Occipital Lobe Research Paper

OCCIPITAL LOBE:

The occipital lobe encompasses the posterior portion of the human cerebral cortex and is primarily responsible for vision. The surface area of the human occipital lobe is approximately 12% of the total surface area of the neocortex of the brain. Direct electrical stimulation of the occipital lobe produces visual sensations. Any damage to the occipital lobe results in complete or partial blindness or visual agnosia depending on the location and severity of the damage. Vision begins with the spatial, temporal, and chromatic components of light falling on the photoreceptors of the retina and ends in the perception of the world around us. The occipital lobe contains the bulk of machinery that enables this process. However, our perception

Information from the retina leaves the eye through the optic nerve, which leads to the optical chiasm. Here, the fibers from the nasal side of the fovea in each eye cross over to the opposite side of the brain while the others remain on the same side. The result is that the mapping from external visual fields to the cortex is crossed. Visual information from the left half of the visual field (from both the right and the left eyes) goes to the right half of the brain (right hemisphere), whereas all the information from the right visual field goes to the left hemisphere. The vertical meridian representation of the two hemifields is joined via a large fiber system called the corpus callosum. From the optic chiasm there are two separate pathways that lead to the brain. The smaller one goes to the superior colliculus, a nucleus in the brainstem, which then projects to the thalamic pulvinar nucleus. The larger pathway goes through the lateral geniculate nucleus (LGN) of the thalamus and to the occipital cortex or primary visual cortex.

It forms the walls and lips of calcarine fissure in medial surface of occipital lobe.

There is a definite localization of retinal projections upon visual cortex. In fact, the point to point projection of retina upon visual cortex is well established. The peripheral retinal representation occupies the anterior part of visual cortex. Macular representation occupies the posterior part of visual cortex near occipital pole. [5]

EFFECTS OF LESION AT DIFFERENT LEVELS OF VISUAL PATHWAY:

Injury to any part of optic pathway causes visual defect and the nature of defect depends upon the location and extent of injury. Loss of vision in one visual field is known as anopia. Loss of vision in one half of visual field is called hemianopia. [5]

Effects of Lesion of Visual Cortex:

Lesion of upper or lower part of visual cortex leads to inferior or superior homonymous hemianopia. [6]

Macular sparing:

In all the conditions mentioned above, total blindness does not occur because, the macular vision is not lost. This phenomenon in which the macular vision is retained (unaffected) in conditions of hemianopia is called macular sparing. Macular sparing occurs because of the following