Sight is one of the most important human senses. It gives us the ability to understand the world around us and to navigate within our environment. As we look around, our eyes are constantly stimulated by the light generating an electrical impulse via a unique set of cells situated in the retina. These are known as photoreceptors. The electrical signals coming from retinal corresponding points of both eyes will travel along neurons to the brain. They ultimately end in a region known as the visual cortex located in the occipital lobe of the brain, to give birth to visual perception. As a result of the organization of the visual pathway, the left half of the brain processes visual information from both eyes about the right side of the world. The right side of the brain processes visual information from both eyes about the left side of the world.
1. What is a visual field defect and how can we assess it?
A blind spot in the visual field, or scotoma, within the normal field of one or both eyes, is known as a visual field defect. It can be monocular when it affects the field of one eye only. Or binocular when the defect affects both eyes. Hemianopia is a defect that affects half of a visual field. Therefore, patients with monocular hemianopia are unable to perceive objects to the side of the visual midline. The defective vision may involve either half of the nasal (inner side) or the temporal (outer side) visual field.
Due to the frontal position of our eyes in the orbits, the visual fields of the right and left eye overlap significantly. Therefore, visual field defects may not be evident without specific testing of each eye separately. Perimetry testing can accomplish precise assessment of visual field defects. During this examination, the affected individual is asked to fixate his/her gaze straight ahead. They will then indicate whether they see special tests lights projected on an illuminated domed screen.