A recent study has unveiled new insights into the neural mechanisms underlying treatment-resistant depression. By recording stereotactic electroencephalography signals (sEEG) from patients’ brains, the team identified specific abnormalities in how depressed individual process emotional information. This study, published in Nature Mental Health, provides a promising step towards understanding and potentially treating this challenging condition.
Depression is a common but serious mental health disorder characterized by persistent feelings of sadness, hopelessness, and a lack of interest or pleasure in daily activities. It affects millions of people worldwide and can significantly impair one’s ability to function at work, school, and in personal relationships.
While many individuals with depression respond well to standard treatments, such as medication and psychotherapy, a significant subset of patients do not experience sufficient relief from these approaches. This condition is known as treatment-resistant depression. It is defined as the failure to respond to at least two different antidepressant treatments administered at adequate doses and durations.
The new study conducted by the researchers at Baylor College of Medicine aimed to explore the neural basis of an emotion-processing bias observed in individuals with depression. This bias leads to a stronger response to negative information compared to positive information, which exacerbates depressive symptoms. Understanding the neural mechanisms behind this bias is crucial for developing targeted interventions that can better address the unique challenges of treatment-resistant depression.
“There has been a big question in the field about whether there was a physiological abnormality we could measure related to depression, as people had historically thought of it as a disorder of the ‘mind’ rather than one of the ‘brain’ and its cells. In this study, we were able to capture very sensitive data from awake, behaving human subjects that demonstrate a physiological basis for treatment-resistant depression,” said study authors Kelly Bijanki, an associate professor, and Xiaoxu Fan, a postdoctoral fellow.
For the study, sEEG electrodes were implanted in specific regions of the participants’ brains, particularly the amygdala and prefrontal cortex (PFC). These regions were chosen due to their known roles in emotion processing and regulation. The electrodes provided high spatial and temporal resolution recordings of brain activity, allowing the researchers to observe detailed neural responses to emotional stimuli.
The study included 12 epilepsy patients and 5 patients diagnosed with treatment-resistant depression. The epilepsy patients served as a control group since they were already undergoing stereotactic electroencephalography (sEEG) monitoring for seizure localization. The treatment-resistant depression patients had not responded to at least four different antidepressant treatments and were recruited as part of an early feasibility trial.
Participants were asked to rate the emotional intensity of human face photographs displaying various expressions, ranging from very sad to very happy. This task was designed to evoke and measure their neural responses to both positive and negative emotional stimuli. The emotional intensity ratings were recorded using a computer interface, ensuring precise synchronization with the brain activity data captured by the sEEG electrodes.
The researchers found that individuals with treatment-resistant depression exhibited a heightened and prolonged response in the amygdala when viewing sad faces compared to the control group. This increased activity began around 300 milliseconds after the sad faces were presented, indicating an overactive bottom-up processing system.
The treatment-resistant depression group also showed a reduced amygdala response to happy faces at a later stage (around 600 milliseconds). This finding suggests a diminished ability to process positive emotional stimuli, which may play a role in the persistent low mood characteristic of depression.
The researchers observed increased alpha-band power in the prefrontal cortex of the treatment-resistant depression patients during the late stage of processing happy faces. Alpha-band power is thought to reflect inhibitory processes in the brain.
Additionally, there was enhanced alpha-band synchrony between the prefrontal cortex and the amygdala, indicating stronger top-down regulation of the amygdala by the prefrontal cortex in these patients. This suggests that the prefrontal cortex may excessively inhibit the amygdala, contributing to the reduced emotional response to positive stimuli.
“sEEG can provide data with high temporal resolution and reliable anatomical precision of signal sources,” Bijanki and Fan told PsyPost. “With the help of sEEG, our results clearly revealed that different neural mechanisms are responsible for the biased negative and positive emotion processing in TRD patients.
The study also explored the effects of deep brain stimulation on neural responses in treatment-resistant depression patients. After deep brain stimulation was administered to the subcallosal cingulate and ventral capsule/ventral striatum regions, the neural responses to emotional stimuli in the patients showed significant changes.
The amygdala response to both sad and happy faces increased, and the alpha-band power in the prefrontal cortex decreased during happy-face processing. Furthermore, the alpha-band synchrony between the prefrontal cortex and the amygdala during happy-face processing was reduced, bringing the neural activity patterns closer to those observed in the control group.
“Treatment-resistant depression has a signature in the firing pattern of neurons in the brain, especially during an emotional task,” Bijanki and Fan explained. “We see the brain being perhaps overly sensitive to negative emotional information in depression patients, and we see evidence of increased top-down inhibition from a moderating brain region that may explain the abnormality. Further, we see after therapeutic brain stimulation, this pattern is normalized. We hope with further study this signal may help clarify the mechanism of depression and suggest new potential treatments.”
The small sample size limits the ability to generalize the findings. Additionally, using epilepsy patients as controls, who may have varying levels of depressive symptoms themselves, might affect the comparison. Future research should aim to include larger and more diverse samples to validate these findings.
The researchers also plan to explore how these neural markers can be used to evaluate the effectiveness of depression treatments. “We hope to use the biased emotional processing signature as a biomarker to evaluate the effects of depression treatments and as an indicator of the severity of depression symptoms in future patients,” the researchers said.
The study, “Brain mechanisms underlying the emotion processing bias in treatment-resistant depression,” was authored by Xiaoxu Fan, Madaline Mocchi, Bailey Pascuzzi, Jiayang Xiao, Brian A. Metzger, Raissa K. Mathura, Carl Hacker, Joshua A. Adkinson, Eleonora Bartoli, Salma Elhassa, Andrew J. Watrous, Yue Zhang, Anusha Allawala, Victoria Pirtle, Sanjay J. Mathew, Wayne Goodman, Nader Pouratian, and Kelly R. Bijanki.