2.0 Visual System Flashcards
1
Q
What is the wavelength of visible light?
A
390nm → 700nm<br></br>(violet) → (far red)
2
Q
What are the 4 variables that the visual system can analyse?
A
1) Intensity<br></br>2) Wavelength<br></br>3) Space<br></br>4) Time
3
Q
Illuminance vs Luminance
A
<b>Illuminance</b> = Quantification of light from light source (lux)<br></br><br></br><b>Luminance</b> = Quantification of light reflected from objects (cd/m²)
4
Q
Define reflectance:
A
A property of a material that dictates the proportion of reflected light from an illuminating source
5
Q
Define constrast
A
Contrast is defined as the ratio of incremental light intensity to the mean background intensity (∆I/I)
6
Q
What is pointspread function?
A
<b>Pointspread function = blurred image of a point source on the retina</b><br></br><br></br>Diameter of pointspread function = wavelength of light/diameter of aperture
7
Q
4 factors which may limit the optical quality/visual resolution:
A
1) Diffraction<br></br>2) Chromatic aberration<br></br>3) Spherical aberration<br></br>4) Glare
8
Q
Define diffraction
A
Spreading of waves as they pass through an apperture.<br></br>Diffraction causes pointspread function
9
Q
Define Spherical aberration
A
<b>In a spherical surface, rays towards the edge are refracted more</b>
10
Q
Define Chromatic aberration
A
<b>Different colours are refracted by different amounts (thus focused at different depths)</b><br></br>Human eye = well focused for green. Poor focus for blue
11
Q
Define Glare
A
Small particles in optical media scatter light in all directions → ↓ contrast of image
12
Q
Define Emmetropic
A
Object at infinity is sharply focused
13
Q
Define Ametropic
A
Inability to sharply focus at an object at distance infinity
14
Q
Define Myopia
A
Short sight (inability to focus on distant objects)<br></br>Light is focused at a point before the retina → ↑ pointspread function
15
Q
Define Hypermyopia
A
Long sighted. Inability to focus on nearby objects
16
Q
Define Presbyopia
A
A progressive ↓ in the ability of the eye to focus on near objects<br></br><br></br>Caused by ↓ accomodation because of ↓ lens elasticity
17
Q
Define Grain (receptor spacing)
A
To see two points as separate points, receptor spacing needs to be at least 1/2 the width of the pointspread function<br></br><br></br>(this is usually achieved in the fovea)
18
Q
What is the receptor spacing in fovea?
A
0.5 arc mins apart
19
Q
What is cataracts?
A
Clouding of the lens
20
Q
What is the power of a lens?
A
The strength of the lens<br></br><br></br>Expressed in <b>dioptres</b>
21
Q
How are the lens and cornea supplied with metabolites?
A
They are avascular. Supply is via the aqueous humour<br></br><br></br>(secreted by ciliary body and drains in the canal of schlemm + trabecular meshwork)
22
Q
What is glaucoma?
A
A reduction in the outflow of aqueous humour → ↑ intraocular pressure
23
Q
What is accommodation?
A
<b>Lens changes its focal length to focus on objects at different distances</b><br></br><br></br>Due to combination of:<br></br>1) Radial elastic ligaments (suspensory ligaments)<br></br>2) Circular ciliary muscle<br></br><br></br>Near reflex accomodation requires:<br></br>1) Constriction of pupils<br></br>2) Convergence of the two eyes (to fixate on new target)
24
Q
What are the two muscles that control pupillary size? <br></br>What is their innervation?
A
1) Sphincter pupillae (PSNS)<br></br><br></br>2) Dilator pupillae (SNS)
25
What is the control pathway from the retinal to the pupillary sphincter (light reflex)?
Retina → Pretectum (midbrain) → Bilateral Edinger-Westphal nucleus → CN III → Ciliary ganglion → Pupillary sphincter
26
What is Argyll-Roberston pupil?
Characteristic of neurosyphilis
Pupil does not react to light but does react to accomodation
Pupil does not react to light but does react to accomodation
27
What are Muller cells?
Retinal glial cells that act as optical wave guides.
28
How wide is the fovea?
1.5mm
5 degrees
5 degrees
29
Features of foveola:
Middle (260um/1 degree) of fovea
Avascular
No rods
Centre of foveola has a cone spacing of 0.5min arc
Avascular
No rods
Centre of foveola has a cone spacing of 0.5min arc
30
Features of parafoveal region:
20 degrees either side of fovea
Peak rod density
Most sensitive to mesopic + scoptic vision
Peak rod density
Most sensitive to mesopic + scoptic vision
31
Where is the blind spot? (how big is it?)
