Vision

Question 1

What does the lens do to what you see? what must your brain then do to counteract this?

Answer 1

The image is inverted through the lens. The brain must flip it to make sense of it

Question 2

What is the measure of lens strength?

Answer 2

diopteres. A measure of lens focusing power

Question 3

How do you work out the power of a lens?

Answer 3

power = 1/focal point in metres X D
1D = a focal point of 1m away

Question 4

As the lens changes shpae, what does this do to its strength?

Answer 4

It changes

Question 5

What 2 factors determine lens strength?

Answer 5

Curvature and refractive index.

The differences in refractive index of different mediums combined with the lens curvature determines lens strength

Question 6

Where in the eye does refraction occur?

Answer 6

Initially at the cornea, then a little more at the lens

Question 7

How is focal length changed?

Answer 7

Focal length is changed by altering the shape of the lens, which is achieved through the ciliary muscle. Contraction of the muscles cause the lens to become rounder by relieving the ligament tension.
Contracting the ciliary muscles cause lens power to increase and shortens focal length for closer objects

Question 8

What are 1. Nearsightedness and 2. farsightedness called?

Answer 8

near = myopia
far = hypermetriopia

Question 9

What does it mean about the eye if you have myopia? What lens corrects this?

Answer 9

Eyeball is too long. There is too much focusing going on. A concave lens corrects this

Question 10

What does it mean about the eye if you have hypermetriopia? What lens corrects this?

Answer 10

Eyeball is too short. There is not enough refraction going on. Another lens of the same shape corrects this

Question 11

What does presbyopia mean?

Answer 11

the lens seizes up with age - it no longer bulges when ciliary muscles contract. Your near point moves further away.

Question 12

What is the first mean of adaptation to changing light levels that enter the eye?

Answer 12

Pupil diameter

Question 13

Changing pupil diameter can alter the amount of light captured by…

Answer 13

about 16 times

Question 14

What are the 2 pupillary muscles and what do they each do to the pupil?

Answer 14

Dilator muscle - larger

Sphincter pupilae - smaller pupil

Question 15

What are the 3 other benefits of smaller pupil size?

Answer 15

• Greater depth of field
• Reduced spherical aberration
• Reduced glare (scattering of light)

Question 16

What does a ophthalmoscope do?

And how can an optician use this?

Answer 16

Shines a light directly onto someones retina. It has an adjustable lens to bring the retina into focus.
Optician uses this to estimate the required spectacle strength needed

Question 17

The optic nerve carries all information from the what? where does this pass through? what does this result in?

Answer 17

from the retina
through the optic disk
results in a blind spot

Question 18

What are the 5 retinal cells?

Answer 18

Cones & rods
Horizontal cells
Bipolars
Amacrines
Ganglion cells

Question 19

What is the retina’s design flaw?

Why is this?

Answer 19

• Photo-receptors are on the outside
  light must pass through the other cell structures before reaching the photo-receptors.= scatter
  Why –> with the receptors being adjacent to the pigment epithelium may help to minimise reflectance/scatter. Also it is closer to blood supply which is required for the high metabolic rate

Question 20

The structure in front of the foveal receptors are pushed to one side (like the ganglion cells) why is this?

Answer 20

this is to reduce light scatter and therefore increases acuity

Question 21

Where in the rods and cones are the photopigment contained?

Answer 21

in the disks of the outer segment

Question 22

Where do the disks move to to get regenerated?

Answer 22

move outwards and are regenerated there

Question 23

The density of your photoreceptors in the human retina is precisely tuned to what?

Answer 23

The pointspread function of the eye

Question 24

what is the pointspread function of the eye?

Answer 24

Determines minimum separation of 2 points before they are perceived as 2 separate things

Question 25

How does density of receptors affect acuity?

Answer 25

If receptors are largely spread apart, the lower density reduces acuity. If receptors are tightly packed than it is pointless to have that many because the optics are not good enough. Density should be perfectly matched to the eye optics

Question 26

What is the mane of the photopigment in rod cells?

Answer 26

rhodopsin

Question 27

what happens when light strikes the photopigment?

Answer 27

it gets bleached.

Question 28

Once a photoreceptor is bleached, what happens to it?

Answer 28

it registers its signal and cannot register another one until it has been regenerated

Question 29

What happens if all the pigment becomes bleached?

Answer 29

You can no longer detect light. On a sunny day, all the rhodopsin becomes bleached then you rely on your cone cells.

