What is Parietal Lobe ?
Parietal lobe is a portion of the brain that is situated behind the frontal lobe and above the occipital lobe. The word has its origins in a Latin word ‘paries’ (meaning wall) as it overlays the parietal bone. It joins together the messages of senses from various modalities especially determination of direction and spatial sense.
Structure of Parietal Lobe
The structure of parietal lobe can be segregated into 3 different parts:
- The Central Sulcus: The central sulcus divides the parietal lobe from the frontal lobe
- The Parieto-Occipital Sulcus: It divides the parietal and occipital lobes and
- The Lateral Sulcus: It separates parietal from the temporal lobe.
There are two hemispheres in the parietal lobe, which is divided by medial longitudinal fissure.
Right behind the central sulcus and the front portion of the parietal lobe is the post central gyrus, the secondary somatosensory cortical region. Separating this and the back part of parietal cortex is the post central sulcus. The back portion of parietal cortex has further divisions. They are:
- Superior parietal lobe and
- Inferior parietal lobe.
Both the above are divided by the intraparietal sulcus. Intraparietal sulcus along with gyri is important for control of eyes and limbs movements and on the basis of cytoarchitectural and functional distinctions it is subdivided into:
- Medial i.e. MIP
- Lateral i.e. LIP
- Ventral i.e. VIP and
- Anterior i.e. AIP
Functions of Parietal Lobe
The parietal performs vital functions in joining the sensory information from different sections of the body, knowing numerical facts and its relation, and in the handling of items.
Parts of parietal lobe involve visuospatial processing. Posterior parietal cortex is usually known as the ‘dorsal stream of vision’. Dorsal stream of vision is known in two ways namely:
- “Where” stream, which is the same as spatial vision and
- ”How” stream, like in vision for action.
First the posterior parietal cortex gets visuals input and next with the help of motor signals the movements of hands, arms and eyes are controlled.
The subdivisions and its functions of intraparietal sulcus are as:
- Medial intraparietal: The neurons of this part helps in encoding the place of a reach target in nasal centered coordinates.
- Lateral intraparietal: It has neurons which represent the saliency of spatial places and concentration to the same places. Oculomotor system uses it to target eye movements as and when appropriate.
- Anterior intraparietal: The neurons in this part respond to the shapes, sizes and direction of articles that are taken hold of. The neurons of anterior intraperietal help in grabbing and manipulation of objects with the help of motor and visual inputs. When anterior intraparietal and ventral premotor work together it results in visuomotor transformations for movements of the hands.
- Ventral intraparietal: Sensory information is received from various senses like vestibular, auditory, visual, somatosensory, by the ventral intraparietal part.
The parietal lobe has two divisions namely:
- Right hemisphere: Right hemisphere is vital for mostly left-handers and is focused to complete images and understand maps, i.e., it is important to spatial relationship. If any injury occurs to this part of the lobe then it leads to loss of descriptions and visualization of spatial relationship.
- Left hemisphere: Left hemisphere is vital for right-handers and involves symbolic function in mathematics and languages. Injury to the left hemisphere leads to difficulty to understand symbols, mathematics, and read and write over a prolonged period of time.
The parietal association cortex allows people to solve mathematical problems, read and write. The sensory inputs move from the right side of the body to the left side of the brain and then from left side to the right side.
There are many parts of the parietal lobe which are vital for language processing. Right posterior to the central sulcus is the post central gyrus. Somatosensation takes place due to this post central gyrus. This part gets inputs from the somatosensory passed on by the thalamus and corresponds to the information about limbs position, sense of temperature, pain and touch. The remaining part of the parietal lobe which is posterior to the post central sulcus is segregated into 2 parts. They are:
- Upper parts: It is known as the ‘superior parietal lobule’ and
- Lower parts: It is known as the ‘inferior parietal lobule’. It has linguistic significance.
Parietal Lobe Damage
Following are some ways to discern whether a particular person has any faults and what the person is suffering from:
- When a person is unable to read, this is known as ‘Alexia’. Injuries in the angular gyrus of the inferior parietal lobe could be responsible for such inabilities.
- When a person is not able to know and identify parts of the body, this is called as ‘Asomatognosia’
- A person can be asked to do simple calculations on paper. If the person is not able to do it the person has ‘Acalculia’.