The Visual System: Optics and Transduction

Question 1

What part of the electromagnetic spectrum is visible light?

Answer 1

We can see the wavelengths 400-700nm
Determined by the spectral absorbancy of the photopigments in the eye - rods and cones
Wavelength = distance between two crests/ troughs
Wavelength gives us the hue
Wave amplitude (height) gives intensity/ brightness

Question 2

What have observations using ophthalmoscopes found?

Answer 2

Observations of the fundus of the retina find retinal vessels which spread across the eye and the macula (containing the fovea with high density of cones)
Optic disk - where the cells of the retina send their axons together in the optic nerve through the optic disk. No rods or cones. In both eyes there is an area of blindness
Brain fills in this gap with surrounding patterns.
Putting something exactly the size of the optic disc within your visual field in that location makes it seemingly disappear

Question 3

How do the cornea and lens focus the visual image?

Answer 3

Light rays pass through the transparent structures (cornea, lens and fluid compartments) - cornea and lens help to focus the light on the retina at the back so it comes in a single point.

Question 4

How does the aqueous humour function to minimise optic distortion?

Answer 4

Carries oxygen and nutrients to the structures within the eye and bathes them and removes the waste products - normally carried out by the blood but this would interfere with the passage of light

Question 5

How does the vitreous humour reduce optic distortion?

Answer 5

The thick gelatinous substance accounting for 80% of eye volume that helps to maintain the shape of the eye.
Contains phagocytic cells that remove blood and other debris that might interfere with the light transmission.

Question 6

How does the pigmented epithelium prevent optic distortion?

Answer 6

A pigment that will absorb any stray light to prevent it from reflecting back into the eye and distorting the image.

Question 7

How are blood vessels arranged to minimise optic distortion?

Answer 7

Blood vessels avoid the fovea at the center of the macula that provides the greatest detail of visual acuity

Question 8

How is the eye able to move to focus light on the fovea?

Answer 8

The eyes are suspended in the orbits of the skull
Each is moved by 6 extra ocular muscles
Attached to the sclera (tough fibrous outer coating of the eye)
Need to track objects
Must land on the fovea in order to get the highest acuity vision.
Human vision is frontal facing and foveally dominated

Question 9

What is vergence of the eyes?

Answer 9

Simultaneous movement of both eyes with the function of pointing the fovea of each on a nearby object

Question 10

what is convergence of the eyes?

Answer 10

both rotate inward (close objects)

Right eye rotates left and left rotate right

Question 11

What is divergence of the eyes?

Answer 11

both eyes rotate outward

Question 12

How are rods and cones distributed in the retina?

Answer 12

greatest concentration of cones in the fovea (where there are no rods) as visual acuity is the priority
rods peak at 15° of eccentricity and are most dense in the periphery (there are cones here though)

Question 13

What are saccades?

Answer 13

movements of the eyes to small bits of interest to focus the fovea on individual points in order to perceive a scene with greater resolution
constantly make eye movements to fixate scene of interest onto the fovea
Areas that need more visual analysis are looked at more

Question 14

What is the function of the pigment epithelium?

Answer 14

it continually replaces the discs of the photoreceptors that are shed as they move to the tip of the outer segment
tips and outer segments containing photoreceptors are replaced every 12 days
Pigment epithelium regenerates the photopigment molecules that have been exposed to light when the rods and cones have been bleached

Question 15

How are cells in the retina ‘nourished’?

Answer 15

blood supply from the capillaries in the choroid

Question 16

Where is the fovea located?

Answer 16

5’ of visual angle at a region of 1.5mm

Question 17

How is the structure of the fovea different from the rest of the retina?

Answer 17

High density of cones that are tapered - allowing for very close packing
Other cells pushed to the side so that the path of light goes straight to the photoreceptors at the back and is less distorted.
Absence of blood vessels so that the light path won’t be disturbed

Question 18

Why is light refracted in the eye?

Answer 18

It is moving from air to fluid chambers in the eye which have different densities so light moves at different speeds

Question 19

How is the eye adapted to accommodate for refraction?

Answer 19

Need to bend light so that it meets at a single point at the back of the retina
so the surface of the cornea is smooth and the eye is spherical

Question 20

What is the optical power of the eye?

Answer 20

Optical power determines the angle of refraction
P=µ/f
optical power = refractive index of the media of the eye / focal length of the eye
P= 60 diopters
refractive power of the cornea = 42 diopters(non adjustable)
Refractive power of the lens = 18 diopters (adjustable - accommodation of distance of the object)

Question 21

what is visual acuity?

Answer 21

Measure of the spatial resolution of the eye and how well it resolves fine detail
Defines as the angle subtended at the eye by two points which are perceived as separate

Question 22

What is a snellen chart

Answer 22

tests visual acuity
Conditions: in bright light with high contrast (B&W) letters (optotype)
Normal visual acuity 20/20 - letters with gaps about 1 minute of arc (1/60 degree) can just be read.
Under ideal conditions, gaps of 0.5 min can be resolved
Image of a gap of 0.5 min on the retina is 2 microns - the width of an individual foveal cone outer segment
Hence acuity is virtually limited by the mosaic of cones.

