A new study sheds light on the neurobiological aspects of anhedonia and how it is related to altered reward processing in depression patients. The findings, published in Psychological Medicine, indicate that depression patients exhibit alterations in how their brains process social touch, which might contribute to their difficulties in social interactions and feelings of loneliness.
Major Depressive Disorder (MDD) is a prevalent mental disorder that affects many people worldwide and leads to significant disability. One of the core symptoms of MDD is anhedonia, which refers to a lack of interest or pleasure in activities that were once enjoyable. Anhedonia can impact various aspects of pleasure, motivation, and desire, making it an essential area of study for better understanding and treating depression.
The authors of the new study were particularly interested in how anhedonia is linked to the brain’s reward network. Previous neuroimaging studies had shown that individuals with MDD have reduced brain responses to monetary incentives and happy faces in reward-related brain regions, such as the nucleus accumbens, caudate nucleus, putamen, insula, and orbitofrontal cortex.
Furthermore, social interactions are considered natural rewards, and social touch is an integral part of nonverbal social communication and bonding. However, little research had been done on how MDD might affect the processing of rewarding interpersonal touch.
“We have long been interested in the experience and the processing of social touch in individuals with mental illness,” explained Clemens Mielacher, a clinical psychologist at the Klinikum Wels-Grieskirchen who conducted this study as a PhD student at University Hospital Bonn.
“Previously, our lab conducted an fMRI study of social touch in individuals with a history of childhood maltreatment (Maier, 2020, AJP) who reported discomfort with touch and showed altered sensory cortical and limbic sensitivity to touch. When we decided to look at other mental illnesses, major depression was an obvious candidate, because many patients suffer from anhedonia, and we hypothesized that these patients would derive less pleasure from being touched by another person than healthy controls.”
The study included 53 patients with MDD and 41 healthy controls. The participants underwent brain scans using functional magnetic resonance imaging (fMRI) while receiving different types of touch on their shins. The touch stimuli were either slow and gentle (affective touch) or fast and repetitive (discriminative touch). Participants rated the comfort of each touch type, and their brain activity was observed during the touch experiences.
The researchers found that MDD patients reported feeling more aversion to interpersonal touch and experienced it as less comfortable compared to the healthy controls. This suggests that social touch might be less rewarding or pleasurable for individuals with depression.
The fMRI scans revealed reduced neural activation in the reward network, specifically in the nucleus accumbens, caudate nucleus, and putamen, in MDD patients during social touch. These regions are associated with processing rewarding experiences, and their decreased activity suggests altered reward processing in MDD patients.
“Loss of pleasure, or anhedonia, is a core symptom of major depression,” Mielacher told PsyPost. “In our fMRI study, we found that this may extend to the experience of being touched by another person. Patients reported greater aversion to interpersonal touch and showed reduced comfort ratings after social touch compared to healthy controls. In addition, patients with major depression showed blunted neural reactivity in the reward network during social touch.
Surprisingly, even after undergoing a multi-week course of antidepressant treatment, the differences in brain activity during social touch persisted for certain regions like the nucleus accumbens and caudate nucleus. This finding suggests that these altered neural responses might be stable traits associated with MDD and may not be entirely corrected by antidepressant treatment.
“We found that these alterations in social touch processing in the reward network did not normalize after treatment and instead appeared to persist even after other symptoms improved,” Mielacher told PsyPost. “This is consistent with findings of residual symptoms of anhedonia that persist after clinical improvement.”
Non-responders to antidepressant treatment (individuals who did not show significant improvement in their depressive symptoms) had reduced brain activity in the caudate nucleus, anterior insula, and putamen specifically during slow touch. This indicates that the way the brain processes social touch might be related to how well a person responds to antidepressant treatment. Those with more altered reward-associated brain activity might be less responsive to standard antidepressant therapies, the researchers said.
However, further research is needed to understand the precise mechanisms behind these neural alterations and their potential role in treatment responsiveness.
“These are exciting findings, but as always, they warrant independent replication,” Mielacher noted.
The study, “Altered reward network responses to social touch in major depression“, was authored by Clemens Mielacher, Dirk Scheele, Maximilian Kiebs, Laura Schmitt, Torge Dellert, Alexandra Philipsen, Claus Lamm, and René Hurlemann.
