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Psychophysics H > Image formation and the eye > Flashcards

Image formation and the eye Flashcards

1
Q

what is the definition of visual perception

A

it is the mental image interpretation of the optical image of our surroundings

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2
Q

list the three series of processes that make up a visual perception

A
  1. image is formed at the back of the eye
  2. neural processing: starting at photoreceptor level, then behind to the v1 cortex
  3. percept: a perception of whats around
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3
Q

what 2 things work hand in hand to make a visual perception

A

the visual side and the neural side, work hand in hand

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4
Q

list some pathological things that can cause a less than optical image at the back of the eye, and what problem dies this result in

A
  • irregular corneas
  • dystrophies
  • keratoconous
  • cataracts
    can all cause light scatter and produce a less than optimal image at the back of our eyes
    so this cannot produce a perfect visual percept
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5
Q

list the 7 ways of investigating image formation in vivo

A
  • retinoscopy: gets rid of problem of poor image quality by correcting the sph and cyl
  • aberrometry
  • modulation transfer function (MTF)
  • neural contrast threshold (NCT)
  • phakometry (lens)
  • keratoscopy (cornea)
  • OCT (anterior chamber)
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6
Q

name a way of investigating image formation in vitro

A

refractive index profile of cornea and lens (as it is not uniformal)

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7
Q

list the three models of investigating image formation

A
  • reduced eye model - 1 surface
  • paraxial schematic eyes with 3-6 surfaces
  • wide angle eye models with aspheric surfaces and refractive index gradients
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8
Q

list the 5 things that affects optical performance and therefore limits visual perception

A
  • aberrations
  • MTF
  • diffraction
  • light levels - pupil size
  • light scatter
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9
Q

what are the 2 main performances that can have limits on visual perception

A
  • optical performance

- detector performance

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10
Q

list the 3 things that affects detector performance and therefore limits visual perception

A
  • ‘pixels’ (photoreceptors)
  • retinal connections - receptive fields
  • variation with light levels
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11
Q

what type of optical system is the eye

A

a general purpose, very wide angles optical system

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12
Q

what can the eye resolve at high contrast

A

can resolve 36cpd = 6/5 snellen or 20/17

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13
Q

what is 36cpd = 6/5 snellen or 20/17 that the eye can resolve in high contrast equivalent to at the retina

A

equivalent to 120 cycles per mm at the retina

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14
Q

what are we better at resolving over letters

A

better at resolving gratings

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15
Q

what design of the eye gives us wide viewing angles

A

the eye’s concentric design

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16
Q

what does the wavefront aberration W =

A

the optical difference between real and ideal wavefront

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17
Q

what is the ideal wavefront

A

a plane wavefront, where the rays are parallel

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18
Q

what does wavefront aberration measure

A

measures optical performance/quality of the optics of the eye

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19
Q

how can wavefront aberration quantify the measurement of the optical performance/quality of the optics of the eye, and what can it create as a result from this

A

can quantify by adding different amounts/types of aberrations
and can mix them in different amounts to create e.g. an intraocular lens specifically to correct this aberration

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20
Q

what are colour coded maps of wavefront aberrations W, composed of and what do they show

A

composed of/uses different colours, which shows that the optical surface is not flat
the stronger colour changes = stronger changes to the surface = higher aberration

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21
Q

what do stronger colour changes i.e. stronger changes to the optical surface indicate in wavefront aberration colour coded maps

A

higher aberrations

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22
Q

what 2 things does wavefront aberrations W, represent

A

refractive error and higher order aberrations

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23
Q

list the 6 different types of aberrometry

A
  • hartmann-shack
  • laser ray tracing
  • tscherning
  • skiascopy
  • spatially resolved refractometry
  • double pass methods
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24
Q

what is the principles of the hartmann-shack method of aberrometry

A
  • wavefronts that are coming out of the eye is interpreted by small grids of lenses
  • each lens then splits up and produces small foci produced by the lens
  • if get a flat wavefront, it will be a regular grid pattern
  • if wavefront has aberrations, then the spots get displaced
  • and can measure the displacements
  • these wavefronts can be turned into a colour coded map
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25
Q

what type of grid pattern will a flat wavefront produce in the hartmann-shack method of aberrometry

