Occipital lobe
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The occipital lobes are the visual processing center of mammalian brains. The primary visual area is Brodmann area 17, located in the interior portion of the occipital lobe at the calcarine sulcus and sometimes continuing onto the surface of the lobe. The lobes' Peristriate regions are the sites for center visuospatial processing, for color discrimination and for discrimination of movement.
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Anatomy
The occipital lobes are the smallest of four true lobes in the human brain. Located in the rearmost portion of the skull, the occipital lobes are part of the forebrain structure.
The lobes rest on the tentorium cerebelli, a process of dura mater that separates the cerebrum from the cerebellum. They are structurally isolated in their respective cerebral hemispheres by the separation of the cerebral fissure. The front edge of the occipital lobe is separated from the parietal lobe by the Parieto-occipital sulcus. The sides of the lobe merge with the parietal lobes along a vague boundary defined by several lateral occipital gyri, which are separated by lateral occipital sulcus.
The occipital aspects along the inside face of each hemisphere are divided by the calacarine sulcus. Above the medial, Y-shaped sulcus lies the cuneus, and the area below the sulcus is the lingual gyrus.
Function
Retinal sensors convey stimuli through the optic tracts to the lateral geniculate bodies, where optic radiations continue to the visual cortex. Each visual cortex receives raw sensory information from the outside half of the retina on the same side of the head and from the inside half of the retina on the other side of the head.
Cells on the posterior aspect of the occipital lobes' gray matter are arranged as a spatial map of the retinal field. Functional neuroimaging reveals similar patterns of response in cortical tissue of the lobes when the retinal fields are exposed to a strong pattern.
If one occipital lobe is damaged, the result can be homonomous vision loss from similarly positioned "field cuts" in each eye. Occipital lesions can cause visual hallucinations. Lesions in the parietal-temporal-occipital association area are associated with color agnosia, movement agnosia, agraphia and alexia.
Functional Anatomy
The occipital lobe is divided into several functional visual areas. Each visual area contains a full map of the visual world. Although there are no anatomical markers distinguishing these areas (except for the prominent striations in the striate cortex), physiologists have used electrode recordings to divide the cortex into different functional regions.
The first functional area is the primary visual cortex. It contains a low-level description of the local orientation, spatial-frequency and color properties within small receptive fields. Primary visual cortex projects to the occipital areas of the ventral stream (visual area V2 and visual area V4), and the occipital areas of the dorsal stream (visual area V3, visual area MT (V5), and visual area DP.)
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