Optical system
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The eye muscles
Schematic of the human eye
The eye has both internal and external muscles. The internal muscles control
focusing and pupil size. The external muscles direct the eyes to the point
of interest and keep the retinal image in constant slight motion.
Any muscle system is subject to fatigue when it is used continuously without
periods of rest. Although muscles of the eye do not work against forces outside
the eye, they do exert forces against each other, and they become "fatigued" from
periods of continuous ocular activity. As eye muscles are not unique, they
too show evidence of fatigue when visual tasks require added effort and concentration.
And, they too recover their optimal levels of function after rest.
The external muscles of the eyes must work together to keep both eyes centered
on the point of interest. Balance between the muscles is somewhat imperfect
in most people, and there is some tendency for the eyes not to converge correctly
(see phoria under "Common Disorders").
Such eye movements are responses to complex neural stimuli, such as from
the vestibule of the ear, affecting a sense of balance. In general, such movements
are precise and relatively fast, and are not subject to fatigue.
The external muscles exert a non-steady force so that the eye has a continuous
tremor. This produces relative motion between the retina and the image. Without
that motion, the perceived image would fade away. That would be like placing
your hand lightly on a rough surface and feeling the roughness only as long
as you keep moving your hand back and forth. Your perception of the roughness
fades when your hand is still. The ocular unrest described is called "hippus."
When a person wishes to change the point of fixation, there is a short burst
of muscle activity to cause the eye to rotate and simultaneously readjust the
balance of neural enervation to the agonist and antagonist muscle groups so
that the desired point of fixation may be maintained. Changes in ocular convergence,
or even head motion, are considered in the calculations made by the complex
ocular system to produce the proper neural inputs to the eye muscles. This
is all done before the eye reaches its intended target. An eye movement of
10 degrees may be accomplished in about 40 milliseconds.
Timing of eye fixation
Figure 13 shows a plot of the time it takes to move the eye fixation from
one point to another.
The plot in Figure 13 combines the research of Robinson (1964) and Yarbus
(1967) for large and small movements, respectively. Small eye movements are
called "saccades." Movement of the eye from one point to another
is called "version."
A vergence movement is related to the fact that there is a measurable distance
between a person's two eyes. Therefore, when viewing an object that is relatively
near, the visual axes of the eyes are not parallel. They are actually turned
slightly inward so that the axes will intersect at the distance of the object
being viewed.
A vergence movement is the change in that relative alignment of the eyes
so that the point of intersection of the axes is moved closer or farther according
to the distance of objects being viewed. If the VDT visual task requires frequent
excursions and refixations between two or more surfaces (such as a display
and a document), users may wish to minimize any consequent vergence fatigue
by placing the two surfaces (display and document) at approximately the same
distance from their eyes.
The number of eye movements a person makes during an hour of reading a book,
may be as high as 10,000 coordinated eye movements. This rarely results in
visual discomfort, however. Walking is a common activity that places even more
demands on the external muscles of the eye. When the head is in motion, as
when a person is walking, the external muscles are in constant activity to
adjust the position of the eye to maintain a steady fixation point; thus, objects
viewed when walking appear stable. This also does not produce noticeable fatigue.
Fatigue, and a sense of annoyance, may be produced when erroneous visual
signals are sent to the brain, thus disturbing the ability of the ocular system
to maintain a point of fixation or to avoid "errors" during refixations.
This disturbance of the visual field may happen when a person first wears eyeglasses
with a new prescription. The change in the optics may cause a noticeable change
in the angular separation of objects in the visual field and the person has
to learn to adjust for that change.
There is an elastic ring of ligaments, called the "zonula," around
the lens of the eye. In the resting state of the eye, this ring exerts an outward
tension on the equator of the lens. This tends to flatten the lens, thus reducing
its refractive power and placing the eye in a condition for viewing distance
objects. When close objects are to be brought into focus, the refractive power
of the lens must be increased. The ciliary muscle then contracts and, in effect,
reduces the tension that the zonula exerts on the lens. As a result, the lens
becomes more spherical and has greater refractive power. This process is called "accommodation." To
maintain focus on a near object, the ciliary muscle exerts a continuous contracting
force.
