The parietal lobe is a major region of the brain located beneath the crown of the skull, situated directly behind the frontal lobe and above the temporal lobe. It acts as a sophisticated processing center, integrating raw sensory information into a cohesive understanding of the body and the surrounding environment. This area is responsible for processing touch, temperature, pain, and pressure, while also generating a sense of spatial awareness and orientation. Damage to this structure disrupts the brain’s ability to interpret these sensory inputs, leading to conditions that profoundly affect perception and interaction with the world.
Disruption of Basic Somatosensory Perception
Damage to the anterior parietal lobe, specifically the somatosensory cortex, directly impairs the body’s ability to process fundamental physical sensations. Individuals may experience numbness or diminished sensation on the side of the body opposite the injury, struggling to perceive simple stimuli like light touch, vibration, or the difference between hot and cold. This loss also extends to proprioception, the unconscious awareness of where the limbs are positioned in space without needing to look. Without this sense, a person finds it difficult to execute coordinated movements, such as walking or reaching for an object, without constantly monitoring their body visually.
More complex sensory failures arise when damage affects the higher-order processing centers. One deficit is tactile agnosia, or astereognosis, the inability to identify an object by touch alone. For example, a person can feel a coin in their hand, recognizing its texture and weight, but cannot name it without looking. This shows that while basic sensation is present, the parietal lobe cannot integrate those physical characteristics into a recognizable identity.
Hemispatial Neglect: Loss of Awareness
A profound consequence of parietal lobe damage, particularly in the right hemisphere, is hemispatial neglect. This condition is not a problem with vision, but a severe failure of attention where the individual does not acknowledge anything on the side of space opposite the brain injury. For instance, a person with right parietal damage ignores the entire left side of their world, even though their eyes are physically capable of seeing it.
This failure of awareness manifests in striking ways that affect daily life. A patient may only eat the food placed on the right side of their plate, or only shave or apply makeup to the right side of their face. When asked to draw a clock, they might crowd all twelve numbers onto the right side of the circle, neglecting the left half of the space.
In more severe cases, neglect can extend to the body itself, a condition known as asomatognosia. The patient may fail to recognize a limb, such as their left arm, as belonging to them, sometimes asking whose arm is lying in their bed. The individual is genuinely unaware of their deficit, a lack of insight called anosognosia, which complicates rehabilitation efforts.
Impairment of Higher-Order Cognitive Functions
Damage to the dominant parietal lobe, typically the left hemisphere, often results in the loss of abilities requiring symbolic representation and spatial manipulation. This area is involved in functions like mathematics and language, leading to distinct intellectual deficits. One outcome is Gerstmann Syndrome, characterized by a specific combination of four symptoms:
The four components of this syndrome are:
- Acalculia, a difficulty with mathematical calculations.
- Agraphia, an inability to write despite having intact motor function.
- Finger agnosia, the inability to identify or name individual fingers.
- Right-left disorientation, making it difficult to distinguish between the right and left sides of their own body or the environment.
These difficulties are thought to stem from damage to the angular gyrus, a specific area in the inferior parietal lobule.
Another complex deficit associated with parietal damage is apraxia, a disorder of learned, skilled movement. A person with apraxia cannot perform purposeful actions, such as waving goodbye or miming the use of a tool, even though their muscles are not paralyzed or weak. This failure is a breakdown in the brain’s ability to correctly plan and execute the sequence of motor commands necessary for a familiar task.