Chapter 6 - The Eye and Vision Flashcards
What part of the brain processes information from the eyes?
The visual cortex
What part of the eye is the cornea?
The translucent part of the front of the eye which allows light to enter the eye.
How much focussing is the cornea responsible for?
70-80%
What is the liquid in front of the lense of the eye called?
The Aqueous Humour
What is the liquid behind the lense of the eye called?
The Vitreous humour
What is the role of the iris in the eye?
To contract and expand to regulate the amount of light entering the eye.
What is the pupil in the eye?
The hole/gap in the iris, where light enters the eye.
How does an objects distance affect the pupil size?
An object further away requires more light and so a larger pupil and vice versa.
What is the role of the lens in the eye?
It is the final focussing point, and focusses light onto the retina.
What is the role of the ciliary muscle in the eye and what is the result called?
It changes the shape of the lens by contracting.
This is called accomodation.
What is accomodation of the lens affected by?
Aging
Fatigue (tiredness)
What happens during accomodation for distant and close objects?
Distant:
Ciliary muscles relax, suspensory ligaments pull against lens, lens becomes flatter
Close:
Ciliary muscles contract, suspensory ligaments relax, lens becomes thicker and rounder.
What is the retina in the eye?
It is the light sensitive inner lining that contains photoreceptors (cones/rods)
What is the sclera in the eye?
The outer layer that makes the “white” of your eye.
What are cones and rods in the eye and what do they do?
Light sensitive cells/photoreceptors.
They convert light into electrical impulses to be sent to the brain.
What is the fovea in the eye?
The part of the retina that only contains cones and has a very high conventration of them (150000/mm).
It is the part where the most detail can be seen in day light.
What is the distribution of rods and cones on the retina?
At the fovea the concentration of cones is very high but as soon as you move away the concentration decreases rapidly.
Around the edge of the fovea is where the highest concentration of rods are and as you go further away the concentration slowly decreases.
There are no rods in the fovea.
What causes the blind spot in the eye?
The position where the optic nerve joins the eye causes there to be no photoreceptors at that spot.
What are the features of cones in the eye?
Colour sensitive
Work in bright light
Responsible for central vision (acuity and enables detail, colour and movement to be seen)
What are the features of rods in the eye?
See black and white
Sensitive in low light
Very sensitive to movement
Used for peripherals and night vision
Why is night vision affected by hypoxia?
Because the photoreceptors are the second part of the body most hungry for oxygen (brain being first)
What is vision through rods called?
Scotopic vision
What is vision through cones called?
Photopic vision
How long does it take rods and cones to adapt from dark to light conditions?
Around 10 seconds.
How long does it take for rods and cones to adapt in light to dark conditions?
Cones - 7 mins
Rods - 30 mins
What happens to the pupil with changes in light?
More light smaller pupil and vice versa
What is the best way to look at something at night and why?
Look away by about 10-15deg, because the area of most rods on the retina is around 10-15 deg away from the fovea.
What factors affect night vision?
Age
Mild hypoxia
Cabin alt above 5000ft
Smoking (20 cigs/day degrades by 20%)
Alcohol
Vit A deficiency
How can you optimise night vision?
Spend time getting adapted
Dont focus on point being observed
Avoid blinding sources of light
What are the 3 types of eye movement?
Saccade - Jerky, involuntary movement
Attention - moving eyes to place image over fovea
Pursuit - Tracking moving object
When measuring visual acuity, what does 20/20 or 6/6 mean?
The first number represents what you can see at 20 ft (6m) and the second number represents the distance a normal person would be able to see the same thing.
Where on the retina does visual acuity fall to 50%
5 deg from the fovea.
Where on the retina does visual acuity fall to 10%
25 deg from the fovea
What factors affect visual acuity?
Angular distance from fovea
Physical imperfections within the visual system
Size and contours of the object being observed
Distance to object
Contrast of object with background
Relative motion of moving object
Clarity of atmosphere (dust)
Age
Hypoxia
Anemia
Smoking (CO poisoning)
Alcohol
What is the visual field of the eye?
