Supramarginal Gyrus | How Does Your Brain Process Language and Empathy?

The Supramarginal Gyrus (SMG), corresponding to Brodmann area 40, is a critical region located in the parietal lobe of the cerebral cortex. It wraps around the posterior end of the Sylvian fissure, a prominent landmark dividing the frontal and parietal lobes from the temporal lobe. The SMG is a key part of the inferior parietal lobule, working in close concert with the angular gyrus. Its location is strategic, positioning it as a hub for integrating sensory information. The SMG is an association cortex, meaning it processes and synthesizes inputs from multiple senses, including auditory, visual, and somatosensory (touch) information. Its primary roles are multifaceted. In the dominant hemisphere (typically the left for most right-handed individuals), it is indispensable for language comprehension, particularly phonological processing—the ability to understand and manipulate the sounds of language. It helps us decode written words by linking them to their corresponding sounds. Beyond language, the SMG is involved in spatial awareness, perception of limb position, and the identification of gestures and facial expressions. This integration of diverse information allows the SMG to support complex cognitive functions that are fundamental to human interaction and learning.

Phonological processing is the cognitive mechanism that allows us to handle the sound structure of language. The Supramarginal Gyrus is central to this function. When we read a word, the SMG helps to translate the written symbols (graphemes) into their corresponding sound units (phonemes). This process is often described as "sounding out" words and is crucial for both reading and verbal repetition. The SMG also plays a vital role in the phonological loop, a component of working memory. This neural circuit allows us to temporarily store and rehearse verbal information, which is essential for learning new vocabulary, understanding long sentences, and producing fluent speech. The SMG acts as a temporary buffer for these sounds, holding them in memory long enough for the brain's language centers, like Wernicke's area, to process their meaning. Without the SMG's contribution, our ability to read, speak, and learn language would be significantly impaired, as the fundamental link between written text and spoken sound would be broken.

Damage to the Supramarginal Gyrus, particularly in the dominant left hemisphere, results in specific and significant neurological deficits. One of the most common outcomes is a form of receptive aphasia, a language disorder where an individual struggles to comprehend spoken or written words. They may hear or see the words perfectly but cannot process their meaning. Another condition associated with SMG damage is Gerstmann's syndrome, characterized by a collection of symptoms including agraphia (the inability to write), acalculia (difficulty with arithmetic), finger agnosia (inability to identify one's own fingers), and left-right disorientation. Furthermore, damage can lead to ideomotor apraxia, a disorder where a person is unable to perform a commanded action (e.g., "pretend to brush your teeth") despite understanding the command and having the physical ability to perform the movement. This demonstrates the SMG's role in translating verbal commands into appropriate motor actions.

The Supramarginal Gyrus is a higher-order processing center for somatosensory information, which relates to the sense of touch, temperature, pain, and body position. While the primary somatosensory cortex receives the raw sensory signals from the body, the SMG integrates and interprets this information to create a coherent perception. It is essential for stereognosis, the ability to identify an object by touch without looking at it. When you reach into a bag and identify your keys by feel, you are using your SMG. It combines tactile information about shape, texture, and size with stored memories to facilitate recognition. Additionally, it contributes to proprioception—our sense of where our body parts are in space. The SMG helps build a mental body schema, allowing for coordinated and precise movements based on tactile feedback.

Yes, the Supramarginal Gyrus, especially in the right hemisphere, is a critical component of the neural network for empathy. Empathy requires not only feeling what another person feels but also understanding their perspective. The SMG's primary role in this process is to help us overcome our own egocentric bias. It allows us to mentally step outside of our own point of view and infer the thoughts, feelings, and intentions of others. Neuroimaging studies show that the right SMG is activated when individuals are asked to take another person's perspective. It works with other brain regions, such as the temporoparietal junction (TPJ) and the precuneus, to construct a model of another person's mind. When the SMG is underactive or inhibited, individuals tend to project their own feelings and beliefs onto others, reducing their empathetic accuracy. Therefore, the SMG does not generate the emotional component of empathy itself, but it provides the crucial cognitive ability to differentiate between our own emotional state and that of someone else, which is the foundation of true empathetic understanding.