You may not realize that when you use your eyes to see, you are actually seeing in many directions at the same time. What you see directly in front of your eyes is called your central vision. What you see off to the sides is your peripheral vision.
How the eye works
Light enters the eye through the pupil and is focused on the back wall of the eyeball. The lining on the inside of the eye, which is called the retina, is full of special light receptor cells called rods and cones. These cells translate the light image into electrical signals which travel through the optic nerve to the brain, where they are perceived as vision. The rods and cones are concentrated in the center of the back of the eye, with fewer receptors further out. This means your vision is clearest in the center and weaker at the edges. Central vision is used for seeing detail, while peripheral vision is more effective at seeing motion.
Why peripheral vision is important
When we are concentrating on something, we focus on what is directly in front of us, such as the computer monitor or a book we are reading. But our eyes are constantly aware of other things going on around us, such as someone approaching from the side. This is especially important during activities such as driving a car or playing sports. A driver may catch a hint of motion out of the corner of her eye that alerts her to a pedestrian stepping off the curb. Or a basketball player may pass the ball to a teammate coming up on the side that he saw using peripheral vision. People with limited peripheral vision lack this ability to see to the side and are forced to turn their eyes, or even their entire heads to see what is not directly in front of them.
Problems with peripheral vision
Your eye doctor may refer to your peripheral vision as part of your visual field. The visual field is the entire area to the sides in which you can see things when you are looking at a focal point directly in front of you. The doctor may check your visual field by having you tell him what you see beside you while you are looking straight ahead.