The occipital lobe merges anteriorly with the parietal and temporal lobes.
On the medial surface the calcarine sulcus extends forwards and the parieto-occipital sulcus separates occipital and parietal lobes.
OCCIPITAL LOBE FUNCTION
The occipital lobe is concerned with the perception of vision (the visual cortex).
The visual cortex lies along the banks of the calcarine sulcus - this area is referred to as the STRIATE cortex:-----
above and below this lies the PARASTRIATE cortex. —1
Splenium of corpus callosum Medial surface
The striate cortex is the primary visual cortex and when stimulated by visual input relays information to the parastriate - association visual cortex. This, in turn, connects with the parietal, temporal and frontal lobes both on the same side and on the opposite side (through the posterior part of the corpus callosum) so that the meaning of a visual image may be interpreted, remembered, etc.
The visual field is represented upon the cortex in a specific manner (page 136).
IMPAIRMENT OF OCCIPITAL LOBE FUNCTION
A cortical lesion will result in a homonymous hemianopia with or without involvement of the macula, depending on the posterior extent of the lesion.
When only the occipital pole is affected, a central hemianopic field defect involving the macula occurs with a normal peripheral field of vision.
Extensive bilateral cortical lesions of the striate cortex will result in cortical BLINDNESS. In this, the pupillary light reflex is normal despite the absence of conscious perception of the presence of illumination (light reflex fibres terminate in the midbrain).
Involvement of both the striate and the parastriate cortices affects the interpretation of vision. The patient is unaware of his visual loss and denies its presence. This denial in the presence of obvious blindness characterises Anton's syndrome.
Cortical blindness occurs mainly in vascular disease (posterior cerebral artery), but also following hypoxia and hypertensive encephalopathy or after surviving tentorial herniation.
Inability to direct voluntary gaze, associated with visual agnosia (loss of visual recognition) due to bilateral parieto-occipital lesions.
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