| Paper authors: | Tabitha Iseger, Martijn Arns, Jonathan Downar, Daniel Blumberger, Zafiris Daskalakis, Fidel Vila-Rodriguez |
| Year of paper publication: | 2020 |
| Post authors: | Alice Erchov, Lisa Ridgway, Fidel Vila-Rodriguez |
| Download the research article: | Iseger (2020) Cardiovascular differences between sham and active iTBS related to treatment response in MDD |
Introduction
Depression doesn’t just affect the way people feel, it also affects the way people think. Specifically, a lot of people with depression notice that their memory might have become worse: they may have difficulty studying for school exams, misplace their things, or forgetting what they needed from the store. Many studies have looked into why this might be, looking particularly at the structure of our brain’s memory center, the hippocampus. Some have found that those with depression have a smaller hippocampus, and this might impact memory.
Because of these findings, more research has been focused on characterizing the size and structure of the hippocampus and how this relates to memory. However, there have been very few studies looking into how the function of this region may be altered in cases of depression.
As mentioned in another recent article, different areas of our brain are always in “conversation” – coordinating their activity in order to do everything from seeing, talking, breathing, and even dreaming! Areas of our brain that are active or quiet at the same time are said to have “functional connectivity”.
The researchers of the current study were wondering if changes in the brain structure (size) and function (activity) were related to differences in memory among people with treatment-resistant depression, when compared to those without.
Methods
56 people with depression completed this study. Everyone had to have tried, and not benefited from, at least one antidepressant in the past. This means their depression was treatment-resistant.
Brain activity was measured using something called functional magnetic resonance imaging (fMRI). Each person lies down in a tube-like scanner. This scanner uses very strong magnets and tracks how different molecules in the brain react to it. Areas of our brain that are very active use a lot of blood and oxygen for energy, when compared to areas of our brain that are not active. The fMRI detects these differences in blood and oxygen to determine what brian areas are working at any given time. For an additional video that explains how MRIs and fMRIs work, you can see here.
The main area the researchers looked at was the hippocampus. We have two hippocampi, one on each side of the brain, located a little below our temples and close to the middle section. Because some studies have suggested that this complicated part of the brain may be more accurately divided into three parts, each responsible for different functions, this study looked at the anterior (front), intermediate (middle), and posterior (back) sections individually. The anterior section is related to emotional processes, the posterior is related to memory, and the role of the intermediate section is unclear.
Memory performance was measured using a “recall” test. Here, participants are read a list of words several times and are asked to repeat back the words both immediately and after a delay.
Changes in brain function and memory were compared between these 56 people with treatment-resistant depression and 42 “healthy controls” without depression. This way, this study could identify the way brain function and memory was different among those with depression.
Results
- This study replicated past research and found that those with depression had, on average, worse memory than those without
- Unlike past studies, these authors found that the overall size of the hippocampus was not reduced in those with depression
- However, its functionality had changed in the intermediate and posterior regions of the hippocampus
- The posterior hippocampus had altered functional connectivity to the front of the brain, with more synchronized activity between these regions being related to worse memory
- This study clarified the role of the intermediate hippocampus as a “transition” between the anterior and the posterior parts of the same brain region: it appears to be involved in both emotion and memory processes
- The intermediate hippocampus was also the only region that was related to the characteristics of the depression, itself
- The longer someone had depression, the more synchronized this region’s activity was with the emotion center of the brain called the amygdala
- This could go in either direction
- Meaning, it is possible that
- The length of the depression changes the way the brain functions OR
- Altered brain function in this region makes someone more vulnerable to depression
Conclusion
People with treatment-resistant depression tended to have worse memory than those without. However, in this study, poor memory is not directly due to changes in the size or structure of the brain’s memory center. Instead, the functional connectivity of the hippocampus is changed, primarily in the intermediate and posterior sections. Changes in how these regions functioned alongside the front of the brain was related to memory function. This study also found, interestingly, that the more synchronized the intermediate hippocampus was to the emotion center of the brain, the longer someone tended to have depression. More research is needed to clarify which came first: is it that depression changes the way the brain functions, or do changes in brain function make someone more vulnerable to depression or resistant to treatment?