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thumb|Peripheral vision of the human eye

thumb|Field of view of the human eye

Peripheral vision or indirect vision is vision as it occurs outside the point of fixation (i.e., away from the center of gaze) or, when viewed at large angles, in (or out of) the "corner of one's eye". The vast majority of the area in the visual field is included in the notion of peripheral vision. "<dfn title="far peripheral vision">Far peripheral</dfn>" vision refers to the area at the edges of the visual field, "<dfn title="mid-peripheral vision">mid-peripheral</dfn>" vision refers to medium eccentricities, and "<dfn title="near-peripheral vision">near-peripheral</dfn>", sometimes referred to as "<dfn title="para-central vision">para-central</dfn>", vision, exists adjacent to the center of gaze. However, in common usage, peripheral vision may also refer to the area outside a circle 30° in radius or 60° in diameter. In vision-related fields such as physiology, ophthalmology, optometry, or vision science in general, the inner boundaries of peripheral vision are defined more narrowly in terms of one of several anatomical regions of the central retina, in particular the fovea and the macula.

The fovea is a cone-shaped depression in the central retina measuring 1.5&nbsp;mm in diameter, corresponding to 5° of the visual field. The outer boundaries of the fovea are visible under a microscope, or with microscopic imaging technology such as OCT or microscopic MRI. When viewed through the pupil, as in an eye exam (using an ophthalmoscope or retinal photography), only the central portion of the fovea may be visible. Anatomists refer to this as the clinical fovea, and say that it corresponds to the anatomical foveola, a structure with a diameter of 0.35&nbsp;mm corresponding to 1 degree of the visual field. In clinical usage the central part of the fovea is typically referred to simply as the fovea.

In terms of visual acuity, "foveal vision" may be defined as vision using the part of the retina in which a visual acuity of at least 20/20 (6/6 metric or 0.0 LogMAR; internationally 1.0) is attained. This corresponds to using the foveal avascular zone (FAZ) with a diameter of 0.5&nbsp;mm representing 1.5° of the visual field (although often idealized as perfect circles, the central structures of the retina tend to be irregular ovals). Thus, foveal vision may also be defined as the central 1.5–2° of the visual field. Vision within the fovea is generally called central vision, while vision outside of the fovea, or even outside the foveola, is called peripheral, or indirect vision. The parafovea has an outer diameter of 2.5&nbsp;mm representing 8° of the visual field.

The macula, the next larger region of the retina, is defined as having at least two layers of ganglia (bundles of nerves and neurons) and is sometimes taken as defining the boundaries of central vs. peripheral vision (but this is controversial). Estimates of the macula's size differ, its diameter estimated at 6° – 10° (corresponding to 1.7 – 2.9&nbsp;mm), up to 17° of the visual field (5.5&nbsp;mm

A dividing line between near and mid peripheral vision at 30° radius can be based on several features of visual performance.

Visual acuity declines systematically up to 30° eccentricity: At 2°, acuity is half the foveal value, at 4° one-third, at 6° one-fourth etc. At 30°, it is one-sixteenth the foveal value. (Note that it would be wrong to say, the value were halved every 2°, as said in some textbooks or in previous versions of this article.) In dark-adapted vision, light sensitivity corresponds to rod density, which peaks just at 18°. From 18° towards the center, rod density declines rapidly. From 18° away from the center, rod density declines more gradually, in a curve with distinct inflection points resulting in two humps. The outer edge of the second hump is at about 30°, and corresponds to the outer edge of good night vision.

Outer boundaries

[[File:Traquair 1938 Fig 1 modified.png|thumb|Classical image of the shape and size of the visual field For both eyes the combined visual field is 130–135° vertically and 200–220° horizontally.

Flicker fusion thresholds decline towards the periphery, but do that at a lower rate than other visual functions; so the periphery has a relative advantage at noticing flicker. based on data by]]

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.

Functions

The main functions of peripheral vision are:

Cone-rich rim of the retina

The rim of the retina contains a large concentration of cone cells. The retina extends farthest in the superior-nasal 45° quadrant (in the direction from the pupil to the bridge of the nose) with the greatest extent of the visual field in the opposite direction, the inferior temporal 45° quadrant (from the pupil of either eye towards the bottom of the nearest ear). Vision at this extreme part of the visual field is thought to be possibly concerned with threat detection, measuring optical flow, color constancy, or circadian rhythm.

See also

  • Averted vision
  • Bitemporal hemianopsia
  • Depth perception
  • Eye movement
  • Eye movement in music reading
  • Fovea
  • Perimetry
  • Visual field
  • Visual perception
  • Tunnel vision
  • Binocular vision
  • Macular degeneration
  • Glaucoma

References