International Standards:

Vision Requirements for Driving Safety

Appendix 3 - Suggestions for Additional Tests

Clinical tests of visual functions were developed to clarify underlying causes of vision loss. In the context of driving safety, however, we are interested in tests of functional vision that explore not the causes, but the consequences of vision loss.

Based on psychophysical principles, visual function tests typically test only one parameter at a time in a static, uncluttered environment.

Functional vision, on the other hand, typically involves multiple parameters in a dynamic, usually cluttered environment.

There is a need for screening tests that do not require highly trained personnel and that assess functional vision, based on a wider array of visual functions than visual acuity alone.  We encourage the development of tests that may assess a combination of parameters.

The following paragraphs, provided by Colenbrander, provide examples of potential ideas.

It should be stressed again that these tests (like any of the other tests discussed) are screening tests, not tests of driving competence. It is assumed that persons who fail the test in the licensing office will be referred for further assessment. The vision professional will then advise the licensing office. The licensing office may require an on-the-road test (the only test that assesses actual driving competence) and will make the ultimate decision.

Proposed screening test for contrast sensitivity

A simple screening test for contrast sensitivity could be based on the Mixed Contrast reading card (Colenbrander, Fletcher, 2005) ^[38]. This card is a regular reading card with alternating black and gray lines in each paragraph.

It was found that the difference between the number of lines read with high-contrast and with low-contrast provides a simple measure of contrast sensitivity that is independent of visual acuity.

The card is meant for use in general practice where the high-contrast section would replace regular reading cards.  With 10% contrast for the low-contrast lines a difference of 1 or 2 lines is normal; in patients differences up to 10 lines or more have been recorded.

The card uses a reading task rather than letter recognition, since reading involves a larger retinal area and contrast losses do not necessarily start at the fovea.

Patients with contrast sensitivity deficits often feel that "something is wrong", but cannot pinpoint the cause; they appreciate the card as a vivid demonstration of the consequences of contrast sensitivity loss.

A modification of this card could be made with a smaller contrast difference, calibrated so that the black and gray lines would be equally difficult for persons with normal contrast sensitivity. Any person who experiences a greater difficulty reading the gray lines than the black lines would be referred to a vision specialist for further testing. 

Proposal for a combined test for visual field, scanning strategy and reaction time

Existing diagnostic visual field tests are monocular and exclude eye movements. A test of the functional field of view must be binocular, must allow scanning eye movements and must include reaction speed. Clinical field testing equipment cannot accomplish this.

A proposed test would present stimuli in different parts of the (binocularly viewed) visual field; the subject would push a button as soon as the stimulus is seen.  Another stimulus would then be presented after a variable interval.  The score is the sum of the reaction times for all stimuli.  If a stimulus is presented in a scotoma, the reaction time will be prolonged. An inefficient scanning strategy will further prolong the reaction time. Subjects with a generally delayed reaction time will fail also.

A similar test for the central field, presented on a computer screen, has shown good correlations with reading performance. For a wider field the stimulus could be presented with a digital projector or in virtual reality glasses. Failure on the screening test would require referral to a vision specialist.

Proposal for a combined screening test for attention, contrast and night vision

This more elaborate, computer-based test would screen for deficits in attention, contrast and night vision, yet would not require highly trained personnel.

The subject is asked to look at a dim computer display in an enclosure that promotes dark adaptation. If the curtain is not closed properly, stray light makes the test more difficult; thus the subject will want to close the curtain. On the screen a bright dot moves slowly at unpredictable intervals in unpredictable directions. The subject uses a mouse or joystick to move a black disc around to keep the bright spot covered. Fixation and attention will thus stay with the moving spot.

From time to time a dim letter (or other object) appears in a random position. These targets would be above threshold when seen alone. If the bright spot is not adequately covered, it will hinder their recognition. Display time, brightness and size may vary. 

When the subject recognizes the letter or the object, he/she presses a button or clicks the mouse (for timing) and names the letter or object. The computer records the position of the stimuli presented and the reaction time. Since the fixation point can move around to the edges of the screen, the diameter of the field that can be tested is equal to twice the width of the screen.

Interpretation

• Failure to keep the bright spot covered may indicate problems in hand-eye coordination and manipulative skills.
  
  
• Extended reaction times would show up in all responses.
  
  
• Missing targets close to the bright spot (when not covered) may indicate glare problems.
  
  
• Targets detected, but not recognized may indicate focusing problems in night vision (such as night myopia from pupil dilation).
  
  
• Missing the dim targets, but not the brighter ones may indicate dark adaptation/ contrast problems.
  
  
• Missing targets in one area may indicate a scotoma.
  
  
• Missing peripheral targets may indicate restricted attention.
  
  
• Missing targets on one side may indicate hemi-neglect.

Scoring would be done by the computer. A simple printout showing the location of the targets seen and of those missed, would give a clear record to convince the subject and would be kept for documentation in the file. Failures on this screening test would need to be followed up with other clinical tests by a vision professional.

Since this test requires dark adaptation, it will catch problems that only become evident when the pupil is dilated. This may include problems due to the exposure of the edge of a decentered IOL or of a small ablation zone after refractive surgery.

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Also see: Vision Requirements for Driving Safety: Table of Contents

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