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.
Boundaries
Inner boundaries
The inner boundaries of peripheral vision can be defined in any of
several ways depending on the context. In common usage or everyday
language the term "peripheral vision" is commonly 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.
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In common usage, peripheral vision may also refer to the area
technically known as "mid peripheral vision," defined by a circle 30° in
radius or 60° in diameter.
In vision-related fields such as
physiology,
ophthalmology, or
optometry,
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 (using
ophthalmoscope or
retinal photography)
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 (6/3 metric).
This corresponds to 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 field of vision. Vision within the fovea is generally
called central vision, while vision outside of the fovea is called
peripheral 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 (bundles of nerves and neurons) 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 example, only the central
portion of the macula is 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.
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 the decline in 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 (up), 60° nasal (towards the nose), 70-75° inferior (down), and
100-110° temporal (away from the nose and towards the temple). 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
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.
Flicker fusion threshold is higher for peripheral than
foveal vision. Peripheral vision is good at detecting motion (a feature of rod cells).
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.
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 (Gestalt psychology laws),
- delivery of sensations which form the background of detailed visual perception.
