Each cerebral hemisphere is generally divided into five lobes: the frontal, parietal, temporal, occipital and insular lobes. Some texts may also include components of the limbic system as the sixth cerebral lobe.
This article will focus on the occipital lobe. The occipital lobe is the smallest lobe of the cerebral hemisphere and only accounts for approximately 18% of the total neocortical volume. It forms the most posterior portion of the brain, with the occipital pole constituting the most caudal point of the occipital lobe and the cerebrum.
The surface of each cerebral lobe is characterized by a system of cerebral eminences or ridges known as gyri. The occipital lobe is no exception. Separating the gyri are valley-like depressions or grooves known as sulci. The gyri and sulci work together to create the signature wave-like ‘brain’ appearance on the surface of each cerebral hemisphere. These cerebral folds function to increase the surface area of the human brain and also aid in defining the boundaries and landmarks of the lobes of the brain.
|Location||Posterior to parietal and temporal lobes of each cerebral hemisphere|
|Boundaries||Imaginary plane in line with parietooccipital sulcus|
|Principal sulci||Medial surface: Calcarine sulcus (anterior and posterior parts)
Lateral surface: Lateral occipital sulcus, intraoccipital sulcus, transverse occipital sulcus, lunate sulcus
||Medial surface: Cuneate gyrus, lingual gyrus,
Lateral surface: Superior gyrus, middle gyrus, inferior occipital gyrus
|Blood supply and drainage||Arteries: Posterior cerebral artery (occipital branches, parietooccipital artery, calcarine artery)
Veins: Superficial and deep cerebral veins
|Functional units||Primary visual cortex (V1): Brodmann area 17
Visual association cortex (V2,V3,V4): Brodmann areas 18 and 19
|Function||Centre for visual processing|
- Sulci and gyri
- Blood supply and drainage of the occipital lobe
- Function of the occipital lobe
- Occipital lobe lesions
The occipital lobe is located posterior to the parietal and temporal lobes and has both medial and lateral surfaces. This posteriorly located lobe lies over the tentorium cerebelli, while its medial surface faces the falx cerebri.
On the medial surface of the occipital lobe, its border is well defined. The prominent parietoocccipital sulcus separates the occipital from the parietal lobe. It traverses the medial surface at an almost vertical trajectory. At its inferior end the sulcus is continuous with the anterior part of the calcarine sulcus. On the opposite end, it extends slightly onto the superolateral surface of each cerebral hemisphere.
The lateral aspect lacks a clear landmark marking the boundaries of the occipital lobe. Instead, an imaginary line extends from the termination of the parietooccipital sulcus superiorly, to the preoccipital notch inferiorly.
The occipital lobe is separated from the temporal lobe by the same imaginary plane in line with the parietooccipital sulcus. However, some text refer to this part of the boundary stretching between the preoccipital notch of the temporal lobe superiorly to the parieto-occipital sulcus inferiorly as the lateral parietotemporal line. Due to its arbitrarily defined boundaries, the occipital lobe assumes a triangular shape. The occipital lobe extends from the occipital pole all the way to the parietooccipital sulcus.
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Sulci and gyri
Located on both the medial and lateral aspects of the occipital lobe are a number of sulci and gyri. The sulci and gyri of the occipital lobe are relatively inconsistent and their descriptions vary across texts, however there are several consistently recognized structures.
Lateral aspect of the occipital lobe
The superolateral aspect of the occipital lobe presents with three notable gyri: the superior, middle and inferior occipital gyri.
The superior occipital gyrus is the only clearly defined gyrus of the lateral aspect of the occipital lobe. It is continuous with the superomedial margin of the cuneus of the medial aspect of the occipital lobe. Superiorly, the superior gyrus is bounded by the parietooccipital sulcus and inferiorly by either the intraoccipital or transverse occipital sulcus.
The middle occipital gyrus is the largest gyrus of the occipital lobe. Because the middle gyrus stretches between the superior and inferior sulci and covers the major part of the lateral surface, it is sometimes termed the lateral occipital gyrus.
The inferior occipital gyrus is indistinct and sometimes forms part of the middle gyrus. The inferior occipital gyrus is located along the inferomedial aspect of the hemisphere. The posterior aspect of the inferior occipital gyrus is continuous with the lingual gyrus of the medial aspect of the occipital lobe. The superior and inferior occipital gyri are typically consistent, however the middle occipital gyrus has been reported to be frequently undefined. The superior, inferior and middle (if present) occipital gyri converge posteriorly to form the most caudal portion of the cerebrum, the occipital pole.
Formed as a continuation of the intraparietal sulcus is the intraoccipital sulcus which extends and terminates at the transverse occipital sulcus. The intraoccipital sulcus separates the superior occipital gyrus from the middle occipital gyrus. The transverse occipital sulcus is located on the posterolateral aspect of the occipital lobe and also extends from the intraparietal sulcus.
