Peripheral vision is the range of visual perception that lies outside the central stereoscopic vision of the eyes. In a person with normal sight, visual stimuli can be perceived outside the central focus of vision in a field of over 200 degrees from side to side and over 130 degrees from top to bottom even if an object in this area cannot be directly identified by either shape, color or detail. Rapid movement in this field usually results in a reflex of turning the head in the direction of movement to aid in perception and, if the movement is headed towards the eyes, the blink reflex.
The structure of the retina concentrates the cells that perceive detail and color in the central portion (the fovea) while cells that are more sensitive to changes in light levels and movement are concentrated towards the outer range.
Several conditions can affect peripheral vision, either permanently (such as macular degeneration) or temporarily (alcohol or fatigue).
Peripheral vision can be measured either automatically or manually. In both cases, the patient is asked to stare straight ahead and a visual stimulus is moved into the peripheral field. Automatic machines can accurately record the position which results in a stimulus.