Parietal lobe

Lobus parietalis

The lobe between the central and parieto-occipital sulci, containing the primary somatosensory cortex (postcentral gyrus) and posterior parietal association cortex for spatial attention, body schema, praxis, reading and calculation.

Overview

Primary somatosensory areaBrodmann 3, 1, 2 (postcentral gyrus)

Angular / supramarginal gyriBrodmann 39 / 40

The parietal lobe lies behind the central sulcus and above the posterior part of the lateral sulcus. It is separated from the occipital lobe medially by the parieto-occipital sulcus and laterally by an imaginary line from that sulcus to the preoccipital notch. Its subdivisions are:
• the postcentral gyrus;
• the superior parietal lobule;
• the inferior parietal lobule, made of the supramarginal gyrus (capping the end of the lateral sulcus) and the angular gyrus (capping the end of the superior temporal sulcus);
• on the medial surface, the posterior paracentral lobule and the precuneus.

The primary somatosensory cortex (areas 3, 1, 2) in the postcentral gyrus receives thalamocortical input from VPL (body) and VPM (face) and is mapped as the sensory homunculus, with the same layout as the motor strip. Each subarea is specialised:
• area 3a: proprioception;
• area 3b: cutaneous touch;
• area 1: texture;
• area 2: size and shape.
Behind it, the somatosensory association cortex (areas 5, 7) builds a sense of objects held in the hand (stereognosis) and of the body in space.

The posterior parietal cortex is the hub of the dorsal "where/how" visual stream. It integrates vision, touch and proprioception to guide reaching and grasping and to direct spatial attention, especially in the right hemisphere. In the dominant hemisphere the angular and supramarginal gyri belong to the language network and support reading, writing, calculation, phonological processing and praxis.

Location

Superior and posterolateral part of each hemisphere, deep to the parietal bone; between the frontal lobe (in front), the occipital lobe (behind) and the temporal lobe (below).

Parts & subdivisions

Primary somatosensory cortex (areas 3, 1, 2)

Postcentral gyrus. Contralateral touch, pressure, vibration, proprioception, pain and temperature; somatotopic sensory homunculus (large lips, tongue, hand).

Somatosensory association cortex (areas 5, 7)

Superior parietal lobule. Stereognosis, body schema, sensorimotor integration for reaching.

Supramarginal gyrus (area 40)

Phonological processing and praxis (dominant); with the angular gyrus, part of the "Geschwind territory" of the language network.

Angular gyrus (area 39)

Reading, writing, calculation, left–right orientation and finger naming (dominant hemisphere).

Posterior parietal cortex

Spatial attention and the dorsal visual stream; strongly right-lateralised for attention to both hemifields.

Precuneus

Medial parietal cortex; visuospatial imagery, episodic memory retrieval, self-referential processing; a hub of the default mode network.

Anatomy

Arterial supply

Middle cerebral artery (lateral surface: postcentral gyrus face/arm area, inferior parietal lobule — inferior division branches). Anterior cerebral artery (medial: leg area of the paracentral lobule, anterior precuneus). Posterior cerebral artery (parieto-occipital branch: posterior precuneus).

Function

Conscious perception and localisation of somatic sensation

Stereognosis and two-point discrimination

Body schema and awareness of limb position

Spatial attention and visuomotor guidance of reaching (dorsal stream)

Reading, writing and calculation (dominant hemisphere)

Praxis: planning skilled learned movements

Clinical significance

Cortical sensory loss

With the primary sensations largely intact, the patient has astereognosis (cannot identify an object by touch), agraphaesthesia (cannot identify numbers traced on the skin), impaired two-point discrimination, poor localisation and sensory extinction to bilateral simultaneous stimuli.

Neglect and Gerstmann syndrome

• Non-dominant (right) parietal lesions cause contralateral hemispatial neglect: the patient ignores the left side of space and body, bisects lines off-centre and draws half a clock. Anosognosia, dressing apraxia and constructional apraxia often accompany it.
• Dominant (left) angular gyrus lesions cause Gerstmann syndrome: finger agnosia, acalculia, agraphia and left–right disorientation, often with alexia.
• Parietal involvement of the upper optic radiation gives a contralateral inferior quadrantanopia ("pie on the floor").