive disorder (MDD) and obsessive-compulsive disorder (OCD). These drugs block serotonin from being absorbed back into the blood, leaving a higher level of the “feel-good chemical” in the brain.
While SSRIs are effective in helping to alleviate severe MDD or OCD symptoms, many of those who take the drugs report “blunting,” meaning they are unable to experience positive or negative emotions such as happiness or sadness and no longer find things as enjoyable as they used to.
In a study published on Jan. 23 in Neuropsychopharmacology researchers at the University of Cambridge, in collaboration with the University of Copenhagen, recruited 66 healthy volunteers and divided them into two groups. One group (of 34) was given a placebo, while the other group (of 32) was given escitalopram, an SSRI known to be one of the best-tolerated by MDD patients.
After three weeks, both groups were asked to complete a series of tests to assess cognitive functions including learning, memory, executive function, reinforcement behavior, and decision-making.
Compared with the placebo group, the 32 volunteers who took escitalopram were found to be less responsive to reinforcement learning—less able to learn from the feedback from interactions with their surroundings.
The researchers used a “probabilistic reversal test” to assess reinforcement sensitivity. During the test, participants were asked to select from two stimuli, A or B. If they selected A, they would receive a reward four out of five times; if they selected B, they would only receive a reward one time out of five. This rule was not known to participants, who would have to discover it on their own. At some point in the experiment, the probabilities would switch, and participants would have to learn the new rule.
The results showed that the escitalopram group performed significantly worse than those in the placebo group—they became less likely to use positive and negative feedback to guide their actions.
Researchers said this reduced reinforcement sensitivity could help explain the “blunting” side effect.
“In a way, this may be in part how they work,” Dr. Barbara Sahakian, the study’s senior author and a psychiatry professor at the University of Cambridge, said in a press release. “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,” she added.
By Bill Pan