Question 30

Explain the process of phototransduction for rhodopsin

Answer 30

1. Photopigment bleaching --> 2 molecules form to rhodopsin (retinal and opsin). Light photons interact with rhodopsin causing the 2 molecules to split (bleaching) 2. Cell membrane is hyperpolarised due to released opsin activating the enzyme PDE, which converts cGMP to GM, closing Na+ channels - hyperpolarization 3. Neural output of ganglion cell is modified --> rod/cone hyperpolarisation results in less neurotransmitter release. This modulates the membrane potential of the bipolar cell. Changing the firing rate of the ganglion cells

Question 31

How can the eye detect single photons AND work in bright sunlight?

Answer 31

scotopic & photopic vision

Question 32

what are the properties of scotopic vision?

Answer 32

``` low light vision rods only high sensitivity low acuity Non foveal in eye ```

Question 33

what are the properties of photopic vision?

Answer 33

``` Suited for high luminance (a lot of light) cones only low light sensitivity high acuity foveal + peripheral in eye ```

Question 34

What are the 4 mechanisms for how to cope with change in luminance?

Answer 34

1. pupil size 2. Switching between cones and rod cells 3. 'Dark adaptation' - bleaching 4. 'Field adaptation' (light adaptation)

Question 35

what is the cone/rod to ganglion cell ratio and what does this mean?

Answer 35

1 cone to 1 ganglion at the fovea - higher acuity | 75,000 rods to 1 ganglion cell in periphery- lower acuity

Question 36

Why is foveal vision very poor at night?

Answer 36

Because foveal vision is entirely dependent on cones

Question 37

What is dark adaptation?

Answer 37

Copes with large changes in light. Visual sensitivity gradually increases over about 20 minutes in the dark. Photopigment progressively regenerates following a bright light at time 0. Cones adapt faster but rod ultimately take over since their sensitivity threshold is much lower

Question 38

What is field adaptation?

Answer 38

there is a very quick change in sensitivity when background luminance changes It copes with fast changes in light and prevents saturation of cone cells at high light. Involves an automatic gain control process

Question 39

How does information processing change after the photoreceptor

Answer 39

- retinal processing partially reduces scenery information only to things on interest. like changes in both spatial and temporal things

Question 40

what is the concept of a receptive field?

Answer 40

The ganglion cell is the final output of the retina. You shine a light on the retina to determine the ganglion receptive field. It is the space around the ganglion of the photoreceptors that will alter the firing rate of that specific ganglion

Question 41

There are 3 channel receptor systems for colour vision. What are they?

Answer 41

1. theoretical 1 channel receptor system 2. theoretical 2 channel receptor system 3. 3 channel system

Question 42

what is the 1 channel receptor system?

Answer 42

monochrome system | responds purely to stimulus intensity

Question 43

What is the 2 channel receptor system?

Answer 43

responds to 2 aspects of stimulus intensity 2 degrees of freedom can discriminate colours and how bright or dark

Question 44

what is the 3 channel receptor system?

Answer 44

red green and blue 0 more discriminative power

Question 45

In the colour triangle. What are the 2 ways colour is defined and what do they mean?

Answer 45

saturation - strength of the colour | Hue - the colour itself

Question 46

In the colour triangle, why is there a missing segment at the green end?

Answer 46

No wavelength can stimulate green cones alone without activating red or blue at the same time

Question 47

There are 3 systems of colour opponency that are for the neural processing of colour what are they?

Answer 47

ganglion cells respond to combinations of colour, not red/green/blue alone 1. Red/green 2. Yellow/blue 2 Brightness (black/white)

Question 48

What are the 2 mechanisms for colour constanty (colours look the same despite large changes in wavelength on illuminating light)

Answer 48

1. Highly stimulated colour channels will tend to adapt and become less responsive/sensitive to this to recognise the right colour 2. Contextual cognitive cues (you know a banana is yellow)

Question 49

What are the disorders of colour vision?

Answer 49

Colour blindness Monochromaats Dichromats Anomalous Trichromats

Question 50

What can monochromats detect in vision?

Answer 50

Only have rods or cone cells, not both

Question 51

Whats are the sub categories of being a dichromat and what does this mean for their vision

Answer 51

- Only have 2 of the 3 cone cells Proranopes - no red Deuteranopes - no green Tritanopes - No blue

Question 52

Anomalous Trachromats can usually only see what?

Answer 52

Only red or green

Question 53

What is the visual pathway?

Answer 53

Left visual field enters the right brain.