Emotional 'blunting' caused by common antidepressants explained in new study

Scientists have worked out why common anti-depressants cause around half of users to feel emotionally 'blunted'. In a study published recently, they show that drugs affect reinforcement learning, an important behavioral process that allows us to learn from our environment.

A team led by researchers at the University of Cambridge, in collaboration with the University of Copenhagen, recruited healthy volunteers and administered escitalopram, an SSRI known to be one of the best-tolerated, over several weeks and assessing the impact the drug had on their performance on a suite of cognitive tests.

In total, 66 volunteers took part in the experiment, 32 of whom were given escitalopram while the other 34 were given a placebo. Volunteers took the drug or placebo for at least 21 days and completed a comprehensive set of self-report questionnaires and were given a series of tests to assess cognitive functions including learning, inhibition, executive function, reinforcement behaviour, and decision-making.

The key novel finding was that there was reduced reinforcement sensitivity on two tasks for the escitalopram group compared to those on placebo. Reinforcement learning is how we learn from feedback from our actions and environment.

In order to assess reinforcement sensitivity, the researchers used a ‘probabilistic reversal test’. In this task, a participant would typically be shown two stimuli, A and B. If they chose A, then four out of five times, they would receive a reward; if they chose B, they would only receive a reward one time out of five. Volunteers would not be told this rule, but would have to learn it themselves, and at some point in the experiment, the probabilities would switch and participants would need to learn the new rule.

The team found that participants taking escitalopram were less likely to use the positive and negative feedback to guide their learning of the task compared with participants on placebo. This suggests that the drug affected their sensitivity to the rewards and their ability to respond accordingly.

Reference:

Langley, C, Armand, S, et al. Chronic escitalopram in healthy volunteers has specific effects on reinforcement sensitivity: A double-blind, placebo-controlled semi-randomised study. Neuropsychopharmacology; 23 Jan 2023; DOI: 10.1038/s41386-022-01523-x