Parietal Lobe | Where in the Brain Do We Process Touch?

The primary somatosensory cortex is a critical brain region located in the parietal lobe. To be precise, it occupies the postcentral gyrus. The brain's wrinkled surface, the cerebral cortex, has ridges called gyri and grooves called sulci. The postcentral gyrus is the specific ridge situated immediately posterior to the central sulcus, a major groove that separates the frontal lobe from the parietal lobe. This strategic location allows it to receive sensory information directly from the thalamus, a central hub for sensory data. The cortex is divided into distinct areas known as Brodmann areas, and the primary somatosensory cortex corresponds to areas 3, 1, and 2. Each area processes different aspects of touch. For example, area 3b is concerned with basic touch sensations, while area 1 helps in identifying the texture of an object. This precise anatomical organization ensures that sensory signals from the body are efficiently received, processed, and interpreted, forming the basis of our sense of touch.

The somatosensory cortex is the main receptive area for the sense of touch and other bodily sensations. Its fundamental role is to process afferent somatosensory input and contribute to the integration of sensory and motor signals necessary for skilled movement. This involves several distinct sensory modalities. The first is tactile sensation, which includes pressure, vibration, and texture. The second is thermoception, or the sense of temperature. The third is nociception, the perception of pain. Finally, it processes proprioception, which is the sense of the relative position of one's own parts of the body and strength of effort being employed in movement. By integrating these different streams of information, this cortex allows us to build a detailed and dynamic map of our body in space and its interaction with the environment. This is essential for everything from buttoning a shirt to navigating a crowded room.

The somatosensory cortex is organized in a highly specific manner, often visualized as a "sensory homunculus." This is a distorted representation of the human body, where the size of the body parts is proportional to the amount of cortical space dedicated to processing their sensations. Areas with high tactile acuity, such as the fingertips and lips, have a much larger representation than less sensitive areas like the back or thighs. This topographical map ensures that we have finer sensory discrimination in the parts of our body that interact most with the world. The map is contralateral, meaning the left hemisphere's somatosensory cortex processes signals from the right side of the body, and vice versa.

Damage to the somatosensory cortex, often caused by a stroke or injury to the parietal lobe, leads to specific sensory deficits. One common condition is astereognosis, the inability to identify objects by touch alone, even though basic sensation is intact. A person with this condition could feel the shape, weight, and texture of a key in their hand but would not be able to recognize it as a key without looking. Other deficits can include a loss of proprioception, leading to clumsy and uncoordinated movements, or deficits in localizing pain or temperature. These conditions highlight the cortex's critical role in integrating basic sensations into a coherent and usable perception of reality.

Phantom limb pain is a phenomenon where individuals who have had a limb amputated continue to feel sensations, including intense pain, as if the limb were still there. This experience is a direct result of the brain's neuroplasticity, specifically within the somatosensory cortex. The area of the cortex that was dedicated to the amputated limb is no longer receiving input. Over time, neurons from adjacent cortical regions, such as those representing the face or upper arm, can "invade" this silent area. When these adjacent areas are stimulated, the brain interprets the activity as originating from the missing limb. For example, a touch on the cheek might be perceived as a touch on the phantom hand. This cortical remapping demonstrates that the brain's sensory map is not fixed and can reorganize itself in response to drastic changes in the body, sometimes with distressing consequences like chronic pain.