The parietal lobe occupies about one quarter of each hemisphere and is involved in two primary functions: 1) sensation and perception and 2) the integration and interpretation of sensory information, primarily with the visual field. Thus, the parietal lobe is responsible for integrating sensory input to form a single perception (cognition) on the one hand, while also forming a spatial coordinate system to represent our world, on the other hand. There is a range of clinical manifestations following injury to parietal lobe, such as an inability for understanding spatial relations. In this article we will discuss the anatomy and function of the parietal lobes, as well as its clinical relevance.
The parietal lobe is located between the frontal and occipital lobe and above the temporal lobe on each cerebral hemisphere. Its borders are as follows:
- Anterior border - formed by the central sulcus (of Rolando)
- Posterior border - formed by the imaginary line extending between the parieto-occipital sulcus (superiorly) and the preoccipital notch (inferiorly).
- Inferior border - formed by the lateral fissure (of Sylvius)
- Superior border - formed by the medial longitudinal fissure that separates the two hemispheres
Running almost parallel with the central sulcus is the post-central sulcus. Both sulci demarcate the post-central gyrus, located about 6.5 cm posterior to the bregma of the skull. Because the marginal sulcus (or ascending band of the cingulate) points directly to the post-central gyrus on the superior surface of the hemisphere, it serves as an important landmark for identifying the gyrus (particularly on MRIs).
The post-central gyrus is usually connected with the pre-central gyrus of the frontal lobe with a tiny, horizontal gyrus at the base of the central sulcus, called the subcentral gyrus. Together, all three of these gyri which surround the central sulcus, are referred to as the central lobe.
Just posterior to the post-central gyrus, the parietal lobe is divided into a superior and inferior parietal lobule with the intraparietal sulcus. This sulcus originates about at the midpoint of the post-central sulcus and extends posteriorly, parallel to the medial longitudinal fissure. The inferior parietal lobule continues to the parieto-temporal intersection, and is formed by the supramarginal gyrus and the angular gyrus. The “U”-shaped supramarginal gyrus surrounds the posterior end of the lateral fissure, while the angular gyrus is found at the posterior tip of the superior temporal sulcus.
On the medial surface of the hemisphere, the parietal lobe forms the posterior part of the paracentral lobule, which is demarcated by the pre-central sulcus anteriorly and the marginal sulcus posteriorly. Just posterior to the paracentral lobule is the precuneus lobule, which extends from the supramarginal sulcus to the parieto-occipital sulcus.
The primary sensory areas, such as the post-central gyrus, of the parietal lobe show a granular-type histology. In these areas the normal 6-layers of the cortex are not evident, as the layers II and IV that contain predominantly sensory granular cells (external and internal granular layer) are much more pronounced, compared to the layers III and V which contain predominantly motor pyramidal cells (external and internal pyramidal layer). The association cortical areas of the parietal lobe, however, demonstrate all 6 cell layers of the cortex.
The lateral surface of the parietal lobe is supplied by the medial cerebral artery (one of the three branches of the internal carotid artery). Another of the internal carotid artery branches is the anterior cerebral artery, which supplies the medial surface of the parietal lobe. The posterior cerebral artery supplies the posterior surface of the medial parietal lobe.
Functional Cortical Mapping
The parietal lobe is involved in the perception of sensation, including touch, temperature, pain and proprioception, as well as in the advanced perception of visual and auditory information.
In general, the parietal lobe is involved in the following functions:
- Sensation of touch (pain, temperature, etc)
- Information processing
- Spatial orientation
- Coordination of movement
- Visual perception
- Computation (math)
Primary Somatosensory Area
Postcentral gyrus forms the primary somatosensory cortex and is referred to as Brodmann area 3, 1, 2. This gyrus receives the sensory information from all the sensory receptors that provide information related to temperature, pain (spinothalamic pathway), vibration, proprioception and fine touch (dorsal column pathway).
The parts of the body are neurologically mapped on the somatosensory cortex. This pictorial, somatotopic representation of the human body on the post-central gyrus, is referred to as the cortical sensory homunculus devised by Wilder Penfield. The sensory map consists of an upside-down representation of the body, running superior-inferiorly along the post-central gyrus. The point-for-point correspondence of the body on the gyrus, results in a grotesque disproportionate figure, with large hands, lips and face, compared to the rest of the body. This is because that areas that are finely controlled or have acute sensation, have larger portions of the somatosensory cortex.
Parietal Association Cortex
The superior parietal lobule forms the association cortex of the parietal lobe, and plays an important role in planned movements, spatial reasoning and attention. The intraparietal sulcus can be further divided into a lateral, medial, ventral and anterior area. The lateral area is responsible for our eye movements in response to a stimulus in space. The medial area helps us to determine how far and where we need to reach in relation to our nose. The ventral area is an area that receives a number of sensory modalities; these include auditory, visual, vestibular and somatosensory information. Finally, the anterior area enables us to interpret the size, shape and position of objects we are about to grasp. The anterior and ventral areas work together to enable visual motor coordination of hand movements.