Question 23

What does the iris do

Answer 23

Controls the size of the pupil and the amount of light entering the eye
Reduce optical areas and increase the depth of focus when constricted
When you close the eye down there are fewer rays of light coming in that can have aberrations which would cause blurring on the back of the eye.
Control amount of light and the optical errors .’. Can make ability to focus on something better. Need to balance

Question 24

What is the accommodation reflex?

Answer 24

Change the thickness of the lens depending on how far away an object is so that the rays are refracted to the same single point

Close object
rays are diverging
need more refraction
ciliary muscles contract
suspensory ligaments relax (as tension is released)
lens bulges 

Distant focus
rays are parallel
ciliary muscles are relaxed
pulls suspensory ligaments taunt 
lens is pulled flat

reflex mediated by parasympathetic fibres and monitored by the visual cortex

Question 25

How does the close focus change with age

Answer 25

The ability to accommodate to near vision changes with age
The near point - the closest an object can be and still be in focus
The most contracted the ciliary muscles can be and the most rounded the lens can be
Approx 25mm - declines with age (require reading glasses, unavoidable) Presbyopia
Highest in babies, as people age ability to accommodate for near vision gets worse
Near point recedes so object must be held further away in order to focus
Either due to decreased elasticity in lens or loss of power in ciliary muscles

Question 26

What is myopia?

Answer 26

Short sightedness
Near objects in focus and distant objects blurred
Size on blur depends on how open the pupil is
rays converge too early resulting in a cross over causing blur in long distance vision
genetic or lack of natural daylight
One of the strongest and most replicated risk factors is educational attainment
Very young children who look at books close up rather than playing outdoors
Increased time outdoors is effective in preventing the onset of myopia but is not effective in slowing the progression in eyes that are already myopic

Question 27

What is hypermetropia?

Answer 27

long sightedness
Near objects are blurred, distant objects are focused
Rays of light crossover behind retina causing blur
Corrected with convex lens so there is already some convergence of rays of light before entering
Easily fixed by glasses/ contact lenses

Question 28

How does the environment effect the development of vision?

Answer 28

The visual environment exerts a powerful influence on refractive state by controlling the axial length of the eye during the postnatal developmental period.

Question 29

What is an astigmatism?

Answer 29

Results from the difference in the degree of curvature in different planes
Normal cornea has a spherical surface - refracts light to a single point on the back of the retina
Astigmatic cornea has a rugby shaped surface - multiple focal points causing a blur on the back of the retina. A horizontal plane might be in focus and a vertical plane may be blurry for example.
Causes blurred vision in both distance and near vision
Tested using an astigmatism chart - different lines with different angles
Not all the lines will be in focus
Corrected by spectacles that have a cylindrical component in their curved surfaces
Cylindrical lens allow the different lines to be in focus at the same time.

Question 30

What is a posterior vitreous detatchment?

Answer 30

Associated with the vitreous humour
As you age, the VH becomes less gel like and more watery so it isn’t able to keep its usual shape causing it to move away from the retina at the back of the eye towards the centre of the eye
Causes flashes of light, floaters and cobwebs in the vitreous humour can affect image quality
Small flashes of light: as the vitreous detaches it pulls on the retina, the retina sends electrical charges to the brain, so you see small flashes of light
Floaters: as the VH becomes more watery, small harmless clumps of cells develop which obscure image - can move around or remain static but are very distracting.
Cobweb: as the VH pulls away from the retina may see a thicker outer edge of the VH which can change the way the light moves into the eye which can feel like looking through a cobweb
Different visual effects that people can see - can’t be corrected at higher levels of the brain so its necessary to generate the clear image at the eye itself

Question 31

What are cataracts?

Answer 31

Lens develops cloudy patches which become bigger over time
Causes blurred vision and can lead to blindness
Caused by genetics, aging, environmental, sunlight, dehydration or injury
Light is spread around and obstructed rather than having a clear passage through.
Congenital - number of genes identified including mutations in crystalins
Straightforward to treat by removing the lens surgically and replacing it with an artificial lens. NHS provides distance or near vision correction.

Question 32

What is glaucoma?

Answer 32

Normally we have a production of aqueous humour into the front of the eye and draining out through the canal of schlemme
This normal production and drainage helps to maintain the shape at the front of the eye
If it doesn’t drain pressure within the eyeball increases which damages and squeezes the optic nerve and rods
Lose rod vision first .’. lose peripheral vision first
Once optic nerve is damaged the fibres are permanently damaged
Treatment to prevent more damage by prescribing medication that improves drainage or laser treatment or surgery

Question 33

What is Retinitis Pigmentosa?

Answer 33

Inherited degenerative eye disease causing the loss of photoreceptors
Damage and loss of photoreceptors can reduce the visual field
First sign is loss of peripheral vision as it effects the rods first - leading to tunnel vision.
Eventually leaves to total blindness and there’s no cure.
Caused by the pigment epithelial cells not being able to phagocytose the photoreceptor outer segments as they normally would do.
Accumulation of debris and separation of photoreceptors from the pigmented epithelium

Question 34

Summary

Answer 34

light enters the eye and is refracted
the structure of the eye supports the free passage of light to the retina
a clear image relies on functioning optics
pathological conditions and errors of refraction can prevent the formation of a clear image which cannot be improved at higher levels of the visual pathway
light is received by the retina through its photoreceptors and transduced into electrochemical energy