Week 5 - Vision and driving VTA approach Flashcards
- Adaptation effects of driving :- why are we concerned?
•People drive in weather conditions and at all times of day.
• Speedy visual adaptation is therefore clearly important for drivers.
•Illumination levels are constantly changing too while driving (eg. on entering a tunnel).
•Poor adaptation is one of the factors that particularly affects older drivers.
- Time taken to respond - Driving
•Any moving object has a space scotoma: driving space scotoma decreases
• Driving induces a motion-induced scotoma.
- salient objects in full view can repeatedly fluctuate into and out of conscious awareness when superimposed onto certain global moving patterns.
• Rather than being a failure of the visual system its hypotheses :- ‘a functional product of the visual system’s attempt to separate distal stimuli from artifacts of damage to the visual system itself.
- Flicker in driving:-
• In driving, this is not really a major consideration.
• Flicker can be experienced while driving, usually due to sunlight incident through a regular array of trees.
• This may be a problem for those who are visually sensitive.
- Position in visual field/ visual field size:- driving
• Driving uses most of the visual field, the detection and identification of objects with peripheral vision being a particularly important aspect of the task.
• Recent research has suggested that steering is guided by monitoring the distance centrally (about 1 sec ahead) to estimate road curvature, and by monitoring near (about 0.5 sec ahead) peripherally to judge position in lane.
- Viewing distance
• Driving makes strong demands on convergence and accommodation as the driver must be able to see the instruments on the dashboard as well as obiects in the distance.
- Visual subtense of task detail (size/acuity)
• Good central visual acuity and consequently a clear retinal image are necessary for the early recognition and reading of road signs. It also aids in the early detection of small and hazardous objects (e.g. pedestrians).
- Colour of task:- driving
• Recognition of traffic lights and road signs
• Problems might arise from colour deficient driver confuses the red, amber and green traffic lights
• No evidence to suggest higher accident rate
• Protanopic and deuteranomolous drivers may struggle with tail lights due to red light de-sensitivity
- Motion:- driving
• dynamic visual acuity (DVA) and the ability to perceive lateral motion and motion in depth would appear to be crucial attributes of the driver’s visual system.
- however weak correlation been found between reduced DVA and accident rate.
•The awareness of reduced DVA has also been cited as one of the reasons why some elderly people give up driving earlier than others.
- Stereopsis of driving :-
• Under conditions of poor visibility (e.g. at night) the majority of monocular cues to depth are missing and stereopsis becomes the major cue in depth perception.
- Stereopsis is inoperative beyond about 500 m and is therefore of little benefit in high-speed driving, although it is valuable for nearer tasks, such as parking or child location.
• No correlation has been found between defective stereopsis and increased accident frequency.
• However, where a binocular visual anomaly results in diplopia or a large (>44) vertical phoria there is evidence of an association with poor accident records.
- visual field requirements
• Good peripheral vision is essential for driving.
• A restriction of the visual field can never be fully overcome, although increasing head and eye movements and adding extra mirrors to the car can be of help. A full visual field is important for maintaining the driver’s orientation and in establishing relationships between the many objects in the field of view. Visual fields can be artificially reduced by, for example, thick spectacle frames and car design.
• Various pathological disorders also cause field defects (e.g. glaucoma, retinitis pigmentosa, cataracts).
- training requirements of driving :-
• Drivers must be trained
Other visual factors affecting driving:-
• Vehicle lighting
• Glare
• Vehicles provide their lighting for two purposes - to see and be seen. However, in order for the driver of a vehicle to see the road ahead, the intensity of the headlamps can be high enough to act as a significant glare source to other road users. This is particularly the case for more elderly drivers who suffer more from glare, probably due to the changes in the ocular media and retinal adaptation abilities as they age.
Problems with car light glare and how they can be reduced:-
• Main beam can be controlled for glare. In well lit areas, dipped headlamps can be used. Only use main beams on unlit roads to help in being seen, being dipped when approaching drivers. Headlamp alignment is checked during MOT,
• Vehicle overloading causes headlamps to point up - misdirecting headlamps causing glare: solution is self levelling suspension or headlamps
• Polarised headlamps can be used to remove glare - although cant be 100% polarised otherwise wont see car until too late
Weather and driving:- rain
• Rain
• In wet conditions the way that light is reflected back from the road surface changes.
- specularly reflected to the drivers of oncoming vehicles, producing glare. This is a good reason for reducing speed when driving in wet conditions at night.
Weather and driving:- fog
Fog: Lighting in fog is a real problem. The fog acts to scatter light. This reduces the amount of light being directed in front of the vehicle and increases the amount of back scatter from the fog.
- Both of these factors reduce the contrast of any obstructions, pedestrians, or vehicles that may be in the road. The only answer is to slow down and be very careful. Pedestrians don’t carry fog lamps.
Aids to driving:- (5 total)
• Yellow filters
- Cars lamps fitted with yellow filters less scatter of monochrome yellow, and peak voltage giving maximal visual efficiency
• Signalling
- Break lights as warning to other drivers
- Doesn’t create glare as dimmer than headlamps
• Retrofitting reflectors; Make use of other vehicles lights to aid driving
• “Driving lights” - blue lights inside vehicle by increasing adaptation level of driver
• Anti-reflection coatings on lenses to reduce glare: kept clean
Driving and aging:-
• The night myopia actually reduces with age (as amplitude of accommodation reduces), so other factors must be causing this problem.
• reduced retinal adaptation ability, reduced dynamic acuity and increased scatter from the ocular media.
Night myopia and driving:-
• Due to resting state of accommodation due to no stimulus :- 50-100cm (1-2D)
- Experienced by drivers at night
- Even with slightly lit streets, still some night myopia (-0.3D)
- Even though myopic shift is prevalent, a prescription for night driving is probably not required - due to the measurement being made with a laser equipment.
- Moving from mesopic to photopic conditions, the extra lens power may be a hazard
Vision, driving and accidents evidence?:-
• Correlations hard to find, due to driving being a complicated process and a wide range of variables: lighting, alcohol, road type, traffic speed. Isolating a single part is hard.
• Statistics do show a higher accident rate at night than at day, with youngest and oldest being most common.
- Youngers: lack of experience
- Olders : Deterioration of visual performance
Who assesses children’s vision at primary stage?
• Carried out my skilled operative, who has course of referral for full routine refraction and ophthalmoscopy
- specific tests determined by age of child
What examination technique is used for children where an optometrist tests+ their responsibilities?
• Core examination
- History, ophthalmoscopy, refraction (cyclo or equivalent), cover test, ocular motility, fusion and stereopsis, Red reflex test
• The core examiner should then either:-
- take continuous responsibility managing the child
- transfer core responsibility to another
- or discharge the child
Recommended screening procedures for children : Groups
•Neonates
• Six week old child
• Eighteen-month-old child
• Three to three/half years
• School age children
How is screening done in Neonates:
• Who: parents and GP
• What: parent should observe unusual symptoms/behaviours
- GP screen for defect in eye, ocular adnexa, history, fixation and red reflex
What do you test while screening Six-week-old child:
• Any Fx of squints, amblyopia or large refractive errors
• Any neurological/physical problem or difficult birth history - should be followed up
• All parents made aware of healthy visual development