Sensory Speech Area of Wernicke
Wernicke’s area is important for language development and for comprehension of speech. It functions in language comprehension, semantic processing, language recognition and language interpretation. Wernicke’s area is classically found in the posterior part of the superior temporal gyrus usually in the left cerebral hemisphere (Brodmann area 22), an area which also encircles the auditory cortex. Most neuroscientists also include regions of the inferior parietal lobule, particularly the supramarginal gyrus (Brodmann area 40) and the angular gyrus (Brodmann area 39) in Wernicke’s area. The supramarginal gyrus forms the auditory area of speech, while the angular gyrus, the visual area of speech.
In the optic radiations from the lateral geniculate nucleus of the thalamus, two loops of fibers carry information back to the visual area: Meyer’s loop and the Baum’s loop. Meyer’s loop carries information for the superior part of the visual field, while Baum’s loop carries information from the inferior part of the visual field. Meyer’s loop (superior visual field, inferior retinal field) runs through the temporal lobe. Baum’s loop or parietal optic radiation runs through the parietal lobe to terminate on the upper bank of the calcarine sulcus in the cuneus of the occipital lobe.
Dominant vs non-dominant hemispheres
The parietal lobes control calculation and language on the dominant side, and the sensory visuospatial processing on the non-dominant hemisphere side.
Wernicke’s area (Brodmann area 22) lies in the superior temporal gyrus and overlaps the parieto-temporal junction. This region is responsible for our understanding of speech. Damage to this region will result in a receptive aphasia, which is a fluent form of aphasia. The patient will present with ‘word salad’ i.e. they will be able to form words, but the words will not be in any comprehensible order or syntax. The homologous area of the right cortex, is responsible for our interpretation of body language, and making sense of ambiguous words. Damage to Wernicke’s area may not always result in a receptive aphasia. If the surrounding cortex is intact, and the right corresponding area is intact, there symptoms may be minimal.
This syndrome is usually associated with large bilateral lesions, resulting in deficits in visual attention, as well as motor function. Symptoms include:
- simultanagnosia (the patient isn’t able to interpret to see the whole visual field)
- optic ataxia (the patient isn’t able to move their hands in relation to their visual input)
- optic apraxia (an inability to fixate the eyes).
As Bálint’s syndrome is usually attributed to simultaneous strokes in the same region in both parietal lobes, it is a rare disease. Sudden severe hypotension, which impacts the watershed areas between the parietal and occipital lobes, is most common cause of the bilateral ischaemia. Due to the range of symptoms and manifestations, the condition is often mistaken for blindness related to other disorders.
Parietal Lobe Stroke
Ischaemic strokes are commonly the result of atheroschlerotic emboli. The middle cerebral artery is the largest branch of the internal carotid artery, and a direct continuation of the artery. It is therefore the commonest location of ischaemic strokes. The middle cerebral artery supplies the lateral surface of the parietal lobe (as well as the superior temporal lobe), which is the location of the upper limb and face on the primary somatosensory cortex. Therefore, strokes impacting the middle cerebral artery result in sensory loss of these areas, with sparing of the lower limbs. Motor function of the same areas may also result, as the primary motor cortex is just anterior to the primary somatosensory cortex, and is also supplied in part by the middle cerebral artery.
Stroke in the parietal lobe is associated with various symptoms:
- Hemi spatial neglect is a phenomenon that usually follows damage to the non-dominant parietal lobes (usually the right), usually following a stroke. The patient is still able to see both sides of their visual fields in both eyes, but is not able to interpret the sensory information sent to the brain from one half of the visual field. If a stroke occurs in the right parietal lobe, the patient will ignore the left visual field. If a stroke occurs in the left parietal lobe, the ability of the patient to solve mathematical problems, as well as reading and writing would be impaired.
- The symptom of optic ataxia results in issues with the patient reaching for objects in the contralateral visual field to the affected parietal lobe. Amorphosynthesis is a condition where the patient is unaware of somatic sensations from one side of the body, and is a possible result of parietal lobe stroke.
- If the left lobe is affected, agnosia (a loss of general perception) results. A lesion of the right parietal lobe causes issues with the person’s interpretation of the left side of their visual field, as well as their personal space.
- Apraxia is a disorder of motor control, that usually results from damage to the left parietal lobe.
- Damage to Baum’s loop results in a contralateral lower quadrantanopia, or a ‘pie in the floor’ visual deficit. A lesion of Meyer’s loop results in a contralateral upper quadrantanopia, or ‘pie in the sky’ visual deficit.
This syndrome is related to damage to the inferior parietal lobule in the dominant hemisphere of the brain (usually the left), and is associated with right-left confusion and presents characteristic symptoms, including:
- agraphia (difficulty in writing)
- acalculia (difficulty with math)
- aphasia (language disorders)
- agnosia (difficulty to perceive objects
These symptoms vary in severity between patients. When the supramarginal and/or angular gyri (parts of the inferior parietal lobule) are impacted, the patient’s ability to interpret written or oral language may be impacted.