1
Q
At what candela/meter2 is the photopic system and what cd/m2 is the scotopic system active?
A
In general, only the photopic system is active at a luminance > 3 cd/m2;
at a luminance < 0.03 cd/m2, the scotopic system functions alone.
2
Q
What is the measure of luminance?
A
foot-Lamberts or candela per meter squared (cd/m2)
3
Q
What is the measure of luminous intensity?
A
Candela
4
Q
What are the two factors that limit human vision to 380-780nm?
A
- Limited absorption spectrum of the opsin component
- Transmission, reflection and attenuation of various wavelengths by the ocular media
5
Q
What is Snell’s law?
A
n₁ sin θ₁ = n₂ sin θ₂
6
Q
Is the refractive index of the lens higher or lower in aquatic species c.f. terrestrial?
A
Much higher (1.41 in humans; as high as 1.66 in aquatic species).
7
Q
What are the four possible mechanisms of accommodation?
A
- Changing curvature or position of the lens
- Changing curvature of the cornea
- Changing the distance between the cornea and retina
- Static accommodation- having two or more separate optical pathways of different refractive powers
8
Q
How is accommodation commonly measured?
A
Using infrared photoretinoscopy
9
Q
Describe the innervation/stimulus that leads to disaccommodation
A
Sympathetic stimulation causes relaxation of the CB musculature leading to increased tension on the zonules, resulting in a more discoid lens
10
Q
Does sympathetic stimulation cause accommodation or disaccommodation?
A
Disaccommodation
11
Q
What drug can be used to stimulate accommodation?
A
Pilocarpine
12
Q
Compared to the human, what is the elasticity of the feline lens capsule (%)?
A
5% the elasticity of the human lens capsule
13
Q
What is the term for the anterior/posterior movement of the entire lens during accommodation (used in cats for example)? How is this process made possible in cats?
A
Translation
Meridional (i.e. longitudinal) fibres in the feline ciliary body
14
Q
Parasympathetic stimulation causes translation in the cat. Does this result in anterior or posterior movement of the lens?
A
Anterior movement of the lens by up to 0.6mm (inducing 2-4D of accommodation)
15
Q
What is the accommodative power in cats and dogs?
A
Cats: 2-4D (50-25cm)
Dogs: 1-3D (100-33cm)
16
Q
How do snakes accommodate?
A
They contract the iris resulting in an increased vitreal pressure, which pushes the lens anteriorly
17
Q
Some teleosts have a specific muscle that can cause translation (movement of the lens). What is it called?
A
Retractor lentis
18
Q
What are the three ciliary muscles in chickens?
A
Anterior (Crampton’s), intermediate (Muller’s) and posterior (Brucke’s)
19
Q
What does contraction of Crampton’s muscle result in?
A
Flattening of the peripheral cornea and increased curvature of the axial cornea.
20
Q
In diopters, what is the lenticular and corneal accommodative power in the chicken?
A
Lens: 15-19D
Cornea: 8-9D
21
Q
What is the peak sensitivity of rods?
A
500nm (cyan-green)
22
Q
What is the proprtion of cones in nocternal, crepuscular/arrhythmic and diurnal species?
A
Nocturnal: 0.5-3%
Arrhythmic/crepuscular: 5-10%
Diurnal: 8-95%
23
Q
What are the two assumptions made during retinoscopy?
A
- That the light emerging from the eye follows the same optical path as light entering the eye
- The fundus relfex originates at the level of the outer segments (it actually is from the inner limiting membrane).
24
Q
What is the artefact of retinoscopy?
A
The fundic reflex actually emerges from the innner limiting membrane (not the outer segment).
25
Does aphakia result in myopia (nearsightedness) or hypermetropia (farsightedness)
Hypermetropia
26
What refractive power should a canine IOL have?
+40.0-41.5D
27
What is the refractive error in aphakic dogs?
+14.4-15.2D (equivalent to 20/800 visual acuity).
28
What refractive power should a horse IOL have?
+14D
29
What refractive power should a cat IOL have?
52-53D
30
How is the required IOL power calculated?
Using keratometry and intraocular dimension measurements
31
What is direct (with-the-rule) astigmatism vs indirect astigmatism?
