Peripheral vision

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. "Far peripheral" vision refers to the area at the edges of the visual field, "mid-peripheral" vision refers to medium eccentricities, and "near-peripheral", sometimes referred to as "para-central", vision, exists adjacent to the center of gaze.

Boundaries

Inner boundaries The inner boundaries of peripheral vision can be defined in any of several ways depending on the context. In everyday language the term "peripheral vision" is often used to refer to what in technical usage would be called "far peripheral vision." This is vision outside of the range of stereoscopic vision. It can be conceived as bounded at the center by a circle 60° in radius or 120° in diameter, centered around the fixation point, i.e., the point at which one's gaze is directed. 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 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 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 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. A ring-shaped region surrounding the fovea, known as the parafovea, is sometimes taken to represent an intermediate form of vision called paracentral vision. The parafovea has an outer diameter of 2.5 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 mm), up to 17° of the visual field (5.5 mm). The term is familiar in the general public through the widespread macular degeneration (AMD) at older age, where central vision is lost. When viewed from the pupil, as in an eye exam, only the central portion of the macula may be visible. Known to anatomists as the clinical macula (and in clinical setting as simply the macula) this inner region is thought to correspond to the anatomical fovea. 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. From thereon the decline is steeper. (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.) Color perception is strong at 20° but weak at 40°. 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 The outer boundaries of peripheral vision correspond to the boundaries of the visual field as a whole. For a single eye, the extent of the visual field can be (roughly) defined in terms of four angles, each measured from the fixation point, i.e., the point at which one's gaze is directed. These angles, representing four cardinal directions, are 60° upwards, 60° nasally (towards the nose), 70–75° downwards, and 100–110° temporally (away from the nose and towards the temple). For both eyes the combined visual field is 130–135° vertically and 200–220° horizontally.

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