Separating the inferior occipital gyrus from the superior, or when present the middle occipital gyrus, is the horizontal lateral occipital sulcus (also known as the inferior occipital sulcus).
Medial aspect of the occipital lobe
The surface anatomy of the medial aspect of the occipital lobe is more consistent and clearly defined. The calcarine sulcus extends from the parietooccipital sulcus to the occipital pole. Separating the calcarine sulcus into anterior and posterior parts is the parietooccipital sulcus which emerges superiorly from the calcarine sulcus. This deep fissure divides the medial occipital lobe into the cuneate gyrus (cuneus) superiorly and the lingual gyrus inferiorly. The posterior aspect of the cuneus lies over the calcarine sulcus. The lingual gyrus extends along the entire length of the cuneus. Cortical tissue on either side of the banks of the calcarine sulcus is known as the striate cortex (also known as the calcarine cortex) and forms the primary visual cortex.
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Blood supply and drainage of the occipital lobe
The occipital lobe receives its main arterial supply from the posterior cerebral artery. Originating as a terminal branch of the basilar artery, the posterior cerebral artery passes laterally and winds around the cerebral peduncle to supply the occipital lobe and visual cortices. The posterior cerebral artery gives off a number of branches which supply specific areas of the occipital lobe.
- Occipital branches of the posterior cerebral artery further divide into medial and lateral occipital arteries to supply the posterolateral surface of the occipital lobe, including the cuneate and lingual gyri.
- The parietooccipital artery originates as one of the terminal branches of the posterior cerebral artery but may arise directly, as a branch of the medial occipital artery. This artery similarly extends to supply the cuneus
- The calcarine artery is the other terminal branch of the posterior cerebral artery. This artery may also arise directly from the medial occipital artery. The calcarine artery supplies the visual cortex, inferior cuneus, and part of the lingual gyrus.
Venous drainage of the occipital lobe is carried out by a number of superficial and deep cerebral veins which drain to adjacent sinuses.
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Function of the occipital lobe
The occipital lobe is the visual processing area of the brain. Brodmann areas 17,18 and 19 are located within the occipital lobe and form the visual cortices. Brodmann area 17 (V1) forms the primary visual cortex while V2,V3,V4, or Brodmann areas 18 and 19 form the visual association cortex.
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Primary visual cortex
The primary visual cortex also known as V1, Brodmann area 17, or the striate cortex, is located on either side of the calcarine sulcus on the medial surface of the occipital lobe and extends into both the cuneus and the lingual gyrus. It functions to receive special sensory input from the eyes via the optic radiations, and is, therefore, responsible for integration and perception of visual information.
The primary visual cortex possesses columns of cells preferentially responding to specific visual stimuli, such as line orientation. The cortex is retinotopically organized with the upper half of the visual field represented in the cortex inferior to the calcarine sulcus and the lower half of the visual field represented in the cortex superior to the calcarine sulcus.
Visual association cortex
The visual association cortex constitutes the remaining regions of the occipital lobe and is also known as the extrastriate visual cortex. This region functions to interpret visual images. Fibres from the visual association cortex form the corticotectal tract which projects to the pretectal area and/or superior colliculus. Located within the visual association cortex of the occipital lobe are the second, third and fourth visual areas.
Second visual area
The second visual area, also known as the secondary visual cortex, V2, or the prestriate cortex, occupies much of Brodmann area 18 and in some cases 19. The secondary visual cortex surrounds the primary visual cortex and receives information from it. The information from the primary visual cortex is sent to the secondary visual cortex (Brodmann areas 18 & 19), before being passed to the third and fourth visual areas to finally reach the inferior temporal cortex (Brodmann areas 20 & 21). The secondary visual area is important for color, motion, and depth perception.
Third visual area
The third visual area, or V3, lies adjacent to the anterior aspect of V2 and is also located within Brodmann area 18. This visual area communicates directly with the secondary visual cortex and is functionally important in the visual processing of motion whilst also functioning to link parietal and temporal processing streams.
Fourth visual area
Visual area four, V4, is located anterior to V3 within Brodmann area 19. It communicates and receives information from the secondary visual cortex. The function of visual area four is to interpret colors, orientation, form and movement. Visual area four actively communicates with the inferior temporal cortex (Brodmann areas 20 & 21) of the temporal lobe.
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Occipital lobe lesions
Lesions to various regions of the occipital lobe may lead to visual impairment, focal seizures and sensory/motor deficit.
Lesions to the primary visual cortex leads to blindness in the corresponding area of the visual field. Bi-lateral lesions to the striate cortex of the occipital lobe may lead to cortical blindness. In some cases cortical blindness may be accompanied by visual anosognosia or Anton’s syndrome which is a neuropsychological disorder where an individual actively denies or is unaware of their blindness.
Injury to the visual association cortex causes impairment in visual interpretation and processing. Bi-lateral lesions to the visual association cortex may result in spared vision but the inability to process and recognise objects. This pathology is known as apperceptive visual agnosia.
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