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High Uncorrected Phorais Lead to Fatigue

The general subjective symptoms associated with "eyestrain" due to the continuous effort expended to maintain fusion in all visual activities (including driving) are a feeling of tiredness and discomfort in the eyes, varying from a dull ache to actual pain; headaches of various types; and rapid fatigue and blurring of vision sometimes leading to actual confusion or temporary diplopia with giddiness and even nausea and digestive disturbances; torticollis (head tilting) which results from a vertical imbalance (hyperphoria). Most of these symptoms can be induced artificially by the wearing of prisms by a normal person.9,10

Judgment of direction also suffers in heterophoria.10 The faculty of steropsis and the judgment of distances tend to be impaired. It is generally held that an exophore underestimates distances and an esophore overestimates.27 This could explain some of the rear-end accidents that are so common on the road.

Corrective lenses and/or orthoptic training (vision training) to encourage habits of binocularity can compensate for, improve or correct a heterophoria. Thus, if the motor vehicle inspector finds a heterophoria, the applicant for a license should be referred in an effort to have this condition corrected. If the inspector or eye practitioner does not check or examine for this function, the driver of a car operates under a false sense of security.

What does a high, uncorrected hetero

phoria mean to the operator of a motor vehicle? The above mentioned subjective symptoms lead to fatigue and confusion. Fatigue brings on a dazed condition where diminished attention and speed of reaction (both sensory and motor) make the driver prone to accidents.

In a screening of 3,000 drivers, 16.9% failed minimum requirements for lateral phorias and 13% failed for vertical phorias.26 Effect of Fusion

Binocular fusion may be defined as the process by which the visual portion of the cerebral sensory area combines the sensory impulses initiated by the two somewhat disparate retinal images of an object of regard into a single perception, so that only one tri-dimensional object is seen.9

Obviously, fusion is a binocular function. One must have two impulses or messages going into the occipital cortex of the brain for a process of assimilation, integration and other reflex functions, with a resultant single picture to be perceived and interpreted. Fusion is absent when a driver possesses only one eye. It is also absent when a person suppresses or suspends vision in one eye.

The act of suspension of vision in one eye is quite common. It does not always occur, but may be found under conditions of stress. This is a protective mechanism of binocular situation under stress. The suspension may be in the area of the macula only, or it may be total. Suppression of vision in one eye can be found in tropias. Again, this is a protective mechanism of the brain to stop diplopia.

Even when two eyes are apparently straight but, for some reason which may be muscular or neurological, the eyes cannot coordinate or the brain cannot fuse the two images because of a gross dissimilarity, the impulse from one eye to the brain is cut off. If this situation is permitted to continue, nerve fibers from the macular area may atrophy, with a resultant amblyopia. Confusion and inefficiency in the visual process are due at times to a situation in which the basically dominant' eye is more optically deficient than the other, thus making the second eye arti

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ficially dominant. This is often correctible with glasses.

Clinically, fusion is classified in three grades: Grade I. Simultaneous Perception (superimposition); Grade II. True Fusion With Some Stereopsis; Grade III. True Fusion With Stereopsis.10

It is probable that very close to 100% of all drivers could achieve Grade III fusion, with the proper care of an eye practitioner if even some fusion is present. The visual process develops in the infant in the above successive levels. If the eye doctor finds an impairment of fusion, he traces it back to its developmental level and institutes training from that point.

Now, what does the fusion faculty have to do with the safe operation of a motor vehicle on our highways? It can be safely said that good fusion is the cumulative result of an efficient visual process. If fusion is impaired somewhere along the visual pathway, from the eyelids to the occipital area of the brain, something is wrong.

In the majority of people, faulty fusion can be improved. First and foremost, however, it must be found. Fusion ability can be discovered only by the professional practitioner who is examining for it. An impaired fusion faculty may be compared to a headache, because its presence may indicate a malfunction.

Effect of Color Vision

Most authorities state that the ability of the eyes to interpret color is dependent upon the possession of certain color receptors in the retina. In the absence of these color receptors, the individual is color blind.

Genetically, color sense has developed from black and white (and the various shades of gray in between) to blue and yellow perception and then to red and green perception. Light as we see and use it is made up of the various visible wave lengths of the spectrum. The various wave lengths can be separated and measured. The eye is most sensitive to the D line of sodium, a shade of yellow.

Lack of color perception must be considered from two aspects: 1) color blindness,

where the retinal receptors are absent; and 2) color ignorance, where the individual has not learned to interpret or discriminate col

or.

About 4% of the male and 0.4% of the female population are congenitally color blind. Color blindness may be acquired through disease or injury to the retiną. Partial color blindness can be caused by the excessive use of tobacco or alcohol.11

Many color blind people learn the normal colors of objects through experience and under circumstances with which they are familiar. A color blind or color deficient driver must know that he is color blind or deficient. The color deficient may be trained to distinguish color. The color blind cannot be so trained. Color blind patients who drive, know they are color blind and compensate accordingly. We might add that a few take their clues from other drivers, especially when uncertain about the color of traffic signals.

We believe motor vehicle inspectors should continue to check for color vision simply to inform the driver if color blindness exists but not deprive a license for this visual anomaly.

Stereopsis and Depth Perception

As noted above, stereopsis, the ability to see tridimensionally, is the highest development of the binocular organism. Monocular vision or substandard vision in one or both eyes is actually a "depth conception", whereas, good binocular vision is truly “depth perception with stereoscopic vision".12

A monocular individual, or one seeing with substandard vision, depends greatly on experience to determine distance and size. These experiences are: 1) Psychological (a. aerial perspective; b. distribution of light and shade; c. the overlapping of contours; d. geometric perspective; e. interpretation of size.) 2) Physical-parallax.

