Tests for detail vision and contrast vision test the foveal area only, the center of gaze where we are looking. We have surprisingly low visual acuity in parts of the visual field that are not at the center of gaze; vision off to the side is coarser and blurrier, colors are less vivid and you don’t see details very well. But you normally remain unaware of this because we instinctively direct our center of gaze to what we we are looking at.
Your visual field is limited; you don’t see 360 degrees. You don’t have “eyes in the back of your head.” You can see the edges of your visual field by focusing your sight on a point in front of you and then moving a hand, wiggling a finger, further and further to one side. If you keep your focus on the point in front of you, you will see your finger disappear outside the border of your visual field. The human visual field is usually a little more than 180 degrees, or 90 degrees on each side from your central fixation gaze.
While central (foveal) vision allows you to see the fine details of objects you are looking at directly, peripheral vision is used to find objects and detect threats and is important for general orientation and balance. When you move through your environment and around obstacles, go up or down stairs, reach for something, ride a bicycle, or do any physical activity, you’re using peripheral vision.
When your vision becomes diminished on one side, you may find yourself bumping into door frames or furniture, and driving can be dangerous when you cannot see traffic in the next lane or cars entering from side streets.
Peripheral vision is also important for tasks such as reading. If, for example, you have reduced vision on the right side if may be difficult for you to read a line of text, as your ability to quickly see the next word will be diminished. You might not be able to even read long words. With poor vision on the left side you might have difficulty finding the beginning of the next line.
Most of our peripheral vision is subconscious. Driving, walking down the street, eating a meal, reaching for something all use visual information in our visual field that is processed sub-consciously, never reaching consciousness.
Because much of this process is subconscious we are not spontaneously aware of vision loss in our visual field, and this as well makes testing more difficult.
As well, peripheral vision is an essential part of detail vision!, We scan the periphery to find what what we are looking for and then we turn our gaze to it.
Disease processes that affect the retina and/or the optic nerve often lead to decreased or missing vision in localized areas of the visual field. These are called scotomas. While scotomas (or scotomata) are usually referred to as “blind spots,” a scotoma may be an areas of complete visual field loss (when even the brightest stimuli are not seen) or it may be an area of impaired vision or relative vision loss (when dim stimuli are missed, but bright stimuli are seen). These areas or islands of loss or impairment of vision may appear as dark, light, or blurred areas in the field of vision. The visual field may seem to have holes into which objects disappear; one second you see an object and then when you move your eyes it disappears.
Not so obvious
Surely if you have a blind spot in your visual field you would know it, you would see it. In fact every human has a blind spot in each eye, the spot representing the place on the retina where the optic nerve exits the eye, an area with no photoreceptor cells.
Why don’t you see these gaps in your visual field? We see with two eyes, and most of the time the left eye sees what’s happening in the right eye’s blind spot and vice versa. If both blind spots overlap while looking at an object the brain fills in the spot with information from the visual field around the blind spot.