Altered action-related modulation of visual size perception in schizophrenia: Associations with avolition and exploratory effects of supramarginal gyrus stimulation
This study demonstrates that individuals with schizophrenia exhibit altered action-related modulation of visual size perception compared to healthy controls, a deficit that correlates with avolition and is preliminarily linked to the function of the left supramarginal gyrus.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The human brain does not simply record the world like a camera; it actively constructs our experience of reality by blending what we see with what we know about how to move and act. This constant, invisible integration of sight and action is known as affordance. When you look at a cup, you do not just see a cylindrical shape; your brain instantly recognizes it as something you can grasp, lift, and drink from. This connection between an object's physical properties and your ability to use it shapes how you perceive the world around you. In most people, this process happens automatically and seamlessly, influencing even basic judgments like how big or small an object appears. However, in schizophrenia, a complex mental health condition, the link between perception and action can become disrupted. This breakdown may help explain why some individuals struggle with goal-directed behavior or feel a lack of motivation, symptoms known as avolition. Understanding how the brain combines the idea of "what an object is" with "what I can do with it" offers a window into the mechanics of these symptoms and the neural circuits that might be failing.
A team of researchers at Shenzhen Kangning Hospital and other institutions set out to test whether this specific type of action-based perception is altered in people with schizophrenia. They focused on a classic visual trick called the Ebbinghaus illusion, where a central circle surrounded by larger circles looks smaller than the same circle surrounded by smaller circles. The researchers wanted to see if the identity of the surrounding objects could change how big the central circle appeared, specifically if the brain recognized those objects as tools for a specific action. They used two types of tools: a table-tennis paddle and a wooden hammer. The logic was that a paddle is naturally paired with a ball, just as a hammer is paired with a nail. If the brain automatically processes these relationships, seeing a paddle next to a ball should trigger a different perceptual response than seeing a hammer next to a ball. To test this, they showed twenty-seven patients with schizophrenia and twenty-seven healthy volunteers a central ping-pong ball surrounded by four paddles or four hammers. The handles of these tools faced either left or right, adding another layer of complexity to see if the direction of the handle influenced the perception of size.
The results revealed a clear split between the two groups. In the healthy volunteers, the brain automatically used the action relationship to adjust perception. When the central ball was surrounded by paddles, the healthy participants perceived the ball as smaller than when it was surrounded by hammers. This suggests that their brains recognized the paddle-ball pairing as a strong action unit, which subtly shrank the perceived size of the ball. In contrast, the patients with schizophrenia did not show this effect. Their perception of the ball's size remained the same regardless of whether it was surrounded by paddles or hammers. This indicates that the automatic integration of paired-object action information into visual processing is impaired in schizophrenia. The study also found that the severity of a patient's avolition, or lack of motivation, was directly linked to this deficit. Patients who scored higher on measures of avolition showed a weaker difference in how they perceived the ball with paddles versus hammers. The more severe their lack of motivation, the less their brain seemed to use the action relationship to alter their visual experience.
To explore the biological mechanism behind this finding, the researchers conducted a separate experiment with sixteen healthy participants using a technique called transcranial magnetic stimulation, or TMS. This method uses magnetic pulses to temporarily and safely disrupt activity in a specific part of the brain. The researchers targeted the left supramarginal gyrus, a region in the parietal lobe known to be involved in tool use and understanding how objects relate to one another. When they stimulated this area, the healthy participants lost the ability to perceive the size difference between the paddle-surrounded and hammer-surrounded balls. The effect vanished, making their perception look much like that of the patients with schizophrenia. This suggests that the left supramarginal gyrus plays a causal role in processing these action-related relationships. Without this region functioning normally, the brain cannot automatically use the context of paired tools to adjust visual size.
These findings suggest that the difficulty some people with schizophrenia face in staying motivated or engaging in goal-directed behavior may be rooted in a fundamental breakdown of how the brain connects objects to actions. It is not just a matter of willpower or a general lack of interest; rather, the brain may fail to automatically generate the sense of "what this object is for" when looking at the world. If the brain cannot seamlessly integrate the idea of a tool and its use, the environment may feel less inviting or actionable, potentially contributing to the withdrawal and apathy seen in the disorder. While the study does not prove that fixing this specific brain region will cure avolition, it points to a specific neural pathway—the left supramarginal gyrus and its connections—that could be a target for future therapies. By understanding that the perception of size and the perception of action are deeply intertwined, researchers can begin to design interventions that help restore the natural flow between seeing the world and knowing how to act within it.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.