A

a regular grid pattern of spots

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26
Q

what type of grid pattern will a wavefront with aberrations produce in the hartmann-shack method of aberrometry and how can this be quantified

A

the spots on the grid get displaced, and can measure these displacements in order to quantify the wavefront aberrations

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27
Q

list the 4 properties of aberrations in normals

A
  • a lot of inter subject variability with spherical aberration
  • we do not have much monochromatic aberration
  • we do have chromatic aberrations
  • in normals they are good and realistic - but in pathology these findings can change a lot
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28
Q

what is a complete wavefront aberration a combination of

A

the cornea and lens (both contribute to a complete wavefront aberration)

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29
Q

in which types of people is wavefront aberration much lower in and why

A

in young eyes
as the lens compensates for the corneal spherical aberration, but in older patients the compensation reduces = spherical aberrations in higher

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30
Q

which type of spherical aberration NOT change with age

A

corneal spherical aberration

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31
Q

what does the whole eye spherical aberration increase with, and what trend does this show

A

increases with age, shows a positive trend

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32
Q

what causes spherical aberration to increase with age

A

the reduction in compensation by the lens

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33
Q

what is still variable in spherical aberration with age

A

a lot of individual variability

34
Q

what causes off axis aberrations

A

astigmatism = significant off axis aberrations

35
Q

what do we get when we go to increasing eccentricity i.e. away from the fovea

A

oblique astigmatism/aberrations as we go from +/- eccentricity from the fovea = U shape
is very common in our optical systems

36
Q

what is peripheral aberrations and what does it affect

A

is a peripheral image quality and partially affects myopia progression

37
Q

what are produced that can be used to prevent off axis aberrations to control myopia progression

A

contact lenses

38
Q

what does the pupil form

A

and approx. circular entrance pupil

39
Q

what do all apertures/edges do with light and what does this cause

A

diffract light

this causes an imperfect image which is present all the time and we can’t do anything about it

40
Q

what will the image of a point source be with no aberrations present (and only diffraction present)

A

point source = an airy pattern/disc

41
Q

what is diffraction a potential limiter of

A

of what we can see

42
Q

what is point resolution

A

how close two points can be seen as two separate points

the shapes i.e. bright centres with rings around them cause diffraction

43
Q

what does working out the minimum angle of resolution according to the rayleigh criterion state

A

diffraction predicts we will get 6/6 vision acuity with a 2mm pupil and as it gets larger, we will get aberrations

44
Q

which pupil diameter does the rayleigh criterion state will start producing aberrations

A

> 2mm

45
Q

what does diffraction have interactions with

A

photoreceptors

46
Q

what is always present, even if our optical system is perfect

A

diffraction

47
Q

what can aberrations have an effect on with large pupil sizes

A

contrast, the MTF decreases

48
Q

what is MTF

A

the measure of the quality of the optical system

49
Q

what does MTF not account for

A

changes in the photoreceptor sensitivity/neural response or ambient light levels

only purely about the quality of the optics of the eye

50
Q

which diameter pupil pupil does not have too much of an effect on image quality

A

3 and 4 mm pupil

51
Q

which diameter pupil does a decline in image quality start to show

A

5mm pupil (the decline in image quality jumps from 3/4mm to 5mm i.e. is non linear)

52
Q

what is evident in a 6mm pupil

A

more aberrations are present = poorer image performance

shows a non-linear change, which is shown in the difference in the curves in the graph