The accommodated lens is in constant motion. When viewing a steady target
one meter from the eye, the lens may be expected to oscillate over a quarter
of a diopter range at a rate of about three to four times a second. Even under
steady viewing conditions, such as when reading a book, the lens of the eye
is very active (Campbell and Whiteside, 1950). Considering this normal activity
of the accommodation response under static viewing conditions, it is not surprising
that researchers have not been able to find any connection between the frequency
of changing accommodation and visual fatigue.
There are various stimuli for the accommodation response. The act of converging
the eyes so that the two lines of sight intersect at a point near the person
will cause the lenses of the eyes to accommodate. That stimulus for accommodation
may not be very precise, however. Both the microfluctuations of the lens and
the chromatic aberration may serve as cues for the accommodation response (Toates,
1972). If the optics of the eye are not in adequate focus, the microfluctuations
will produce changes in the resolution of the retinal image. These changes
may serve as a cue for the accommodation response. This only holds, however,
for blur caused by the optics of the eye. But, if the image being viewed is
itself defocused, there will be no change in the accommodation response (Fincham,
1951).
A schematic showing chromatic aberrations produced by a non-color corrected
lens system. If the distance between the lens and the retina were A, the
outer ring would be red; if B, magenta (a mixture of blue and red); and if
C, blue
Chromatic aberration may also serve as a stimulus for accommodation. This
is somewhat more complicated than the resolution aspect as a cue for accommodation,
but, as in the case of resolution, the visual system may distinguish between
chromatic aberration produced by the lens system of the eye and blur caused
by a defocused display image or the spectral composition of the target being
viewed.
When the eye is in a state of acceptable focus, there is an order to the
color fringes on the retina, the chromatic aberration. If the display, or projected
image, is defocused, the order of the color fringes will not change. Looking
at a white circle projected on a dark background with the eye in a state of
best focus, the outer color fringe of the retinal image will be magenta. If
the projected image is defocused, this outer color fringe will still be magenta.
If that same retinal image is defocused by the lens system of the eye, however,
the outer color fringe will change. If the lens is too strong, the outer fringe
will be changed to blue, and that will signal the system to relax accommodation.
If the lens system is too weak, the outer fringe of that image will be changed
to red and the visual system will be signaled to increase accommodation.
Accommodation is key to comfortable viewing and may be described as a condition
where the image on the retina is no more than 0.5 diopter out of focus and
where no more than two-thirds of the available accommodation is needed to maintain
that degree of focus. Paper, keyboards and display objects are viewed at distances
from the eye of 30 to 70 centimeters (12 to 28 inches), which is within the
normal accommodation range. Normally, VDT displays can be viewed comfortably.
If, however, VDT users find the visual task difficult, the situation is normally
remedied by appropriate eyewear. See "Common Disorders" and "Eyewear," discussed
later.
After viewing a near object for a length of time, the lens may not be able
to return immediately to its proper setting for infinity. This has sometimes
been called temporary myopia, and may be noticeable for several minutes by
some people.
Two different sets of muscles control pupil size: sphincter pupillae and
dilator pupillae. When light levels are increased, sphincter pupillae cause
the iris to close, reducing pupil size; when light levels are reduced, the
dilator pupillae cause the iris to open, making the pupil larger. The two muscle
groups operate at different speeds and are controlled by different neuromechanisms.
Under any given lighting condition, when the pupil size has stabilized, the
pupil is in a 'resting state." Even in this state, however, the pupil
is in constant motion, much like the accommodated lens of the eye. When viewing
a steady light level (luminance), the pupil constantly changes over a range
of about +/- 10% of the average diameter at a rate of about two times a second.
As mentioned earlier, these normal fluctuations of the pupil are called "pupillary
unrest," or "hippus."
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