Vision Flashcards
What does the lens do to what you see? what must your brain then do to counteract this?
The image is inverted through the lens. The brain must flip it to make sense of it
What is the measure of lens strength?
diopteres. A measure of lens focusing power
How do you work out the power of a lens?
power = 1/focal point in metres X D 1D = a focal point of 1m away
As the lens changes shpae, what does this do to its strength?
It changes
What 2 factors determine lens strength?
Curvature and refractive index.
The differences in refractive index of different mediums combined with the lens curvature determines lens strength
Where in the eye does refraction occur?
Initially at the cornea, then a little more at the lens
How is focal length changed?
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
What are 1. Nearsightedness and 2. farsightedness called?
near = myopia far = hypermetriopia
What does it mean about the eye if you have myopia? What lens corrects this?
Eyeball is too long. There is too much focusing going on. A concave lens corrects this
What does it mean about the eye if you have hypermetriopia? What lens corrects this?
Eyeball is too short. There is not enough refraction going on. Another lens of the same shape corrects this
What does presbyopia mean?
the lens seizes up with age - it no longer bulges when ciliary muscles contract. Your near point moves further away.
What is the first mean of adaptation to changing light levels that enter the eye?
Pupil diameter
Changing pupil diameter can alter the amount of light captured by…
about 16 times
What are the 2 pupillary muscles and what do they each do to the pupil?
Dilator muscle - larger
Sphincter pupilae - smaller pupil
What are the 3 other benefits of smaller pupil size?
- Greater depth of field
- Reduced spherical aberration
- Reduced glare (scattering of light)
What does a ophthalmoscope do?
And how can an optician use this?
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
The optic nerve carries all information from the what? where does this pass through? what does this result in?
from the retina
through the optic disk
results in a blind spot
What are the 5 retinal cells?
Cones & rods Horizontal cells Bipolars Amacrines Ganglion cells
What is the retina’s design flaw?
Why is this?
- 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
The structure in front of the foveal receptors are pushed to one side (like the ganglion cells) why is this?
this is to reduce light scatter and therefore increases acuity
Where in the rods and cones are the photopigment contained?
in the disks of the outer segment
Where do the disks move to to get regenerated?
move outwards and are regenerated there
The density of your photoreceptors in the human retina is precisely tuned to what?
The pointspread function of the eye
what is the pointspread function of the eye?
Determines minimum separation of 2 points before they are perceived as 2 separate things
How does density of receptors affect acuity?
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
What is the mane of the photopigment in rod cells?
rhodopsin
what happens when light strikes the photopigment?
it gets bleached.
Once a photoreceptor is bleached, what happens to it?
it registers its signal and cannot register another one until it has been regenerated
What happens if all the pigment becomes bleached?
You can no longer detect light. On a sunny day, all the rhodopsin becomes bleached then you rely on your cone cells.
Explain the process of phototransduction for rhodopsin
- Photopigment bleaching –> 2 molecules form to rhodopsin (retinal and opsin). Light photons interact with rhodopsin causing the 2 molecules to split (bleaching)
- Cell membrane is hyperpolarised due to released opsin activating the enzyme PDE, which converts cGMP to GM, closing Na+ channels - hyperpolarization
- 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
How can the eye detect single photons AND work in bright sunlight?
scotopic & photopic vision
what are the properties of scotopic vision?
low light vision rods only high sensitivity low acuity Non foveal in eye
what are the properties of photopic vision?
Suited for high luminance (a lot of light) cones only low light sensitivity high acuity foveal + peripheral in eye
What are the 4 mechanisms for how to cope with change in luminance?
- pupil size
- Switching between cones and rod cells
- ‘Dark adaptation’ - bleaching
- ‘Field adaptation’ (light adaptation)
what is the cone/rod to ganglion cell ratio and what does this mean?
1 cone to 1 ganglion at the fovea - higher acuity
75,000 rods to 1 ganglion cell in periphery- lower acuity
Why is foveal vision very poor at night?
Because foveal vision is entirely dependent on cones
What is dark adaptation?
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
What is field adaptation?
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
How does information processing change after the photoreceptor
- retinal processing partially reduces scenery information only to things on interest. like changes in both spatial and temporal things
what is the concept of a receptive field?
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
There are 3 channel receptor systems for colour vision. What are they?
- theoretical 1 channel receptor system
- theoretical 2 channel receptor system
- 3 channel system
what is the 1 channel receptor system?
monochrome system
responds purely to stimulus intensity
What is the 2 channel receptor system?
responds to 2 aspects of stimulus intensity
2 degrees of freedom
can discriminate colours and how bright or dark
what is the 3 channel receptor system?
red green and blue 0 more discriminative power
In the colour triangle. What are the 2 ways colour is defined and what do they mean?
saturation - strength of the colour
Hue - the colour itself
In the colour triangle, why is there a missing segment at the green end?
No wavelength can stimulate green cones alone without activating red or blue at the same time
There are 3 systems of colour opponency that are for the neural processing of colour what are they?
ganglion cells respond to combinations of colour, not red/green/blue alone
1. Red/green
2. Yellow/blue
2 Brightness (black/white)
What are the 2 mechanisms for colour constanty (colours look the same despite large changes in wavelength on illuminating light)
- Highly stimulated colour channels will tend to adapt and become less responsive/sensitive to this to recognise the right colour
- Contextual cognitive cues (you know a banana is yellow)
What are the disorders of colour vision?
Colour blindness
Monochromaats
Dichromats
Anomalous Trichromats
What can monochromats detect in vision?
Only have rods or cone cells, not both
Whats are the sub categories of being a dichromat and what does this mean for their vision
- Only have 2 of the 3 cone cells
Proranopes - no red
Deuteranopes - no green
Tritanopes - No blue
Anomalous Trachromats can usually only see what?
Only red or green
What is the visual pathway?
Left visual field enters the right brain.
