Visual Field testing is one way to measure your central or peripheral (side) vision. Your head is held still on a head rest in a device shaped like a large bowl. An eye patch is placed over one of your eyes. You will be asked to look straight ahead and to press a button when you see a light. A spot of light is presented in different intensities in different locations to determine your ability to detect the light spot. The shape and size of blind spots and other defects can help reveal the characteristics and severity of your disease. The resulting data is used to determine the location and severity of vision loss associated with various eye diseases. Visual defects can be caused by disorders of the optic nerve, retina, or visual centers in the brain. The visual field test can be repeated over time to determine changes in your disease and to help to make treatment decisions.