Structural reductions of Auditory–Temporoparietal Association Cortex in Cannabis Users: Relevance to Neural Systems Implicated in Auditory Hallucinations

The human brain is a master of prediction. It constantly builds a model of the world, anticipating what we will see, hear, and feel next. When we speak to ourselves internally, the brain sends a signal to the auditory system saying, "I am the one making this sound," which allows us to recognize our own thoughts as our own and not as voices coming from outside. This ability to distinguish between self-generated experiences and external reality is a delicate process. When this system falters, a person might hear voices that have no external source, a symptom known as an auditory hallucination. These experiences are a hallmark of psychotic disorders like schizophrenia, but they can also occur in people without a diagnosed illness. Scientists have long known that using cannabis increases the risk of developing psychosis, yet the specific biological changes in the brain that link the drug to these hallucinations have remained somewhat of a mystery.

A new study by researchers at the University of Texas Rio Grande Valley has taken a closer look at this connection by examining the physical structure of the brains of people who use cannabis. The team focused on a specific network of brain regions that are critical for hearing, understanding language, and integrating sensory information. These areas, located along the sides of the brain, act as a processing hub where sound is combined with context and memory. The researchers hypothesized that if cannabis use contributes to the risk of hearing voices, it might leave a visible mark on the thickness of the tissue in these specific regions. By comparing the brain scans of cannabis users with those of non-users, the study aimed to see if the drug was associated with a thinning of these crucial neural circuits, and whether this physical change was linked to how people make decisions about rewards.

To investigate this, the researchers analyzed data from the Human Connectome Project, a massive database of brain scans and behavioral tests from healthy adults. They identified 330 individuals who had used cannabis at least three times in their lives and matched them perfectly with 330 non-users based on age and sex. This careful matching ensured that any differences found were likely due to cannabis use rather than other factors like age or gender. Using high-resolution MRI scans, the team measured the thickness of the cortex—the outer layer of the brain—in five key areas known to be involved in hearing and language: the superior temporal gyrus, Heschl's gyrus, the banks of the superior temporal sulcus, the supramarginal gyrus, and the angular gyrus. They also looked at how these individuals performed on a task designed to measure impulsivity, specifically their willingness to wait for a larger reward later versus taking a smaller reward immediately.

The results revealed a clear pattern of structural change. The brains of cannabis users showed significantly thinner tissue in almost all the auditory and language-related regions examined. This thinning was observed on both sides of the brain in the superior temporal gyrus, the angular gyrus, and the supramarginal gyrus. It was also found in the banks of the superior temporal sulcus and in the right side of Heschl's gyrus, which is the primary area for processing sound. The differences were consistent and measurable, with the thinning being most pronounced in the supramarginal gyrus and the banks of the superior temporal sulcus. Importantly, the study found that this thinning was specific to the thickness of the tissue itself, not the overall size of the brain regions, suggesting a change in the microscopic structure of the brain's layers rather than a loss of the region as a whole.

Alongside these physical changes, the cannabis users also displayed a behavioral tendency toward impatience. When faced with choices between a smaller immediate reward and a larger delayed one, the users were more likely to choose the immediate option, a behavior known as steeper delay discounting. While this difference was a trend rather than a massive statistical divide, it aligned with the physical findings. The researchers also discovered a link between the brain's structure and this behavior: in both users and non-users, a thicker right Heschl's gyrus was associated with a greater ability to wait for a larger reward. This suggests that the physical integrity of the primary auditory cortex might play a role in how the brain values future outcomes, connecting the ability to process sound with the ability to exercise self-control.

The study also looked at how these changes might evolve over time. Among the cannabis users, older individuals showed more rapid thinning in the right supramarginal gyrus and the right banks of the superior temporal sulcus compared to non-users of the same age. This suggests that the effects of cannabis use on these specific brain regions might accumulate with age, potentially making the brain more vulnerable to the kinds of errors in perception that lead to hallucinations. The researchers noted that the age at which a person first started using cannabis did not show a significant link to these structural changes in this particular group, likely because the study did not include enough people who started using the drug as very young teenagers.

These findings support the idea that cannabis use may alter the brain's predictive machinery. The regions that showed thinning are essential for distinguishing between what we generate internally and what comes from the outside world. If the tissue in these areas becomes thinner, the brain's ability to accurately predict and suppress self-generated sounds might be compromised, potentially leading to the misattribution of inner thoughts as external voices. The fact that these structural changes are also linked to impulsive decision-making suggests a broader disruption in how the brain assigns importance to different signals, a mechanism that is central to both addiction and psychosis.

While the study provides strong evidence of an association, the researchers are careful to note that it cannot prove that cannabis causes the thinning or the increased risk of hallucinations. It is possible that people with naturally thinner auditory cortices are more likely to start using cannabis, or that other factors play a role. Furthermore, the study relied on self-reported use and did not account for the potency of the cannabis or how recently it was used. However, the consistency of the findings across a large, well-matched sample offers a compelling glimpse into the neurobiology of psychosis risk. The study suggests that even in people without a diagnosed mental illness, cannabis use is linked to measurable changes in the brain's auditory and language networks, changes that mirror those seen in clinical psychosis. This points to a potential biological pathway where the drug may erode the brain's ability to distinguish reality from imagination, a process that could be exacerbated by age and repeated use.