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Human basic neuroscience > Visual system- optics and transduction > Flashcards

Visual system- optics and transduction Flashcards

1
Q

How much of our cerebral cortex does our visual system use

A

over 1/3

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2
Q

What actually is light

A

Electromagnetic radiation

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3
Q

What is the wavelength range of visible light

A

400-700nm

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4
Q

What does the wavelength of light determine

A

Hue- red light has the longest wavelength, violet the shortest

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5
Q

What does the wave amplitude of light determine

A

Intensity or brightness

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6
Q

What primarily determines the width of the visible light spectrum

A

The spectral absorbances of the photopigments in the eye

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7
Q

What allows us to view the back of the eye

A

Opthalmoscope or fundus camera

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8
Q

What is the macula

A

A spot at the centre of the retina, focused on clear straight-ahead vision

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9
Q

What is within the macula

A

The fovea- very high cone density, used for high acuity vision

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10
Q

What is the optic disk

A

Where cells of retina send their axons together in the optic nerve through the optic disk to the brain

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11
Q

What is the retina

A

Light sensing part of the eye

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12
Q

What causes our blind spot

A

The optic disk has no rods and cones as it is where the optic nerve leaves the eye

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13
Q

Why don’t we see our blind spot

A

The brain fills in the hole with surrounding patterns

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14
Q

Where is the aqueous humour vs vitreous humour

A

Aqueous humour- between the iris and the inner surface of the cornea
Vitreous humour- between the lens and optic nerve

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15
Q

What does the aqueous humour do

A

Carries O2 and nutrients to the structures within the eye, bathes them, and removes waste products (blood would prevent transparency)

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16
Q

What is vitreous humour

A

Gelatnious substance that accounts for 80% of the eye’s volume, fills the space between the lens and optic nerve

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17
Q

What does the vitreous humour do

A

Helps maintain the spherical shape of the eye, contains phagocytic cells that remove blood and debri that might interfere with light transmission

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18
Q

What is choroid

A

A pigment within the pigmeted epithelium that helps absorb any stray light to prevent it reflecting back into the eye and distorting the image

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19
Q

Where are the eyes suspended

A

In orbits of the skull

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20
Q

What are the extraocular muscles

A

Attached to the sclera- we use them to move our eye all the time eg tracking objects allows light to come into the fovea for high acuity vision

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21
Q

What do light rays pass through as they travel through the eye

A

Transparent structures- the cornea, lens, fluid compartments

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22
Q

What keeps the vascularity of the eye to a minimum

A

Cleansing function of aqueous humour, function of the choroid in nourishing the retinal layers

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23
Q

Where in the eye do blood vessels avoid

A

Avoid the fovea at the centre of the macula that provides the greatest visual acuity

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24
Q

What features minimise optic distortion

A

Transparet structures, minimised vascularity, no blood vessels in fovea, pigmented epithelium

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25
Q

What does it mean that human vision is frontal facing and foveally dominated

A

When we move our eyes we are bringing the object of interest to bear on the fovea

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26
Q

What is vergence

A

Simultaneous movement of both eyes with the functino of pointing the fovea of each eye on an area of visual interest

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27
Q

What is convergence

A

Inward rotation of both eyes so fixate on an object

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28
Q

What is divergence

A

Outward rotation of both eyes to focus on an object

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29
Q

How are rods and cones distributed in the retina

A

Cones are in the fovea of retina, rods are very high in periphery of the fovea but completely absent in the fovea (Osterberg, 1935)

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30
Q

What are saccadic eye movements

A

Small jerky eye movements as you move your eyes eg across a page- allows you to focus on small bits of interest in the scene with greater resolution

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31
Q

Where are photoreceptors located in the eye

A

At the back of the eye at the back of a 3 neuron relay (so behind bipolar, ganglion cells etc)

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32
Q

Why are photoreceptors at the back of the 3 neuron relay

A

They need pigmented epithelium to continually replace disks (every 12 days) of photoreceptors that are shed as they move to the tip of the outer segment, and to regenerate bleached photopigment molecules

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33
Q

What is the diameter of the fovea

A

1.5mm (5 degrees of visual angle)

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34
Q

How are cones organised in the centre of the fovea to allow close packing

A

Those in the center are tapered

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35
Q

How does the arrangement of retinal nerve bodies in the fovea allow a direct path for light

A

Retinal nerve cell bodies either side of the fovea are shifted aside, so light has a direct path to photoreceptors

