A diffusion MRI-based structural assessment of the striatal compartments, striosome and matrix, in Obsessive Compulsive Disorder
Using diffusion MRI-based connectivity parcellation, this study reveals that patients with Obsessive Compulsive Disorder exhibit a significant shift toward striosome-like structural volume and connectivity, particularly in the bilateral rostral caudate and putamen, suggesting that altered organization of striatal compartments underpins the disorder's pathophysiology.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Inside the deep folds of the human brain lies a region called the striatum, a critical hub for turning thoughts into actions and learning which habits to keep and which to discard. For decades, scientists have known that when this area malfunctions, it can lead to the relentless, repetitive behaviors seen in obsessive-compulsive disorder, or OCD. Yet, looking at the striatum as a single, uniform lump of tissue has left researchers with an incomplete picture. Recent discoveries in neuroscience have revealed that the striatum is actually built from two distinct, interwoven types of tissue, much like a complex fabric with two different threads running through it. One thread, called the matrix, helps with planning and stopping actions, while the other, known as the striosome, is more involved in processing rewards and emotional signals. These two parts connect to different areas of the brain and serve different jobs, but until now, no one had been able to see how they are arranged in the living human brain or whether their balance changes in people with OCD.
A team of researchers set out to map these hidden compartments in the brains of people with obsessive-compulsive disorder and compare them to healthy individuals. Using a specialized type of brain scan that tracks the tiny pathways connecting different brain regions, they developed a way to distinguish between the matrix and the striosome without needing to perform surgery or use chemical stains. By analyzing the connections of thousands of tiny brain units, they could identify which units behaved like the matrix and which behaved like the striosome. This allowed them to measure the volume of each compartment in the brains of 263 people with OCD and 263 matched healthy volunteers. The goal was to see if the delicate balance between these two functional parts had shifted in the disorder, potentially explaining why the brain gets stuck in loops of worry and repetitive action.
The results revealed a clear and significant difference in the brains of those with OCD. In the healthy control group, the two tissue types maintained a standard proportion, but in the OCD group, the balance had tipped. The researchers found that the volume of tissue behaving like the striosome had increased, while the volume of tissue behaving like the matrix had decreased. This shift was not a small fluctuation; in the front part of the striatum, specifically in the caudate and putamen regions, the volume of striosome-like tissue was 24.3 percent larger in people with OCD compared to healthy controls. The change was most pronounced in the caudate, where the shift toward striosome-like volume was two and a half times greater than in the putamen. This suggests that the brain's internal architecture has physically altered, with the emotional and reward-processing side of the striatum expanding at the expense of the planning and inhibition side.
To understand where this change was happening, the researchers looked at how different parts of the brain connect to the striatum. They found that the shift toward more striosome-like tissue was concentrated in the front, or rostral, part of the striatum. This area receives signals from brain regions involved in emotional processing and reward prediction. In contrast, the back part of the striatum, which connects to areas responsible for motor control and stopping actions, showed no such shift. The study also examined the specific connections from ten different brain regions that feed into the striatum. In eight out of ten of these connection zones, the volume of striosome-like tissue was higher in the OCD group. This indicates that the imbalance is widespread across the brain's emotional and decision-making circuits, rather than being isolated to a single tiny spot.
The researchers also looked closely at the distribution of these changes to guess what might be happening at the microscopic level. They considered several possibilities, such as whether the striosome tissue had simply multiplied in number or if the entire structure had become more tightly packed. However, the pattern of the data pointed most strongly to a specific structural change: the branches of the striosome tissue appear to have become thicker. Imagine a web of fine threads running through a sponge; if those threads grow wider, they take up more space within the sponge, pushing the surrounding material aside. The data showed an increase in the volume of the most strongly defined striosome tissue, alongside a decrease in the tissue that was purely matrix, which aligns with the idea that the striosome branches have expanded in diameter. This physical expansion could mean that the signals from the reward and emotion centers are becoming louder and more dominant, potentially overwhelming the brain's ability to stop or filter out unwanted thoughts.
This study does not claim to have solved the mystery of obsessive-compulsive disorder, nor does it suggest that the brain changes are the sole cause of the condition. The researchers note that their method relies on tracing connections, which is an inference rather than a direct photograph of the tissue, and that the resolution of the scans is still too coarse to see the individual cells. However, the findings provide the first concrete evidence that the internal balance of the striatum is altered in living humans with OCD. By showing that the brain's architecture shifts toward the compartment associated with habit formation and reward, the study offers a new anatomical explanation for why these behaviors become so persistent. It suggests that the repetitive actions of OCD may stem from a physical expansion of the brain's "go" signals, making it harder for the "stop" signals to take hold. This insight opens a new path for understanding the disorder, moving beyond general brain scans to look at the specific, interwoven structures that govern our ability to control our own minds.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.