Authors' Note: Lesions in the occipital, or posterior portion of the brain's cerebral hemispheres are notorious for producing visual disruption. Each side of this sensitive area of brain tissue is targeted at integrating one half of the patient's visual field (to left or right). So for example, a tumour in the right side of the occipital lobe interrupts the signals arriving from the nerve fibres in the right side of the retina in both eyes; the patient's ability to see objects in the well-defined semi-circular zone to his left is eliminated in a fashion that is homonomous or congruent - both eyes are affected similarly. The resulting pattern of contralateral loss of visual sensation (homonomous hemianopsia) may be mapped by a test known as perimetry (visual-field analysis).
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