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36
Q

What does it mean to say light refracts

A

It changes direction as a result of entering a medium with a different density

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37
Q

What is the purpose of refraction of light in the eye

A

Bend the light entering the eye so it forms a single point on the back of the retina

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38
Q

What formula gives the optical refractive power of the eye

A

P= uf
u is refractive incidence of the media of the eye
f= focal length of the eye

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39
Q

What real numbers are plugged into the formula to give the optical refractive power of the eye

A 
f= 22mm
u= 1.33
P= 60 diopters
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40
Q

What is the refractive power of the cornea

A

42 diopters (non-adjustable)

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41
Q

What is the refractive power of the lens

A

18 diopters (adjustable via accomodation)

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42
Q

What is spatial resolution

A

How well fine details can be resolved

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43
Q

What is visual acuity

A

A measure of spatial resolution, the angle subtended at the eye of two points which are perceived as separate

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44
Q

How can visual acuity be tested

A

Snellen chart- test in bright light with high contrast, achromatic letters, with glasses if needed

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45
Q

What is normal visual acuity

A

20/20- at 20 feet, someone can read a line of letters labeled 20/20
Letters with gaps about 1 min of arc can just be read

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46
Q

What is 1 min of arc

A

1/60 degree

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47
Q

What is visual acuity at best

A

Gaps of 0.5min of arc on the retina can be resolved aka 2 microns

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48
Q

What determines the limits of visual acuity

A

Gaps of 0.5min of arc (2 microns) is the width of an individual foveal cone outer segment, hence acuity is limited by the cones, as light must hit 2 seperate cones for 2 points to be seen as separate

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49
Q

What does the iris do

A

Controls the size of the pupil and the amount of light entering the eye, reducing optical errors and increasing depth of focus

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50
Q

How does the iris reduce optical errors

A

Constriction of the pupil means less rays of light are entering the eye that can have aberrations that will cause blurring on the back of the eye

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51
Q

What do sympathetic vs parasympathetic response cause the pupil to do

A

Sympathetic response- dilation of pupil

Parasympathetic response- constriction of pupil

52
Q

How does the image projected onto the retina differ from the object in the world

A

Image on the retina is inverted due to the optics of the eye

53
Q

What did Stratton say about the importance of inversion of the retinal image

A

Stratton (1897)- inversion is not important as long as the relative spatial positions remain preserved

54
Q

What experiments did Stratton carry out into the importance of inversion

A

Stratton (1897)- wore goggles with inverted lenses so everything he saw looked upside down, after a few days he adjusted to things being upside down, concluding inversion is not important

55
Q

What is the accomodation reflex

A

The increase in power of the lens caused by contraction of the annular ciliary muscle, which reduces tension in radial zonular fibres, allowing the lens to relax to a may convex state

56
Q

By which system is the annular ciliary muscle innervated

A

Parasympathetically innervated

57
Q

Accomodation reflex- what is the state of the rays of light when we look at something distant

A

The rays of light are entering the eye parallel, so when they hit the cornea they form a single point on the back of the eye

58
Q

Accomodation reflex- what happens when a fixated object is distant

A

Relaxed ciliary muscles pull suspensory ligaments taut, so the lens is pulled flat and rays of light come to a single point on the retina- don’t need to be refracted a lot

59
Q

Accomodation reflex- what is the state of the rays of light when we look at something close

A

The rays of light are already diverging when they meet the cornea, so more refraction is needed to bring them to a single point on the retina

60
Q

Accomodation reflex- what happens when a fixated object is close

A

Ciliary muscles contract and release the tension on suspensory ligaments, allowing the lens to bulge out thicker to help refract light rays onto a single point on the retina

61
Q

What stimulates the accomodation reflex

A

The visual cortex monitors the image- if it appears blurred, parasympathetic fibres are activated, and we automatically and instantaneously accomodate via ciliary muscle contraction

62
Q

What is presbyopia

A

Reduced ability of the eye to focus on nearby objects aka reduced ‘near point’ as we age

63
Q

What is our ‘near point’

A

The most contracted ciliary muscles can be ie the closest objects can be that we can still accomodate them
Approx 25cm, but declines with age

64
Q

What are possible mechanisms for presbyopia

A

Decreased elasticity of the lens so it can’t curl enough, loss of power in ciliary muscles they can’t contract enough

65
Q

What is myopia

A

Short-sightedness- near objects in focus but distant objects are blurred

66
Q

What does the size of the blur for far ojects in short sightedness depend on

A

How open the pupil is, because a constricted pupil allows less myopic light rays to enter the eye

67
Q

What causes distant objects to appear blurred in myopia

A

Light rays cross over too early, meaning when they reach the back of the eye the image is blurred

68
Q

How can myopia be treated

A

A corrective concave lens will make light rays diverge outwards before they enter the cornea, meaning they cross over later, on a single point at the back of the eye- spectacles or contacts

69
Q

Study into environment vs genes causing myopia

A

Xiong et al (2017)- systematic review and metanalysis found increased time outdoors is effective in preventing the onset of myopia, but is not effective in slowing progression of already myopic eyes

70
Q

What is hypermetropia

A

Hyperopia, long-sightedness, near objects are blurred but distant objects are in focus

71
Q

What causes hypermetropia

A

Rays of light cross over behind the retina, causing a blur on the back of the retina

72
Q

How can hypermetropia be treated

A

Convex lens causes some convergence of light rays before they reach the cornea, so rays form a clear image and cross over at the back of the eye- spectacles or contacts

73
Q

Treatment for myopia vs hypermetropia

A

Myopia- corrected with negative power

Hyperopia- corrected with positive power

74
Q

What is the effect of the axial length of the eyeq

A

If the eye is too long or short, rays of light will cross over either before or after they hit the retina

75
Q

How is the visual environment important in developing vision

A

Visual environment exerts a powerful influence on refractive state by controlling the axial length of the eye during postnatal development

76
Q

Study showing a cause of eye enlargement

A

Wiesel and Raviola (1977)- if a monkey eye is fused shut, it grows too long, causing myopia

77
Q

Study showing need for near and distance vision in early development

A

Hung et al (1995)- monkeys made to wear spectacle lens that simulated refracrive anomalies developed compensating ocular growth, suggesting both distance and near vision are needed within a normal environment to produce the normal eye length

78
Q

What atigmatism

A

Common cause of blurred vision due to irregularly curved cornea, causing different degree of curvature in different planes and refractive error

79
Q

How does astigmatism cause refractive error

A

When lights travels through the unevenly curved cornea, it comes to focus at multiple different points on the retina eg horizontal plane may focus well while vertical plane will be blurred

80
Q

How can astigmatism be assesed

A

Astigmatism chart with lines at many different angles- for a person with astigmatism, not all the lines are in focus

81
Q

How can astigmatism be corrected

A

Spectacles with a cylindrical component in their curved surfaces- the cylindrical lens allows the different lines to be in focus at the same time

82
Q

How does posterior vitreous detachment affect vision

A

Flashes of light, floaters and cobwebs in the vitreous humour, can affect image quality

83
Q

What causes the flashes of light in posterior vitreous detachment

A

As the vitreous pulls away from the retina it can stimulate the retina which causes flashes

84
Q

What causes the floaters in posterior vitreous detachment

A

As vitreous becomes more watery, debris and clumped microscopic fibres too large to be cleared by phagocytes can float freely, casting shadows on the visual field

85
Q

What causes the cobweb effect in posterior vitreous detachment

A

When the vitreous edge pulls away from the retina, light passes through it differently, and tou may see a thicker outer edge of the vitreous, appearing as cobwebs

86
Q

What is posterior vitreous detachment

A

With age, the gelatinous substance can partially liquify and lose its shape

87
Q

How do visual distortions (eg refractive errors, posterior vitreous detachment) need to be treated

A

Can’t be corrected at higher brain levels, necessary to generate a clear image at the eye itself

88
Q

What is cataracts

A

Condition in which the lens becomes cloudy- patches can become bigger, causing blurry misty vision and eventually blindness

89
Q

Causes of cataracts

A

Genetics, aging, environmental (sunlight, dehydration), injury

90
Q

What can cause congenital cataracts

A

No of genes identified, including mutations in crystallins

91
Q

Why does cataracts cause blurry vision

A

Light is spread around and obstructed by the cloudy patches rather than having a clear passage through the eye

92
Q

Treatment of cataracts

A

Removing the lens surgically and replacing it with an artificial lens

93
Q

How is aqueous humour circulated through the eye

A

Aqueous humour produced and comes out in front of the eye in front of the lens, then drains out through the canal of Schlemm

94
Q

What is circulation of aqueous humour important in maintaining

A

Normal intraocular pressure (IOP) around 16.5mm Hg

95
Q

What causes glaucoma

A

If the eye can’t drain aqueous fluid well enough, pressure inside the eye rises, reducing blood flow and squeezing and damaging the cells that form the optic nerve