Direct: light entering through the vertical meridian refracted more
Indirect: Light entering through the horizontal meridian refracted more
32
Describe static field accommodation
Different refractive power of different pathways of light- e.g. in birds, amphibians and reptiles lower-field myopia allows animals to monitor objects nearby on the ground as well as predators in the sky without active accommodation
33
Describe spherical aberration
Light passing through the peripheral cornea/lens refracted more than at the centre
34
What adaptations to the cornea and lens minimise spherical aberration?
The nucleus has a higher refractive index
The peripheral cornea is flatter
Miosis
35
Describe the relationship of n (in the equation D=n/f) and λ.
n is inversely proportional to λ
Therefore light with a shorter wavelength (e.g. blue light), will result in a higher n and be refracted more than longer wavelengths (e.g. red light)
36
What is chromatic aberration?
The distance between focal points of the short and long wavelengths (measured in diopters)
37
What is GRIN (gradient index)?
The bell-shaped refravtive index curve (i.e. the refractive index of the lens gradually decreases towards the cortex).
38
Do species with multifocal lens have a steeper or shallower GRIN than specieis with a monofocal lens?
Steeper GRIN (gradient index).
39
What adaptations help decrease the effect of chromatic aberrations?
Multifocal lens
Retinal changes- human fovea doesn't contain any blue cones (just red and green). Blue light, which is not focused at the fovea, will not be absorbed in this region.
Also yellow macular pigment (carotenoids, lutein, zeaxanthin) to absorb unfocused short wavelengths
40
How do fish compensate for the lack of refraction from the cornea?
Highly spherical lens with high refractive index (up to 1.66).
41
What is stenopaic vision?
Improved vision through a tiny hole
41
What is spheric aberration?
Light passing through the peripheral lens/cornea is refracted more.
42
What adaptations minimise spheric aberration?
Peripheral cornea is flatter
Nucleus of the lens has a higher refractive index
43
In relation to the equation D=n/f, how is n related to λ (wavelength).
Shorter λ (wavelength) results in higher n (refractive index) which bends light more.
44
How is chromatic aberration determined?
The focal points between short and long wavelengths of light in diopters
45
What adaptations help lessen chromatic aberration?
Multifocal lenses (+/- irregularly shaped pupil).
Retinal: human fovea contains no blue (S-) cones. Macula contains yellow pigment (absorbs short wavelength light).
46
What power IOL is required in a dog?
40.0 to 41.5D
47
What power IOL is required in a cat?
52 to 53D
48
What power IOL is required in horses?
14D
49
What is direct and indirect astigmatism?
Direct (with-the-rule) light entering at the vertical meridian is refracted more
Indirect (against-the-rule): light entering at the horizontal meridian is refracted more.
50
What difference in refraction between the meridians is required for an astigmatism?
>0.5D
51
A horse is naturally astimagtic. By what proportion and is it a direct or indirect astimatism?
1.3D direct astigmatism
52
Describe static accommodation in birds, reptiles and amphibian species?
Lower field myopia allows focus on near objects in lower visual fields and far objects in upper visual fields during emmetropia
53
Describe static accommodation in Guinea Pigs?
The 'ramp-retina'- longer distance from lower cornea to dorsal retina- lower field myopia of -6D
54
How do fish overcome the weak negative power of the cornea in water?
Highly spherical lens with a high refractive index (up to 1.66)
55
Morphologically, what are the four parts of rods and cones?
1. Outer segment
2. Inner segment
3. Nucleus
4. Synaptic terminal
56
What connects the photoreceptor outersegment and inner segment?
The cilium.
57
What makes up the photoreceptor inner segment
Ellipsoid (area rich in mitochondria)
Myoid (Golgi apparatus and endoplasmic reticulum)
57
Where is opsin synthesized and describe how it is transported/incorporated into the outer segment.
In the Golgi apparatus of the inner segment. Transported through the cilium to the outer segment base where it becomes incorporated into the plasma membrane before evagination and invagination.
58
In rods are the discs attached or detached from the plasma membrane?
Detached. In cones, disc remains part of the membrane
58
When the outer segments shed and phagocytosed by the RPE in rods and cones?
Rods: following morning light onset
Cones: following night onset
59
What are the two components of photopigment?
1. The opsin: apo-protein which determines wavelength of light to absorb (in rhodopsin- scotopsin)
2. The chromophore: 11-cis-retinal
60
How is the opsin bound to the chromophore?
A covalent bond
60
What is photoisomerization
Conversion of 11-cis-retinaldehyde into a11-trans-retinal. This changes the arrangement of the opsin molecule, leading to the phototransduction cascade
61
What is dark current?