53
Q

between what diameters is the pupil in a lot of common lighting conditions

A

2-4mm

54
Q

what is the ~ pupil size in moonlight

A

~5mm

55
Q

how much of luminance is a pupil diameter of 6mm equivalent to

A

-2.50 log10 luminance

56
Q

what does light scatter produce more of in comparison to diffraction

A

the spreading of light on a wider scale than diffraction

57
Q

what are the major components of light scatter and which one is the main cause

A

light diffracted/refracted by local changes in refractive index e.g. the cornea and lens

crystalline lens is the main cause of light scatter

58
Q

when is light scatter mainly caused by the crystalline lens

A

in cataracts, the light scatter is caused by the build up of protein aggregates in the lens

59
Q

why does cataract cause adverse affects on vision and reduced contrast

A

because the cataract causes light to flow more widely

60
Q

what is the reason if you can’t see anything in slit lamp due to a cataract

A

due to light scattered out of the eye

61
Q

what does stray light have an effect on

A

vision

62
Q

what is stray light

A

a visually perceived effect of light scatter

63
Q

what is the cause of light scatter

A

light coming from a source e.g. a peripheral source that can hit the lens and the lens scatters the light in lots of different directions, even where you don’t want

64
Q

what can more glare/light scatter cause with contrast and what is the effect on vision

A

can cause reduced/lower contrast
so cannot see e.g. a person as well, so theres a threshold you need to exceed in order to see well enough, but light scatter can push you below that threshold

65
Q

what type of changes with age causes increased light scatter/straylight

A

mostly due to lens changes, not only cataract, but general changes, so can be pre cataract

66
Q

what can be done in order to abolish light scatter/straylight

A

replace the affected media e.g. intraocular lens for cataract surgery or conceal graft

67
Q

when is light scatter/straylight significantly increased

A

in certain pathologies that affect clarity of media e.g. cataract and corneal disease e.g. corneal endothelial dystrophy or scarring involving keratoconous

68
Q

how large is a cone diameter at the fovea

A

~2 microns (varies)

69
Q

what megapixel array is a ~2 micron cone diameter equivalent to

A

72 megapixel array

70
Q

what can reduce a ~2 micron cone diameter from being equivalent to a 72 megapixel array

A

limting factors in the eye e.g. the lens

71
Q

how much higher than a very high resolution camera sensor, is a 72 megapixel array caused from the ~2 micron cone diameters at the fovea

A

3x higher

72
Q

what does our media pixel value change with

A

eccentricity (goes down)

73
Q

list 4 variations with photoreceptors that occur with eccentricity

A
  • increase in cone size at the parafovea
  • cone density falls away from fovea quickly
  • rapid changes in cone and rod density
  • pooling of photoreceptor outputs in the peripheral retina - receptive fields
74
Q

what is a spatial receptive field

A

an area in space that results in a response from a neuron if a stimulus is presented within that area

i.e. if you have a stimulus in that same area, the receptive field will generate the same response

75
Q

how are the receptive fields organised in the centre of the fovea, what effect does this have if you move a stimulus

A

there are lots of receptive fields in the centre of the fovea, so if you move a stimulus slightly, you will notice the difference

76
Q

how are the receptive fields organised in the periphery of the fovea, what effect does this have if you move a stimulus

A

the receptive fields are larger at the periphery, so need larger movements of the stimulus to notice a change

77
Q

what is peripheral vision NOT limited by, and what is it limited by instead

A

NOT limited by optics

it is limited by retinal sampling (RF’s getting larger = being the limiting factor)

78
Q

what 2 things is foveal vision optically limited by

A

diffraction
and
aberration (at lower light levels)

79
Q

how is peripheral vision limited by photoreceptor pooling

A

photo receptor pooling gives receptive fields that exceed the optical point spread function

80
Q

what 2 things increase with peripheral vision/eccentricity

A

aberrations and receptive field sizes

81
Q

what does neural contrast threshold reduce with and why

A

retinal illuminance (our sensitivity goes down with higher light levels) which is expected as the sensitivity of photoreceptors changes

82
Q

what does neural contrast threshold combine with, to give our visual performance

A

combines with the optics of our eyes to give our visual performance

Psychophysics H (11 decks)

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