Vision and Work - Fundamentals and Assessment Tools for Occupational Ergonomics - page 435

compared to both eyes opened (Sheedy et al., 1986). Stereopsis provides the worker with enhanced ability
to properly maneuver objects with respect to one another. Stereopsis can also be important for some
workers with critical intermediate distance tasks — such as forklift drivers.
Individuals without stereopsis or with significantly reduced stereopsis should probably not be placed
in jobs that benefit highly from it. Monocular individuals, of course, have a total loss of stereopsis. The
effects of the loss of stereopsis in everyday task performance have been described by Brady, who wrote a
book based upon his traumatic loss of an eye (Brady, 1972). Likewise, individuals who have two eyes but
are unable to keep them aligned (a condition named “strabismus”) do not have stereopsis. Also, some
individuals are able to keep their eyes aligned but have amblyopia (reduced acuity in one eye) and
hence significantly reduced stereopsis. Likewise, stereopsis is reduced if a worker has reduced acuity in
one eye due to uncorrected refractive error, this situation can normally be resolved by obtaining
proper correction. Various vision screening devices are available to identify individuals with reduced
or absent stereopsis.

18.2.3.5.2 Binocular Alignment — Phoria and Convergence
Even though most workers keep both eyes aligned when viewing an object, many have difficulty main-
taining this ocular alignment resulting in symptoms such as fatigue, headache, blur, double vision, and
general ocular discomfort. These problems are much more common among workers with demanding
near and intermediate visual needs such as viewing
printed text or computer displays. Binocular
alignment at near viewing distances is more
complex than for distance viewing because of the
required ocular convergence and the interaction
between ocular convergence and accommodation.

Phoria Binocular fusion, the sensory process by
which the images from each eye are combined to
form a single percept, requires sensory input from
each of the two eyes. If the eyes are not properly
aligned on the fixation object, double vision will
result. The brain continuously feeds information
back to the eye alignment muscles in order to
maintain ocular alignment and avoid double
vision. Without sensory feedback from each eye
(e.g., occluding an eye as in Figure 18.7), the eyes
assume their “position of rest” with respect to one
another. If the position of rest is outward or
diverged, the patient has exophoria. If it is inward
or converged, the condition is esophoria.
Most workers have at least a small phoria.
Whether a worker experiences symptoms depends
upon the amount of the misalignment, the ability
of the worker to overcome that misalignment,
and the task demands. The symptoms associated
with phoria can be eyestrain, double vision, head-
ache, eye irritation, and general fatigue. Clinical
studies have documented the relationships
between the visual clinical measurements and the
symptoms that are experienced (Sheedy and
Saladin, 1978). Various vision screening devices
enable measurement of the phoria and provide
guidelines for eye care referral.

FIGURE 18.7 Binocular alignment when fusion is
denied by occluding an eye. (a) Orthophoria. (b)
Esophoria. (c) Exophoria.