In dark (inactive) photoreceptors release glutamate. The depolarised state is maintained by constant influx of Na+ and Ca++ (85:15 ratio) and outflow of K+
62
Where do the cations enter the photoreceptor during 'dark current' state?
Through cyclic GMP-gated channels (decresed cGMP at the end of phototransduction leads to closure of these channels leading to hyperpolarization of the photoreceptor, terminating glutamate release).
63
What transports inactive chromophore (a11-trans-retinal) out of the disc and into the cytoplasm?
ATP-binding casette transporter (ACA4)
64
How is a11-trans-retinol moved from the photoreceptor to the RPE?
Via the interphotoreceptor retinoid-binding protein (IRBP).
64
What is the only step of regeneration of the chromophore that occurs in the photoreceptor outer segment?
Reduction of of a11-trans-retinal to a11-trans-retinol by a11-trans-retinol dehydrogenase. Important as a11-trans-retinal has an aldehyde group that can cause damage to the photoreceptors.
65
Once in the RPE, a11-trans-retinol undergoes a 3-step transformation to 11-cis-retinal. Which enzyme is key in this reaction?
RPE65
66
Other than through recycling of the chromophore in the RPE, where else can the cones receive 11-cis-retinal?
From Muller cells (cones are not totally dependent on recycling of chromophore from the RPE and the function of IRBP).
67
What are the three sites of 11-cis-retinal that the rods have access to when moving from scotopic to photopic environment?
1. The rod outer segments
2. Chromophore bound to IRBP
3. Chromophore bound to RPE65 in the RPE
68
Describe the dietary uptake/process for chromophore synthesis.
1. Carotenoids in diet are oxidised to vitamin A (all-trans-retinol) in wall of small intestine
2. Vitamin A esterified and transported to the liver
3. Bound to RBP
4. Transported in serum to choriocapilaris
5. Taken up by specific receptors in outer RPE
6. Processed by RPE
69
Where are the cell bodies and synapses of the horizontal cells?
Cell bodies in the outermost inner nuclear layer (INL)
Synapse exclusively in the outer plexiform layer (OPL).
70
Horizontal cells connect photoreceptors laterally in the OPL. They contribute to lateral inhibition. What is lateral inhibition?
Increased sensitivity of photorecptors to changes of illumination (brightness contrast) and colour
71
What are the two classes of horizontal cell?
HI/B or L (luminosity) type: with axons; synapse with rods and cones
HII/A or C (colour)-type cells: no axons
72
Müller cells are ependymoglial cells. What does this mean?
They have both a structural support and metabolic role.
73
What are the functions of Müller cells?
1. Structural support- radial cells that extend from inner to outer limiting membrane
2. Encircle intraretinal bvs and neurons
3. Sheath dendritic processes insulating them chemically and electrically
4. Surround photoreceptors insulating them from each other and keeping them aligned
5. Synthesise and store glycogen
6. Uptake of K+ (K+ 'sink') released by photoreceptors
7. Uptake and metabolise glutamate, GABA and dopamine
8. Activated by inflammatory mediators contributing to the immune response of the retina following injury
74
Where are amacrine cells found and what is their function?
In the innermost inner nuclear layer (displaced amacrine cells in ganglion cell layer)
Functions: create contextual effects and contribute to vertical and lateral communications in the retina
1. Center surround antagnoism enhancing motion detection relative to background
2. Mediate gain control
3. Alter kinetics of bipolar cell output
75
What cells do the vertical and horizontal pathways involve?
Vertical: photoreceptor- bioplar- RGC
Horizontal: photoreceptor-horizontal cell- amacrine cell- RGC
76
What are the 3 traditional ways of classifying RGCs?
1. Morphology
2. Physiologic responses
3. Information processing
77
RGCs can be classified based on information processing into the luminance (magnocellular) and resolution (parvocellular) pathways. What is the function of these pathways?
Magnocellular (luminance): process and relay info. on changes in light level and motion
Parvocellular (resolution): fine detail and colour
78
When defined by physiologic response, what are X and Y RGCs?
X-cells (correspond to β-cells in cats) receive input from bipolar cells directly
Y-cells receive input from amacrine cells
79
When RGCs are classified by the cell they output to, where is the output of M-, P- and K- cells?
M-cells: magnocellular layers of LGN
P-cells: parvocellular layers of LGN
K-cells: koniocellular layers of LGN (? relay blue light/rapid movement response).
80
Are α- or β- cells larger and where is the distribution in the retina?
α- cells are larger, peripheral retina, converge signal from many amacrine cells
β- cells (midget cells in primate) are in the central retina- small size permits denser packing, input from fewer cones (1:1:1 cone:midget bipolar: β- cell in foveal region).
81
Describe the function of:
1. β-, X-, or parvo-cells
2. α-, Y- or magno-cells
1. smaller, more numerous. Input from cone system + bipolars. Small diameter axons- slow conducting. Sustained (tonic) response. Relay colour and fine detail
2. Larger, fewer in number. Input from rod system + amacrine cells. Larger, faster axons. Transient (phasic) responses. Relay subtle changes to illumination (contrast) and stimulus motion (temporal resolution).
82
What proportion of RGCs are α- and β- in the cat? Describe the remaining γ- (W-) and δ- (delta) cells.
α- 3-5%
β- 45-50%
γ (W)- small cell bodies and azons- serve cone system (high resolution photopic vision)
δ- >20 types (G4-23 based on size). 50% project to dorsal colliculus
83
Where is the RGC dendritic tree smallest?
In the area centralis (AC). Increasing size toward periphery (peripheral RGCs extend over 10x larger areas than in AC)
84
Where do ipRGCs project to?
Mainly the hypothalamic suprachiasmatic nucleus (SCN) but also geniculate and olivary pretectal nuclei
85
Describe the OFF-center bipolar receptors
OFF-center cells have excitatory, ionotropic (AMPA kainate) glutamate receptors- i.e. in the dark, when cone glutamate release is high, the cells are depolarised (excited). In the light, when glutamate release decreases, the OFF-bipolar becomes hyperpolarised
86
What is a 'sign-conserving synapse'?
A cone with an OFF-bipolar as both cells become hyperpolarised in response to light (photoreceptor stimulation, bipolar cell inhibition).
87
Describe ON-bipolar receptors
Inhibitory, metabotropic glutamate receptors. Cells are inhibited (hyperpolarised) in dark due to glutamate release by cones. Light leads to decrease glutamte, removal of this inhibition and depolarisation of the bipolar cell
88
Are excitatory, ionotropic (AMPA kinate) glutamate receptors found in OFF or ON- center bioplar cells?
OFF-center bipolars
(explanation in the dark, when cone glutamate release is high, the cells are depolarised (excited). In the light, when glutamate release decreases, the OFF-bipolar becomes hyperpolarised)
89
Are inhibitory, metabotrophic glutamate receptors found in OFF or ON- center bioplar cells?
ON-center bipolars
(explanation: light leads to hyperpolarisation of cone, lower glutamate, removal of inhibition and therefore depolarisation (activation) of the bipolar= sign-reversing synapse).
90
What is a sign-reversing synapse (cone-bipolar cell)?
ON-center bipolars
(explanation: light leads to hyperpolarisation of cone, lower glutamate, removal of inhibition and therefore depolarisation (activation) of the bipolar= sign-reversing synapse).
91
What kind of receptors do rod bipolar cells have?
inhibitory metabotropic glutamate receptors (similar to ON-center bipolar cells)
Therefore light stimulation results in hyperpolarisation of rod, lower glutamate and loss of inhibition of rod bipolar, leading to depolarisation (excitation).
91
How do rod bipolars synapse with RGCs?
Indirectly via amacrine cells
92
What are the main exictatory and inhibitory neurotransmitters in the neuroretina?
Excitatory: glutamate and acetylcholine
Inhibitory: GABA
Other: dopamine and seratonin from amacrine cells
93
Why is the optic nerve considered a pure white matter tract?
Contains only RGC axons and no other neuronal cell bodies (does contain important glial cell populations).
94
Which cells form the myelin sheath + nodes of Ranvier around the optic nerve?
Oligodendrocytes
95
What is the function of astrocytes around the optic nerve?
K+ homeostasis
Transport and store metabolites (mainly glycogen)
96
What are microglia and what is their function in the optic nerve?
CNS-specific macrophages that scan the tissue for signs of damage or distress
97
What is the term meaning that the precise spatial arrangement of the retina is maintained within the optic nerve?
Retinotopic
98
What % of optic fibers remain ipsilateral (don't decussate) in the following species:
Humans
Horses
Dogs
Cats
Humans: 50%
Cat: 33%
Dog: 25%
Horse: 15%
99
What proportion of RGC axons go to the LGN?
80-90%
100
Where do RGC axons synapse?
In the LGN with LGN interneurons and projection cells
101
What is the primary visual cortex named?
V1 (visual area 1) or the striate cortex
102
Which LGN pathways terminate in V1 layers 4A, 4Ca, and 4Cb, respectively?
Koniocellular → 4A, Magnocellular → 4Ca, Parvocellular → 4Cb.
103
What do layers 1-3 of V1 contain?
Excitatory neurons that project to each other and to the extrastriate visual areas
104
What do layers 5 and 6 of V1 do?
Communicate with superficial layers and feed back to the LGN
105
Where do incoming thalamic (LGN) axons synapse in V1 (cells and layer)?
In glutamate-containing simple cells in layer 4
106
What is the % of thalamic (LGN) axons that provide excitatory synapses in layer 4 of V1?
5%
107
What is a V1 hypercolumn?
Contains both column types (dominance and orientation). Gives info on stimulus orientation, movement, direction and binocularity
Interspaced between columns are colour opponent cells ('blobs') which have an antagonistic center surround regarding colour
108
What is Brodmann area 17 also called, and what key histologic feature gives the “striate cortex” its name?
V1 (primary visual cortex); the striae of Gennari (a myelinated stripe of LGN axons).
109
Which thalamic nucleus provides the primary input to V1?
LGN
110
Where is V1 located in cats?
Posteromedial cortex, from the crown of the lateral gyrus to the superior bank of the splenial sulcus.
111
Where is V1 located in dogs?
At the junction of the marginal and endomarginal gyri.
112
How many cortical layers does V1 have, and which layer receives direct LGN input?
Six layers; LGN input mainly to layer 4.
113
What is the principal neurotransmitter of the simple cells in V1 layer 4?
Glutamate.
114
Which V1 layers (1–3 vs 5–6) project to extrastriate areas and which send feedback to the LGN?
Layers 1–3 → extrastriate areas; Layers 5–6 → feedback to LGN.
115
Do simple cells in V1 layer 4 tend to be monocular or binocular, and what receptive-field property do they preserve?
Monocular; they preserve center–surround antagonism ortientation.
116
What is a cortical “column” in V1?
A vertical unit spanning all six layers that processes input from a specific retinal receptive field.
117
Are columns serving central retina larger in size than those serving peripheral retina? If not, how is central vision prioritized?
No; more columns are devoted to central retina.
118
What is an “orientation column” in V1?
A column in which all cells respond best to the stimulus orientation.
119
How does eye dominance vary from the center to the periphery of an ocular dominance column?
Strongest at the center, gradually becomes more binocular at the edges.
120
What are “blobs” in V1, how are they identified histologically, and what information do they process?
Cytochrome oxidase-stained regions; process color information.
121
What kind of opponency do blob cells exhibit?
Color opponency with antagonistic center–surround responses.
122
What is visual area 2 (V2) also called, and where is it located relative to V1?
Parastriate cortex (Brodmann area 18); surrounds V1 in a concentric crescent.
123
Name one feature that separates V2 from V1 at their border.
Reversal of retinotopic map (also histology and connectivity differences)
124
What is MT (also known as V5), and what visual attributes does it primarily process?
Middle temporal area; processes motion, speed, and binocular disparity.
125
Which stream (dorsal or ventral) primarily handles motion, direction, stereoscopic depth, and visually guided actions?
Dorsal stream.
125
After V2, into which two major functional streams does visual processing diverge, and what is each broadly responsible for?
Dorsal stream → “where/how” (motion, location, visuomotor); Ventral stream → “what” (object recognition, form, color).
126
Which stream (dorsal or ventral) primarily handles object recognition, including form, color, orientation, and shape?
Ventral stream
127
Which LGN pathway contributes predominantly to the dorsal stream, and to what areas does this stream commonly relay?
Magnocellular pathway; relays through V3 and V5.
128
Which LGN pathways feed the ventral stream, and through which area does it commonly relay before higher processing?
Both magnocellular and parvocellular; relays through V4.
129
What kinds of deficits are associated with lesions in dorsal vs ventral visual pathways?
Dorsal → spatial/motion/visuomotor deficits; Ventral → colour and object recognition deficits.
130
In V1 mapping, which visual hemifield (ipsilateral or contralateral) is represented?
Contralateral.
G6 C1: Basic Vision Science
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