These experiences constitute a learned process of spatial localization. A monocular person actually sees his space world flat. His learning, almost from birth, to localize objects in space permits him to find these objects and to orientate to his space world.

Parallax is an optical phenomenon which makes a near body seem to move in a direction opposite to the motion of the eye, while a more remote object appears to move in the same direction; the relative degree of motion being conditioned by the space interval between the two objects. Therefore, when one moves one's head or eye slightly to the side, he is able to localize an object in space by the apparent movement of other objects both closer to (against motion) and further away (with motion) than the object of regard. When we adjust the muscles within and outside our eyes, we have a clue to depth through kinesthetic (muscle) sense.

Now, let us consider stereoscopic vision. This is achieved by the stimulation of disparate areas of the retina. In addition to this, we must also consider the separation of the two eyes which makes us see two images, one with each eye, but at a slightly different angle. Werth, Stralton, Jaensch, Langlands, et. al. have found that stereoscopic vision is a very fine and accurate function of the eyes.12

As noted before, lack of stereopsis does not preclude the absence of depth perception as we know it. Many drivers, who are monocular individuals under conditions of stress and fatigue, are accident free. It would give great assistance to the driver to be warned of an impaired judgment of distance. In discussions with patients found to have faulty stereoscopic vision, we find they argue when they are told to be careful while driving and allow sufficient room for safety between their car and the car in front of them. They say they have no difficulty. Again, it is experience and knowing limitations. No matter how vociferous the argument, these patients will generally allow more room between cars in the future.

Referral But Not Refusal

In a study on depth perception testing by motor vehicle inspectors, Ryan found that stereopsis was being measured. No minimum requirements were in use, so there were no license refusals but rather referrals to optometrists or ophthalmologists for further consultation.13

If depth perception is to be a salient factor in passing or rejecting an applicant for licensure, then depth perception tests, such as the Howard-Dolman test, must be employed rather than stereoscopic targets in screening instruments.

Depth perception tests of some sort must be used by motor vehicle inspectors. The records show many rear-end accidents. If the driver is warned about fair or poor depth perception, he could compensate by control of his speed and the interval between cars. If depth perception is poorly learned or developed, we believe the applicant should be rejected and referred to an optometrist or an ophthalmologist for vision training. Once it is learned, he may, upon re-application, be awarded a driver's license. In a screening program for depth perception, 22.3% failed the Howard-Dolman test.26

Effect of Visual Fields

The visual field is that portion of space in which objects are visible at the same moment during steady fixation of the gaze in one direction. The eye moves constantly, and with every eye movement, there is a change in the visual field. The effects of visual impressions last longer than an instant with a resultant larger field in normal use of the eyes.

We are mainly interested in the lateral or temporal field of a driver.1 In a normal eye, one should be able to discern movement at 90° or more to the side. No large scale studies have been conducted on the problem of what the minimum angular visual field should be for drivers.

Hockenbeamer15 states that the normal, stationary "side vision" of 180° or more, is reduced as follows when moving: 20 miles per hour, 104°; 30 mph, 96°; 40 mph, 70°; 60 mph, 42°. This is due to the blurring of stationary objects close to the side of the car which are not allowed enough time to create an impressionable, clear stimulus on the peripheral retina.

Danielson1 states that the HarringtonFlock Method of screening for central visual field defects is more important than peripheral field defect detection because central

vision may be absent and the driver may be unaware of this condition.

The visual field is limited normally by the anatomical structures that surround the eye. Thus, the deep-set eye will have a reduced field. And several pathological conditions reduce the field. It is important for the individual to know that he has a reduced field and there must be consultation to screen out active pathology that would reduce the field below requirements. We would favor a 70° lateral field in one eye and 140° total lateral field as a standard. The number of persons who would be referred to an eye practitioner by this standard would be very small.

The conclusion of Kites and King1s is that while not enough research has been done on visual fields in relation to the driver, there is a relationship between speed and size of the visual field which the driver requires for

ments. The best adjustment is not always easy to find. There may be a conflict of reflexes with a consequent rapid shifting to overcome the difficulties.

If it is true that “eyestrain" is due to extra demands on the neuro-muscular adjustment mechanism, then it should follow that "eyestrain" increases when conditions for the use of the eyes are made worse. The eyes should, by more efficient adjustments, compensate to a measureable degree for these conditions. The result should be nearly equal work done but greatly increased fatigue. This was proven by Luckiesh and Moss.19 Under conditions of fatigue, reactions become more superficial. In a fatigued state experimenters have noticed momentary lapses scattered through the performance of uniform tasks.20 Driving of a motor vehicle is considered a uniform task in which boredom and fatigue reduce overall efficiency.

Physical fatigue and ocular fatigue naturally would occur at the same time. At night under normal conditions with the approaching glare of oncoming headlights, there is a constant fluctuation in the size of

the pupil to decrease or increase the amount of light entering the eye. The searching movement to compensate for varying light intensities and the adjustments of the pupil, increase fatigue.21 If, in addition to this, an eye is already making compensations for a refractive error, muscular imbalance, or poor coordination, a definitely hazardous situation for the driver is the result.27 Research on Night Visibility

More research is being done presently, and has been done in the recent past, on night visibility than all other phases of motorists' vision put together. Outstanding is the work of Oscar Richards of the Biological Laboratories of American Optical Company and of Merrill Allen and Henry Hofstetter of the Indiana University Division of Optometry under a grant from the American Optometric Foundation. Allen and his colleagues are attempting to determine what visual functions are involved in night visibility while driving and to then devise an instrument to measure these functions.

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