Relationship between Visual Functions & Driving Performance

Good Vision → Safe Drive → Save Life

Driving Performance ∝ Visual Functions

80% of the information we received from the environment is visual and rest 20% through non-visual such as ears, skin etc. The commonest visual function tests that are performed at clinics and hospitals are: Visual Acuity, Visual Field, Colour Vision and Contrast Sensitivity.

Different testing procedures and standard is followed by different countries at the time of issuing a Driving Licence. In India, visual acuity and peripheral field of vision(side vision) is tested for driving licence. In other countries more rigorous examinations are used to assess visual functions, such as measurements of visual field, dark adaptation, stereopsis and glare recovery.

There are several factors affecting driver behaviour such as-

• Vision
• Fatigue
• Alcohol
• Vehicle Visibility
• Road Lighting
• Motor Co-ordination
• Accident Proneness
• Attention

Good visual acuity allows the driver more time to react against obstacles and signs, where as a driver with poor visual acuity requires the obstacle to be closer before its significance can be appreciated, leaving less time to react. He or she must be more alert, and will therefore fatigue faster.

Colour vision defects may be hazardous if they cause confusion between red, green and amber colour signal lights. According to Coles and Brown, a red colourblind person requires about 4times the normal intensity to see a red light.

Good peripheral vision is essential as significant increase of traffic day by day.

Stereopsis is important for judging the nearby vehicles.

Many drivers complain about glare during night driving. This is mainly due to dirt & scratches on the windscreen causing scattering of light.

Glare recovery time(the recovery time of retinal sensitivity) not only increases with age but also with the use of tinted spectacles or windscreen.

Alcohol impairs mental efficiency and slows the response to a hazardous situation. It also can cause diplopia(double vision) and blurring of vision.

Precautions to be taken by the Driver: 

1. First time spectacles or contact lens user should be well adjusted before start driving
2. Thick rim frames should be avoided as it may restrict peripheral vision(rimless frame is ideal)
3. Lenses of the spectacles should be high impact resistant(eg- polycarbonate and MR8)
4. If bifocals are required  they should be dispensed with the smallest possible bifocal segment. Progressive lenses are ideal for driving
5. Tinted spectacles should not be worn for night driving

 

Blindness

The term 'Blindness' means inability to perceive light. So, from a practical point of view, a person is said to be blind when he can not perform ordinary work.

In India, economically blind persons are those who, with both eyes open, can not count fingers beyond 3metres in broad daylight.

Major Causes: 

• Cataract
• Refractive Error
• Corneal Opacity

There are 8 million blind & 54 million partially-blind persons in India.

Causes of Blindness according to Age-group: 

1. Infants & Pre-school Age: Congenital cataract & glaucoma
2. School Going Age: Congenital & developmental anomalies, xerophthalmia, trauma
3. Adult Life(20-50yrs): Myopia, neurological disease, industrial injuries
4. Late Adult Life(50-70yrs): Cataract, glaucoma & diabetes
5. Elderly Persons(Above 70yrs): Cataract, glaucoma & Age Related Macular Degeneration

Prevention: In India, 80% of blindness is preventable or curable, but the problems are ignorance, superstition, poverty, illiteracy etc. Some preventive measures are as follows:

• Always use own towel or handkerchief
• Properly clean hands before touching to the eyes
• Try to avoid Kajal or Surma unless prescribed by eye doctor
• Use safety goggles during Diwali or Dussehra or within industrial working environment
• Retinal health check up once in every two years
• Yearly power check up 

Rehabilitation: 20% of blindness is incurable. These persons have to rehabilitated and trained properly so that they can earn their livelihood. Governments & Social Welfare Organizations should take proper initiative regarding education, training & rehabilitation for all blind persons in India so that they feel privileged. 

Visual Perception

Visual perception is the ability to perceive our surroundings when light falls on the retina. These are 4types:

1) Light Sense

2) Form Sense

3) Sense of Contrast

4) Colour Sense

Light Sense: It is the ability to perceive variations in the degree of light. This is assessed in 2ways:

A) Light Minimum

B) Light Difference or Differential Light Threshold

A) Light Minimum: Light minimum is the minimum amount of light energy which, when projected on the retina, causes a visual sensation. 

It varies greatly with the degree of adaptation of the eye at the time of observation. When the eye is dark-adapted(adaptation in dim light), the light minimum is very small as compared to when it is observed in a light-adapted(adaptation in bright light) eye.

• Dark adaptation is delayed by prolonged exposure to bright light.
• Delayed dark-adaptation occurs in Retinitis Pigmentosa(degenerative disease of Retina) & in case of vitamin-A deficiency 

B) Differential Light Threshold: The smallest difference in light intensity that can be appreciated.

Form Sense: The ability to discriminate between the shapes of the objects. This can be assessed by Visual Acuity(resolving power of the eye)

Sense of Contrast: It is the ability to perceive slight changes in luminance between regions which are not separated by definite borders. 

Contrast Sensitivity is reduced in many ocular diseases such as Glaucoma, Macular disease, Cataract, Refractive errors etc. 

In case of poor contrast sensitivity, the person may face the following problems:

• Difficult to see facial expression
• Difficulty in navigating steps of the staircase.
• Problem in night driving

Colour Sense: It is the ability to distinguish among different colours. The appreciation of colours occur only in photopic vision(bright light vision).