Vision is a complicated sensory ability that starts at the eye but ends in the brain. Disturbances anywhere along the line can cause visual disturbances. Ophthalmologist and optometrists treat the vast majority of these conditions, many we have heard of including refractive problems (the need for eyeglasses), cataracts, glaucoma, macular degeneration, as well as a host of other conditions. There is, however, a number of less common conditions that affect vision but where the problem lies in the brain. These have some unusual and sometimes bizarre symptoms and often cannot be well treated per se, but the discovery of the cause can lead to prevention for further visual deterioration.
First, a short anatomy lesson. Light enters the eye through the opening called the pupil, is focused by the lens and then hits the retina. The retina is the back of the eye where the light sensing cells are. The retina is actually a very complicated 10-layered structure but one layer called ganglion cells send out their fibers and merge together to form the optic nerve which then passes into the brain itself. Although the front of the optic nerve can be seen by looking through the eye with a simple device, it is actually a nerve tract of the brain. This optic nerve then passes backward, makes some detours, crosses and uncrosses and finally ends up in the very back portion of the brain called the occipital lobes. As these nerve fibers pass through the brain, they receive and give off branches from other structures and damage to these, although not directly in the visual pathway, can produce a number of unusual symptoms. A lesion however in the occipital lobe can cause a visual defect as profound as damage to any structure in the eyeball itself. Some of these conditions also have loss of other abilities such as reading, writing or recognizing objects.
Alexia is the inability to read and usually means the loss of the previous ability to read. Sometimes this is called pure word blindness. While inability to read can be caused by many things, if these have been ruled out (such as macular degeneration or cataracts) then a brain lesion is possible and this is usually in the left occipital lobe. The lesion in or near the comparable right occipital lobe can produce a condition called prosopagnosia. Here a person cannot recognize faces unless clear given clues such as a prominent facial feature or voice. They can, however, see and recognize other objects………
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Source: Ravalli Republic
Image: Bio Science Technology