Tucked into the lateral surface of the parietal lobe is a prominent groove known as the intraparietal sulcus (IPS). The IPS is not a single unit but a collection of subregions acting as a hub. It sits between brain areas that process sensory information, like vision and touch, and those that control motor actions. This position allows the IPS to integrate these information streams, helping us perceive and interact with the world.
The Brain’s Number Center
One of the most studied functions of the intraparietal sulcus is its role in understanding numbers. This region is fundamental to our intuitive “number sense,” an ability to approximate quantities without consciously counting. For instance, when glancing at a basket of apples, the IPS allows for a rapid estimation of how many are there.
The IPS is also engaged during formal mathematical tasks. Functional imaging studies show activation in this area when people compare numerical magnitudes, such as determining if 8 is greater than 6, or when performing arithmetic calculations. The left IPS is more active during tasks involving learned mathematical symbols, while the right IPS is more involved in non-symbolic tasks like estimating.
The IPS is a core component of the brain’s numerical processing network. Neurons within the IPS show tuned responses to specific quantities, reacting more strongly to a preferred number. This neural behavior highlights its role in representing the concept of “how many,” bridging the gap between seeing objects and understanding their numerical value.
Directing Spatial Attention and Action
The intraparietal sulcus is instrumental in how we perceive and navigate our physical environment. It helps create a “salience map,” a representation of our surroundings that highlights objects of interest. This internal map guides our attention, directing our eyes toward specific items in a cluttered visual scene.
The IPS is also involved in visuomotor coordination, which is the integration of visual information with motor commands to guide actions. When you reach out to pick up a cup, the IPS calculates the object’s position relative to your body and guides your hand toward it. This function is necessary for actions like catching a ball or using a fork.
Different parts of the sulcus contribute to this process. The anterior intraparietal area (AIP), for example, is associated with shaping the hand for grasping based on an object’s size and shape. Other subregions are more involved in planning the reach itself or coordinating eye and hand movements.
Holding Visual Information in Mind
The intraparietal sulcus also contributes to visuospatial working memory. This system is a temporary mental “scratchpad” for holding and manipulating visual and spatial information for short periods. It is the process used to remember the layout of a room you just left or to visualize the steps of a route before driving.
The IPS helps maintain this visual information by sustaining neural activity related to the remembered items. For example, if you are asked to remember the location of a dot on a screen after it disappears, regions within the IPS remain active during the delay period. This capacity is also used for more complex cognitive tasks, such as mentally rotating an object.
The IPS plays a role in this process by acting as an internal attention system that can focus on stored representations. While there is some evidence for specialization, with the right IPS more engaged for visuospatial tasks, the overall pattern indicates a bilateral involvement. The superior part of the IPS, in particular, helps control the precision of our mental representations.
Impact of Intraparietal Sulcus Damage
Damage or atypical function in the intraparietal sulcus has consequences that reflect its roles. Difficulties with mathematics, known as developmental dyscalculia, are associated with dysfunctions in the IPS. Individuals with this condition often show reduced gray matter or atypical activation patterns in this region during numerical tasks. This can manifest as trouble with estimating quantities, comparing numbers, and learning arithmetic.
Damage to the IPS, often from a stroke, can also lead to difficulties with spatial awareness and action. One such condition is optic ataxia, an inability to accurately reach for objects under visual guidance. A person with optic ataxia can see an object but cannot coordinate their hand movement to grasp it.
In cases of bilateral damage to the parieto-occipital regions, which include the IPS, a more complex disorder called Bálint’s syndrome can emerge. This syndrome includes optic ataxia along with simultanagnosia, an inability to perceive more than one object at a time. A person with simultanagnosia might see a single tree but be unable to recognize the entire forest.