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 visual field
that 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 exists in the middle of the visual field, 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, generally the fovea.
The fovea is a cone-shaped depression in the central retina measuring 1.5 mm in
diameter, corresponding to 5° of the field of vision. 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, only the central portion of the fovea is 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 field of vision. 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 the part of
the retina in which visual acuity is at least 20/20. This corresponds to the
foveal avascular zone 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 field of vision. Vision within the fovea
is generally called central vision, while vision outside of the fovea 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 field of vision. The macula, a
region of the retina defined as having at least two layers of ganglia is
sometimes taken as defining the boundaries of central vs. peripheral
vision. The macula has a diameter of 5.5 mm and corresponds to 18° of the field
of vision. When viewed from the pupil, as in an eye exam, only the central
portion of the macula is visible. Known to anatomists as the clinical macula
this inner region is thought to correspond to the anatomical fovea.
The dividing line between near and mid peripheral vision at 30° radius is based
on several features of visual performance. Visual acuity declines by
about 50% every 2.5° from the center up to 30°, at which point visual acuity
declines more steeply. Color perception is strong at 20° but weak at 40°. 30° is
thus taken as the dividing line between adequate and poor color perception. 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 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° superior, 60° nasal, 70°–75°
inferior, and 100°–110° temporal. For both eyes the combined visual field is
130°–135° vertical and 200°–220° horizontal.
Characteristics The loss of peripheral vision while
retaining central vision is known as tunnel vision, and the loss of central
vision while retaining peripheral vision is known as central scotoma.
Peripheral vision is weak in humans, especially at distinguishing detail,
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. Rod cells are unable to
distinguish color and peak in density in the near periphery, while cone cell
density is highest in the very center, the fovea, and from there declines
rapidly. 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.
Peripheral vision is also relatively good at detecting motion.
Central vision is relatively weak in the dark since cone cells lack sensitivity
at low light levels. Rod cells, which are concentrated further away from the
retina, operate better than cone cells in low light. This makes peripheral
vision useful for detecting faint light sources at night. In fact, pilots are
taught to use peripheral vision to scan for aircraft at night.
The distinctions between foveal 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 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. Functions
The main functions of peripheral vision are:
recognition of well-known structures and forms with no need to focus by the
foveal line of sight, identification of similar forms and
movements, 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 with the greatest extent of
the visual field in the opposite direction, the inferior temporal 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. See also
References