Hemianopsia is a condition that causes you lose the ability to see half of your visual field. When the outer or peripheral vision is missing, the condition is called bitemporal hemianopsia.
When you view the world, you may think that you are seeing one large image. But in the structure of your eye, you are actually seeing thousands of tiny pieces of the image which are assembled by the brain into the broad image that you perceive. The total of what you see is called your visual field. It includes what you are looking directly at (central vision) and all the things you can see around the edges (peripheral vision).
In normal vision, each eye sees a part of the picture with some overlap in the middle. The brain assembles the image to create the complete view. You can illustrate this for yourself by holding your hands to your eyes to simulate looking through binoculars. You see two circles of whatever is in front of you. Now cup your hands against your face so you are blocking the outer half of both eyes. You will be able to see only part of what you could see before. This is the effect bitemporal henianopsia has on normal vision. A person with this condition can see what is directly in front of her, but does not see anything on the outer edges, or in the peripheral vision.
How the optic nerve works
To understand why bitemporal hemianopsia happens, you need to understand how images get from your eyes to your brain. Inside the eye, light strikes the retina which converts the image to a signal that can be sent through the optic nerve to the brain. The optic nerve is made up of many strands that keep track of what part of the retina is sending each tiny piece of the image. This allows the brain to reconstruct the image with all the pieces in the correct order.
The optic nerve for the each eye also keeps track of what the eye sees on the inside (nasal) half and the outside (temporal) half of the visual field. You can imagine the optic nerve as four strands – two for each eye. These four strands come together at an intersection known as the optic chiasm.