The Posterior Parietal Cortex (PPC) is a highly interconnected brain region that integrates various types of sensory information. It plays a role in understanding our surroundings and interacting with the world. The PPC processes what we see, hear, and feel, using this information to guide movements and actions, which is important for daily navigation and coordinated tasks.
Locating the Posterior Parietal Cortex
The Posterior Parietal Cortex is situated in the upper-back portion of the brain, within the parietal lobe. This region lies behind the central sulcus and is positioned between the visual cortex and the somatosensory cortex, which processes touch and body sensation.
The PPC is present in both the left and right hemispheres of the brain, with each side contributing to different aspects of cognitive processing. Anatomically, the intraparietal sulcus divides the PPC into two main sections: the dorsal superior parietal lobule and the ventral inferior parietal lobule. The inferior parietal lobule is further divided into structures such as the supramarginal gyrus and the angular gyrus.
Key Roles in Cognition and Action
The Posterior Parietal Cortex is involved in functions that bridge sensory input and motor output. It receives information from the visual, auditory, and somatosensory systems, combining these inputs to create a comprehensive understanding of our environment and our body’s position within it. This integration allows for effective interactions with the world.
Spatial Awareness and Navigation
The PPC plays a role in spatial reasoning, helping us understand where objects are relative to ourselves. It helps us perceive and interpret the spatial relationships of objects, determining distances, directions, and our own location. For example, when walking through a crowded room, your PPC helps you avoid bumping into furniture or other people by continuously updating your position and the position of obstacles around you.
This region is also involved in navigation, allowing us to form mental maps and find our way around familiar or new places. This function allows you to remember the layout of your home or navigate a city street without getting lost.
Attention
The PPC is involved in directing and shifting attention, particularly spatial attention. It helps filter relevant sensory information from distractions, allowing us to focus on what matters in our environment. For instance, if you are searching for your keys on a cluttered desk, your PPC helps you selectively attend to the visual features of the keys while ignoring other items.
Different parts of the PPC contribute to different aspects of attention; the temporoparietal junction, for example, is involved in stimulus-driven attention, which is when something unexpected grabs your focus. Meanwhile, the superior parietal lobule shows activation when you intentionally shift your attention. Maintaining sustained spatial attention, like focusing on a specific spot, is associated with the right PPC.
Motor Planning and Execution
The PPC plays a role in planning and guiding movements, especially those that are visually guided. It helps convert visual information into motor commands, a process known as visuomotor transformation. This allows for accurate reaching and grasping movements, such as picking up a cup or hitting a target.
Subregions within the PPC are specialized for planning different types of movements, including eye movements (saccades), reaching movements, and grasping movements. For example, the medial intraparietal area (MIP) and anterior intraparietal area (AIP) are involved in reaching and grasping, respectively. This planning ensures that your hand moves precisely to grab an object, adjusting for its size and orientation.
Sensory Integration
The PPC integrates information from multiple senses, including vision, touch, and hearing. This integration allows the brain to create a unified and coherent perception of the world. For example, when you hear a car approaching and simultaneously see it, your PPC combines these auditory and visual signals to form a complete understanding of the car’s presence and movement.
This multisensory integration is also involved in understanding the relationship between our body and external objects. It helps us process how our body interacts with our surroundings, such as understanding the texture of an object through touch while simultaneously seeing its shape. The PPC also integrates proprioceptive and vestibular signals, which provide information about body position and balance, contributing to a comprehensive sense of self in space.
When the Posterior Parietal Cortex is Impaired
Damage or dysfunction in the Posterior Parietal Cortex can lead to a range of sensorimotor and cognitive difficulties, affecting daily activities. The specific symptoms depend on the location and extent of the damage within the PPC.
A common outcome of PPC damage is spatial neglect, also known as hemispatial neglect. Individuals with this condition fail to notice or respond to stimuli on one side of space, typically the side opposite to the brain lesion. For example, a person with right-sided PPC damage might ignore food on the left side of their plate or only shave one side of their face. This deficit is more common and severe after damage to the right hemisphere, particularly involving the posterior-inferior parietal cortex.
Another impairment is apraxia, which involves difficulty with skilled movements despite having the physical ability to perform them. This can manifest as problems with drawing or building objects, sometimes referred to as constructional apraxia. For instance, a person might struggle to button a shirt or use a fork correctly, even though their muscles are capable of the movements.
Damage to the left hemisphere’s posterior parietal lobule, specifically the angular gyrus, can result in Gerstmann Syndrome. This syndrome is characterized by a combination of four distinct symptoms: agraphia (difficulty writing), acalculia (problems with mathematical calculations), finger agnosia (inability to identify or name one’s own fingers), and left-right disorientation. A person with Gerstmann Syndrome might struggle to write a grocery list, perform simple addition, or tell their left hand from their right.