Functional Vision Issues

(presented at Dupage EasterSeals, Rosalie Dold Symposium, VillaPark, IL, 1995)

Janice E. Scharre, OD, MA, FAAO

Coordinator, Rehabilitative Service

Illinois College of Optometry

(312) 225-1700

And

Patricia A. Politano, M.A. CCC-SLP/L, ATP

Assistive Technology Unit, University of Illinois at Chicago

312 -413-1473

"Vision is critical to all areas of development and learning. Moreover, the effective and efficient use of vision, including visual function and visual perception, is learned and follows a developmental progression." (Zambone AM. Serving the young child with visual impairments: An overview of disability impact and intervention needs. Inf Young Children 1989; 2(2): 11-23

I. Visual Acuity

ability to determine if something is seen or not
capacity to discriminate fine detail in an object or scene
ability to see objects clearly at different distances

A. How does this process occur?

1. Not all poor vision is due to poor optics

2. The recognition process: detection, resolution, identification

(from: Thibos LN, Bradley A. New methods for discriminating neural and optical losses of vision.

Optom Vis Sci 1993; 70:279-287)

B. Types of VA:

1. detection : is the object there ?

Catford Method: Can you see this dot?

Teller Acuity Cards: Is this a grating of black and white bars, or is it a uniform gray patch? IIIIIIII

2. resolution: Are you able to detect a separation between elements within a pattern?

Broken Wheel Test:

3. recognition/ identification What letter is this? N

Lea Symbols:

C. How is visual acuity interpreted?

can resolve details of an object that creates a visual angle of one minute of arc
angle of one minute of arc is an angle made by a 1 inch target viewed at 100 yards
at 20 feet, the target would be less than 1/10 th of an inch
acuity is measured at near and far
acuity may be reduced due to disease process, structural abnormality or amblyopia

D. Amblyopia

reduced visual acuity even with corrective lenses
not due to structural or pathological reasons.
occurs as the result of an amblyogenic factor such as an uncorrected refractive error or strabismus

E. Signs/Symptoms of a visual acuity problem

holds materials or toys too close to face
unable to see blackboard
difficulty copying from blackboard
frowning while reading, writing or looking at distance
excessive blinking or rubbing of eyes

II. Refractive State:

A. Hyperopia: farsighted; "corrected "with plus or converging lenses

B. Myopia: nearsighted; "corrected" with negative or diverging lenses

C. Astigmatism: rays of light are not focused at a single point of light

D. Anisometropia: difference between the eyes for refractive error

E. Emmetropia: no refractive error

F. Signs/Symptoms of a refractive error problem

similar to visual acuity symptoms
fatigue
headaches, nausea and dizziness
crossed or turned eyes
turning or tilting head to use only one eye or closing one eye
holding materials close to face
excessively sloppy or hard to read handwriting

III. Binocular Vision

ability of the eyes to work together: eye coordination
influences ability to localize objects in space accurately and to perceive depth

A. Stereopsis:

Binocular visual perception of objects are seen in three- dimension
Stereoscopic perception of depth exists because the eyes are separated in space.
Both eyes are needed for stereopsis.
Stereopsis is innate
measured with various stereopsis tests: Lang, Randot E, etc.

B. Disorders in binocular vision may be either in over or underconverging of the eyes or be manifest as strabismus.

Strabismus:
esotropia: in
exotropia: out
hypertropia: up
hypotropia: down
alternating/ unilateral and / or constant/intermittent

C. Signs or symptoms of a binocular vision problem.

past pointing
frequently loses place while reading
uses finger or marker to read
closes one eye or tilts head
headaches, nausea and dizziness
complains of blurring of vision or of double vision
restlessness, fatigue, difficulty concentrating
difficulty with gross motor skills, clumsy
poor hand-eye coordination
poor depth perception

IV. Oculomotor Ability

A. Types

visual attention
visual avoidance behavior
fixation (eye pointing)
visually-directed reaching
scanning (pursuits)
shift of gaze (saccades)
full range of eye movements

B. Signs/Symptoms of an oculomotor problem.

difficulty remembering what is read or seen
loses place while doing written work
difficulty with sequential concepts
persistent word or letter reversals
omitting, repeating and miscalling words or numbers
uses finger or marker to maintain their place while doing work
poor hand-eye coordination

V. Visual Field

A. Definition

part of space where objects are visible at the same time during fixation in one direction/ full area over which objects are seen

B. Abnormalities

visual field defects
field of neglect: unconscious ignorance of everything in a particular field

C. Signs/Symptoms of a visual field problem.

turns or tilts head
ignores objects in a visual field
clumsy, walks into objects/people
poor visual figure-ground perception

VI. Accommodation

A. Process by which the eye adjusts its focal length in response to visual stimuli

B. Adversely affected by age, disease, medications and functional components.

C. Signs/symptoms of a focusing problem:

difficulty remembering what is read
loses place while reading
complains of occasional blur of material
frequently blinks or rubs eyes
fatigue, headaches
short attention span, difficulty concentrating

VII. Color Vision

often inherited; permanent
varies degrees and kinds of color discrimination
may be acquired: due to eye disease

VIII. Contrast Sensitivity

represents how sensitive the visual system is to everything
think in terms of black letters on a white background. 100%
Contrast can be reduced by having gray letters on a white background, dark gray letters on a light gray background etc.
gray letters on an equally gray background- 0%
Visual acuity suffers with any decrease of contrast below approximately 90 % therefore, as the contrast goes down the size of the target need to increase.

VIX. "... limited vision may yield imperfect sensations, which in turn may yield imperfect perceptions that become vague impressions (that are) confusing to the educational process.

Scholl G. Growth and development. in Scholl G "(ed): Foundations of Education for Blind and Visually Handicapped Children and Youth: Theory and Practice. New York : American Foundation for the Blind,1986: 65-84