96
Q

What cells together form the optic nerve

A

Retinal ganglion cells

97
Q

Why is nighttime vision often lost first in glaucome

A

Info from rods is sent on the outside of the optic nerve, meaning glaucoma damages rods first (involved in nighttime vision)

98
Q

How does glaucoma affect vision

A

Reduces the visual field through damage to the optic nerve that is permanent- one of the most common causes of blindness

99
Q

What elevated IOP is glaucoma associated with

A

Over 21mmHg

100
Q

How can glaucoma be treated

A

Can be treated to reduce intraocular pressure to prevent permanent vision loss- medication, laser treatment or surgery

101
Q

What is retinitis pigmentosa

A

RP is an inherited degenerative eye disease that causes severe vision impairment

102
Q

What causes retinitis pigmentosa (RP)

A

Progressive degeneration of initially the rod photoreceptor cells in the retina, and later the cones

103
Q

How does retinitis pigmentosa impair vision

A

First sign is loss of peripheral vision and night vision, eventually leads to tunnel vision and total blindness

104
Q

What do animal models causing the degeneration of photoreceptors in retinitis pigmentosa (RP)

A

Reduced ability of retinal pigment epithelial (RPE) cells to phagocytose receptor outer segments leads to accumulation of debris and seperation of photoreceptors from RPE cells- causes their degeneration

105
Q

Study in animal model showing how RP may be treated

A

Lorach et al (2018)- surgical removal of debris when about half the photoreceptors are lost allows the remaining photoreceptor cells to renew their outer segments and survive 3 times longer than in untreated eyes

106
Q

How is the retina’s output not a faithful reproduction of the light falling on it

A

It is specialised to detect differences in light intensity on different parts of the retina, image processing is already underway

107
Q

What is the pupillary light reflex

A

in bright light, your pupils contract

108
Q

What controls the pupillary light reflex

A

Connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils

109
Q

What does it mean to say the pupillary light reflex is consensual

A

Shining a light into only one eye causes constriction of the pupils of both eyes- if not, often a sign of seriously neurological disorder involving the brain stem

110
Q

What are the benefits of the pupil constricting and dilating

A

Allows light/dark adaption, pupil constriction allows increased depth of focus

111
Q

How does the process of information compression and sorting into parallel pathways begin

A

In the retina, 130 million photoreceptors are refined into an output of 1 million ganglion cells

112
Q

How many rods and cones are in the retina

A

125 million rods, 7 million cones

113
Q

What cells send visual info to the brain

A

Retinal ganglion cells

114
Q

Which cells in the retina use graded potetials vs APs

A

All neurons in the retina use graded potentials, except the ganglion cells that use APs

115
Q

What are the 6 types of retinal neurons

A

Retinal ganglion cells, amacrine cells, horizontal cells, bipolar cells, rods and cones

116
Q

What order do electrochemical signals travel through the retinal neurons

A

Photoreceptors ->bipolar cells -> retinal ganglion cells (->LGN)

117
Q

What is optics

A

The study of light rays and their interactions

118
Q

What determines the angle of refraction

A

The size of the difference betweeen the speed of light in 2 media- the greater the difference, the greater the angle of refraction

119
Q

What is the cornea

A

The glassy transparent external surface of the eye, continuous with the sclera (white of the eye) which forms the tough wall of the eyeball

120
Q

Why aren’t the extraocular muscles visible

A

They are behind the conjunctiva, a membrane that folds back from the inside of the eyeballs and attaches to the sclera

121
Q

Where is the retina thinnest

A

In the fovea

122
Q

How do you describe the different areas of the retina

A

Part of the retina closer to the nose than the fovea is nasal, part near the temple is temporal, above fovea is superior, below fovea is inferior

123
Q

Why is the aqueous humour in the cornea the main contributor to refraction

A

The aqueous humour has simiilar refractive properties to water so light travels significantly slower in the cornea than air, while there is less refraction after this point as the aqueous humour/lens/vitreous humour are all composed largely of water

124
Q

What is the focal distance

A

The distance between the refractive surface and where the paralllel light rays converge- the tighter the cornea curvature, the shorter the focal distance

125
Q

What is the diopter

A

Refractive power of the eye- the reciprocal of the focal distance

126
Q

Around how close to objects have to be to the eye for the lens to change shape to focus the object

A

Closer than 9m from the eye

127
Q

What is the visual field

A

The total amount of space that can be viewed by the retina when the eye is fixated straight ahead

Human basic neuroscience (68 decks)

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