Pupil response can reveal the depths of depression

Can a simple eye response reveal the depths of depression? In a recent study conducted by scientists from the Max Planck Institute of Psychiatry, researchers have discovered that the way our pupils react to anticipation of rewards can differentiate between those suffering from depression and healthy individuals. The study, published in the journal Scientific Reports, suggests that the reduced pupil dilation observed in depressed patients during reward anticipation tasks could pave the way for new diagnostic tools and more personalized treatment approaches.

Pupillometry, the measurement of pupil size and reactivity, serves as the cornerstone of this research. Traditionally used to assess vision and brain function, pupillometry is gaining traction in psychological and psychiatric research due to its ability to offer a window into the brain’s noradrenergic system—a part of the brain involved in arousal and stress responses.

The researchers embarked on this study to explore how this simple, non-invasive measure could illuminate the complexities of major depressive disorder (MDD), a condition that affects millions worldwide and is characterized by persistent sadness, loss of interest in enjoyable activities, and an array of physical problems.

“One of the main aims of my group is to improve diagnostics in psychiatry, make it more objective and causal with physiological tests. We are testing various non-invasive methods, but pupillometry has the benefit that it’s very precise and tracking some processes that are relevant to depression. One of these is the upregulation of arousal in order to get a reward, which is what this study was about,” explained study author Victor Spoormaker, the leader of the Psychophysiology Lab at the Max Planck Institute of Psychiatry.

The study initially involved 201 new participants, all of whom were recruited from a broader research project aimed at classifying mental disorders biologically. After a screening process that excluded individuals on antidepressant medication or with incomplete data, the study focused on 40 unmedicated depressed participants and 30 healthy controls.

Participants underwent a series of assessments, including diagnostic interviews and the Beck Depression Inventory—a questionnaire used to measure the severity of depression. The heart of the study was a specially designed task that measured participants’ pupil reactions as they anticipated rewards, mirroring situations in daily life that provoke excitement or pleasure.

The results revealed that participants suffering from MDD showed significantly less pupil dilation compared to the healthy control group. This reduced dilation was even more pronounced among those who reported higher levels of depressive symptoms, such as an inability to experience pleasure (anhedonia) and a general loss of energy, suggesting a direct correlation between the severity of depressive symptoms and the extent of diminished pupil response.

“The reduced pupil reaction was particularly noticeable in patients who could no longer feel pleasure and reported a loss of energy,” said Andy Brendler, first author of the study.

Further analysis extended these observations by including both depressed and healthy participants, confirming the robustness of the correlation between reduced pupil dilation and depressive symptomatology. This consistency across different participant groups lends strong support to the potential of pupillometry as a diagnostic and monitoring tool for depression.

The study’s findings were not limited to just behavioral observations. Functional magnetic resonance imaging (fMRI) data provided a deeper dive into the neural mechanisms at play, correlating changes in pupil size with activity in specific brain regions known to be involved in emotional regulation and arousal, such as the right insula and medial prefrontal cortex.

However, like all research, this study comes with its limitations. The sample size, while adequate for initial findings, necessitates larger, more diverse cohorts to generalize the results across different populations and stages of depression. Furthermore, the study’s design, focused on unmedicated participants, raises questions about how these findings would translate to individuals currently undergoing treatment for depression.

Future research is also needed to explore the specific mechanisms linking pupil response to the noradrenergic system and how these findings could be integrated into clinical practice for diagnosing and treating depression.

Despite these challenges, the study marks a significant advance in our understanding of depression, paving the way for more personalized and biologically informed approaches to treatment.

“Depression is a very heterogeneous mental disorder with very different symptoms,” Spoormaker told PsyPost. “It is important to tease apart this umbrella term and be more specific about which physiological processes are affected in a certain individual with a depressed mood, so that we can get better in selecting the best fitting treatment. This one test could maybe identify a subset of patients, so ideally, this would be one of multiple tests in a future diagnostic procedure.”

The study, “Assessing hypo-arousal during reward anticipation with pupillometry in patients with major depressive disorder: replication and correlations with anhedonia“, was authored by Andy Brendler, Max Schneider, Immanuel G. Elbau, Rui Sun, Taechawidd Nantawisarakul, Dorothee Pöhlchen, Tanja Brückl, the BeCOME Working Group, Michael Czisch, Philipp G. Sämann, Michael D. Lee, and Victor I. Spoormaker.