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Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision. "Far peripheral" vision exists at the edges of the field of view, "mid-peripheral" vision exists in the middle of the field of view, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze.
Peripheral vision is weaker in humans, compared with other animals, especially at distinguishing color and shape. This is because receptor cells on the retina are greater at the center and lowest at the edges (see visual system for an explanation of these concepts). In addition, there are two types of receptor cells, rod cells and cone cells; rod cells are unable to distinguish color and are predominant at the periphery, while cone cells are concentrated mostly in the center of the retina, the fovea.
Peripheral vision is hard to study in an objective manner, because there is no way to separate the visual detection of the eye from the neural processing of the brain. While the eye can be dissected and examined under a microscope, even if the entirety of the retina is capable of detecting light, that capacity may not be fully utilized or may not be consciously aware within the brain. Certain conditions such as lazy eye can cause suppression of an otherwise usable visual field, while stroke or damage to the corpus callosum can prevent left/right integration.
It is not possible to directly observe what the brain is detecting and comprehending, so research primarily involves perception tests based on reactions of test subjects to simulated stimuli. This testing is commonly carried out by requesting test subjects to focus on an object in front of them and then flashing lights at increasing distances away from the center of the visual field, noting the subject's reactions.
Central vision is relatively weak at night or in the dark, when the lack of color cues and lighting makes cone cells far less useful. Rod cells, which are concentrated further away from the retina, operate better than cone cells in low light. This makes peripheral vision useful for seeing movement at night. In fact, pilots are taught to use peripheral vision to scan for aircraft at night.
The distinctions between foveal (sometimes also called central) and peripheral vision are reflected in subtle physiological and anatomical differences in the visual cortex. Different visual areas contribute to the processing of visual information coming from different parts of the visual field, and a complex of visual areas located along the banks of the interhemispheric fissure (a deep groove that separates the two brain hemispheres) has been linked to peripheral vision. It has been suggested that these areas are important for fast reactions to visual stimuli in the periphery, and monitoring body position relative to gravity.
Peripheral vision can be practiced; for example, jugglers that regularly locate and catch objects in their peripheral vision have improved abilities. Jugglers focus on a defined point in mid-air, so almost all of the information necessary for successful catches is perceived in the near-peripheral region.
The main functions of peripheral vision are: