Peripheral vision, or indirect vision, is vision as it occurs outside the point of fixation, i.e. away from the center of gaze. 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.
The inner boundaries of peripheral vision can be defined in any of several ways depending on the context. In vision-related fields, 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 macula and the fovea.
The fovea is a cone-shaped depression in the central retina measuring 1.5 mm in diameter, corresponding to 5° of the visual field. 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 is achieved. A ring-shaped region surrounding the fovea, known as the parafovea, is considered to represent an intermediate form of vision called paracentral vision.
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 defines the boundaries of central vs. peripheral vision. The term is familiar in the general public through the well known macular degeneration (AMD) at older age, where central vision is lost. A dividing line between near and mid peripheral vision at 30° radius can be based on several features of visual performance. Color perception is strong at 20° but weak at 40°. 30° can thus be taken as the dividing line between adequate and poor color perception. In vision adapted to dark conditions, the sensitivity to light corresponds to the density of the sticks, which peaks at just 18 °.
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.
Peripheral vision is weak in humans, especially at distinguishing details, color, and shape. This is because the density of receptor and ganglion cells in the retina is greater at the center and lowest at the edges, and, moreover, the representation in the visual cortex is much smaller than that of the fovea. The distribution of receptor cells across the retina is different between the two main types, rod cells and cone cells.
- recognition of well-known structures and forms with no need to focus by the foveal line of sight
- identification of similar forms and movements (Gestalt psychology laws)
- delivery of sensations which form the background of detailed visual perception
Extreme peripheral vision
When viewed at large angles, the iris and pupil appear to be rotated toward the viewer due to the optical refraction in the cornea. As a result, the pupil may still be visible at angles greater than 90°.
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. 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.
Source: Wikipedia – December 2020.
Clarification: This text is an excerpt from a published Wikipedia article.