The sensory cortex is the brain’s processing center for touch and other related bodily sensations. This region allows us to interpret our physical interactions with the world around us. Through this part of the brain, raw sensory data from our skin, muscles, and joints becomes meaningful perceptions that guide our movements and awareness.
Primary Functions and Location
The primary somatosensory cortex is a strip of brain tissue located in the parietal lobe, on a ridge known as the postcentral gyrus. It sits just behind the central sulcus, a fissure that separates the parietal lobe from the frontal lobe. Each hemisphere of the brain has a somatosensory cortex that processes information from the opposite side of the body. This means that sensations from the right hand, for example, are interpreted by the left cerebral hemisphere.
This area processes tactile sensations such as pressure, vibration, and texture, allowing you to feel the smoothness of a stone or the roughness of tree bark. The sensory cortex also interprets temperature, distinguishing between hot and cold stimuli, which is why you can sense the warmth of a coffee mug.
Another one of its duties is nociception, the perception of pain, which alerts the body to potentially harmful stimuli. It also manages proprioception, the awareness of the position and movement of your body parts. This internal sense is how you can touch your finger to your nose with your eyes closed or walk without constantly looking at your feet.
The Sensory Homunculus
The organization of the sensory cortex is visualized through the sensory homunculus, which translates to “little man.” This is a neurological map of the body stretched across the surface of the postcentral gyrus. The homunculus represents how the brain processes sensory input from different parts of the body. Sensory signals travel from receptors throughout the body, through the spinal cord and thalamus, before arriving at their designated spot on this cortical map.
A feature of the sensory homunculus is its distorted proportions. The size of each body part on the map is not related to its physical size but to the density of its sensory receptors. This means that areas with high tactile sensitivity, like the hands, fingers, lips, and tongue, are represented by large regions on the cortex. We use these parts extensively to interact with our environment, from picking up small objects to discerning the texture of food.
In contrast, body parts with fewer sensory receptors, such as the back, torso, or legs, are allocated much smaller portions of cortical tissue. This disproportionate mapping explains why a paper cut on a fingertip feels more intense than a scrape on the back. The homunculus provides a visual model for understanding why our sensitivity to touch varies across the body’s surface.
Consequences of Damage
Damage to the sensory cortex can have specific consequences for an individual’s ability to perceive their body and the world. This damage can occur from a stroke, a traumatic brain injury, a tumor, or a neurodegenerative disease. An injury to this area can disrupt the brain’s ability to make sense of touch-based signals, even if the nerves in the body are functioning correctly.
One outcome is a condition called astereognosis, also known as tactile agnosia. A person with astereognosis has difficulty recognizing objects by touch alone, without visual cues. They might be able to feel the physical features of an object, like its weight and texture, but cannot integrate that information to identify what it is. For instance, they could hold a key but be unable to name it until they see it.
Other effects include numbness, tingling, or a loss of sensation in the body part corresponding to the damaged cortical area. Another condition is agraphesthesia, the inability to recognize letters or numbers traced onto the skin. These deficits can impact daily life, making tasks that require tactile feedback, such as buttoning a shirt, very difficult. The specific impairment depends on the location and extent of the lesion within the parietal lobe.
Neuroplasticity and Adaptation
The sensory cortex is not a static structure; it possesses a capacity for change and reorganization known as neuroplasticity. This means the brain can adapt based on experiences, injuries, or changes in sensory input. The cortical map of the sensory homunculus can expand or shrink depending on how much a particular body part is used or stimulated. This adaptability shows the brain is a dynamic organ.
A clear example of this is seen in musicians. Studies have shown that individuals who play string instruments have an enlarged cortical representation for the fingers of their left hand, used for intricate fingering. The amount of this cortical expansion often correlates with the age at which the musician began training, suggesting that long-term practice drives these structural changes in the brain.
Neuroplasticity is also evident in cases of amputation. After a person loses a limb, the area of the sensory cortex once devoted to that limb does not remain silent. Instead, it can be taken over by adjacent cortical regions.
This remapping is thought to be responsible for phantom limb sensation, where an individual feels sensations, including pain, in a limb that is no longer there. For example, the cortical area for the hand is next to the area for the face. In some amputees, touching the face can trigger a sensation in the phantom hand.