Central vision + peripheral vision.
Each eye has:
120deg lateral field
150deg vertical field
What are the features of binocular vision?
Sterioscopic vision
Depth perception but only effective to about 60m.
What is the condition called where vision in each eye differs?
Stereopsis
What cues does the brain use for depth perception?
Colour
Contrast
Size
Rate of movement
What is empty field myopia, what dangers does it pose and what can be done to prevent it?
Where you dont see distant objects due to not being at the appropriate focal point.
You dont identify normally obvious objects as they may be blurred or out of focus.
Use short sharp scans or periodically focus on a distant object such as a wing tip or cloud edge.
What is the natural focus for an eye at rest?
1-2m
What is a good visual scanning technique for flying?
Regular eye movement in overlapping sectors of 10deg over entire field of vision.
What is hypermetropia/hyperopia and how can you correct it?
Long sightedness caused by the focus point of the lense being behind the retina due to a short eye ball.
Can be corrented with a convex lens.
What is myopia and how can you correct it?
Short sightedness caused by the focus point of the lense being infront of the retina due to an elongated eye ball.
Can be corrented with a concave lens.
What is presbyopia?
Long sightedness due to age.
The elasticity of the ciliary muscle becomes less flexible and so accomodation decreases.
What is astigmatism how does it occur and what can be done about it?
Where a misshapen cornea or lens can cause difficulty in resolving fine detail.
It most commonly occurs with age and can be corrected via a tailored lense.
What is cataracts, how can it happen and how can it be fixed?
A cloudiness of the lens in the eyes, which can lead to blindness.
It can happen due to age and also can be brought on by chronic exposure to UV light.
Can be fixed with lazer treatment.
What is Glaucoma, what are it’s symptoms and how can it be treated?
A pressure rise of liquid in the eye.
Symptoms:
Acute pain in eye (extreme cases); Blurred vision; Sensitivity to brightness; Progressivly narrowing visual field; Red dicolouration in the eye.
Can be treated with eye drops or surgery.
Why might glaucoma be unnoticed until it is too late and what can happen if left untreated?
It has insidious onset and concealed progression.
Can damage optic nerve and lead to blindness.
What can cause colour vision defficiency/colour blindness?
Genetics mainly
Other causes:
Underlying health condition (eg. diabetes, glaucoma); Side effect of medication; exposure to harmful chemicals (carbon disulphate, styrene)
What is the Ishihara test?
The test that tests for colour blindness.
What is flash blindness?
Where bright light can cause temporary blindess (eg lightning while night flying)
What should you do to avoid flash blindness while flying near a thunderstorm with lightning?
Turn cockpit lighting to max
Look inside the cockpit
Wear sunglasses
Use blinds or curtains when installed
What is a requirement for a pilot who has to wear corrective glasses/contact lenses?
They must carry a spare pair of glasses while flying.
What are some issues regarding wearing contact lenses?
They can cause damage to cornea in low humidity.
May be dislodged by rubbing the eye, G-force or accidental knock.
Bi-focal contact lenses are prohibited.
At more than 450 kts at low level, how long does it take to go through the visual perception cascade (visual input, brain reaction, perception, recognition)?
About 1 second
At more than 450 kts at low level, how long does it take to go through the visual perception cascade (visual input, brain reaction, perception, recognition) followed by evalutation, decision, action & response?
Around 5-7 seconds.
What are the requirements for sunglasses?
Impact resistant
Thin metal frames
Coated with polycarbonate for strength
Good optical quality
Lumimance transmittance of 10-15%
Appropriate filtration characteristics
Why may it be important to wear sunglasses in the cockpit?
Light at high altitude is more damaging to retina as it contains more blue and UV light.
Are photochromic (light sensitive lenses) allowed to be warn in the cockpit?
Generally, no.
What can be said about wearing polarised sunglasses in the cockpit?
It is discouraged as they can cause distortion from certain luminated cockpit windscreens and may also alter cloud appearance.