At the optic disk
(5 degrees)
(5 degrees)
32
Describe papilloedema and why it occurs:
Papilloedema = swollen optic disk
Seen with ↑ intercranial pressure, because CSF around the optic disk is continuous with brain
Seen with ↑ intercranial pressure, because CSF around the optic disk is continuous with brain
33
Describe the general anatomy of rods and cones
1) Outer segment (transduction)
2) Inner segment (cellular machinery)
Both joined with cilium
2) Inner segment (cellular machinery)
Both joined with cilium
34
Describe the visual pigments in rods
Rhodopsin
GPCR embedded in disk membrane
7 transmembrane domains
Binds to 11-cis retinal (chromophore)
Wavelength peak = 500nm
GPCR embedded in disk membrane
7 transmembrane domains
Binds to 11-cis retinal (chromophore)
Wavelength peak = 500nm
35
Describe the visual pigments in cones
Cone opsins
Bind to 11-cis retinal
Different interactions tune the absorption to different wavelengths (compared to rodopsin)
Bind to 11-cis retinal
Different interactions tune the absorption to different wavelengths (compared to rodopsin)
36
Describe photoisomerisation of the photopigment
- With absorption of single photon the chromophore isomerises from 11-cis to all-trans retinal.
- Photoisomerisation leads to catalytically active form of rhodopsin = metarhodopsin II (R)
- After R has played its role, all-trans retinal dissociates slowly (100-1000s) from the opsin
- Rhodopsin is now in its bleached form and must be regenerated before it can be used again
- Photoisomerisation leads to catalytically active form of rhodopsin = metarhodopsin II (R)
- After R has played its role, all-trans retinal dissociates slowly (100-1000s) from the opsin
- Rhodopsin is now in its bleached form and must be regenerated before it can be used again
37
Where does rhodopsin regeneration occur?
Retinal pigment epithelium (RPE)
38
What are the steps for regeneration?
1) Retinal is reduced to all-trans-retinol
2) Transported out of the photoreceptor into the RPE where it is converted back to 11-cis retinal
3) Transported back to the photoreceptors, rejoins the bleached opsin to form rhodopsin
An alternative pathway via Müller cells allows rapid pigment regeneration in cones.
2) Transported out of the photoreceptor into the RPE where it is converted back to 11-cis retinal
3) Transported back to the photoreceptors, rejoins the bleached opsin to form rhodopsin
An alternative pathway via Müller cells allows rapid pigment regeneration in cones.
39
How long can regeneration take after bright light?
Up to 30 mins for full regeneration
40
How do cones regenerate?
Via an alternative pathway via Müller cells
Allows rapid pigment regeneration in cones
Allows rapid pigment regeneration in cones
41
What causes night blindness?
Deficiency of vitamin A (11-cis-retinol)
Retinal is a derivative of Vit A
Retinal is a derivative of Vit A
42
What are the steps of phototransduction?
1) R* activates transducin
2) Transducin activates PDE
3) PDE hydolyses cGMP → 5'GMP
4) ↓cGMP → closes cyclic nucleotide gated cation channels → hyperpolarisation
5) Guanylyl cyclase (GC) resynthesises cGMP to terminate response
2) Transducin activates PDE
3) PDE hydolyses cGMP → 5'GMP
4) ↓cGMP → closes cyclic nucleotide gated cation channels → hyperpolarisation
5) Guanylyl cyclase (GC) resynthesises cGMP to terminate response
43
What 5 proteins are needed for phototransduction?
1) R* (Rhodopsin)
2) Transducin
3) PDE
4) cyclic nucleotide gated cation channels
5) Guanylyl cyclase (GC)
2) Transducin
3) PDE
4) cyclic nucleotide gated cation channels
5) Guanylyl cyclase (GC)
44
What is retinitis pigementosa?
- Progressive hereditary retinal degeneration
- 1 in 3000 people
- There is a gradual onset of night blindness in adolescence, leading to loss of all peripheral vision by adulthood, and in extreme cases total blindness
- 5-10% of cases are caused by mutations in the gene for rhodopsin.
- Other molecules of the transduction cascade are affected in other cases including PDE and the light-sensitive channel
- 1 in 3000 people
- There is a gradual onset of night blindness in adolescence, leading to loss of all peripheral vision by adulthood, and in extreme cases total blindness
- 5-10% of cases are caused by mutations in the gene for rhodopsin.
- Other molecules of the transduction cascade are affected in other cases including PDE and the light-sensitive channel
45
What is photoreceptor light adaptation?
"""A reduction in sensitivity as the steady light intensity increases""
Used to avoid saturation and allow operation over a wide range of background intensities"
Used to avoid saturation and allow operation over a wide range of background intensities"
46
What is the mechanism of photoreceptor adaption?
• Mechanism of photoreceptor adaption:
1. Ca2+ influx inhibits guanylyl cyclase (GC)
2. Less cGMP is remade
3. ∴ photoreceptors saturate more quickly
4. This leads to less light response
• This sets a regulatory negative feedback loop
o Ca2+ enters via cyclic nucleotide-gated channels
o Ca2+ then inhibits GC ⟶ ↓ cGMP ⟶ less Ca2+ entering the cell
o Ca2+ however continues to be extruded via Na+/Ca2+/K+ (NCX) exchanger
o ∴ Ca2+ concentration ↓ ⟶ relieving the inhibition of GC ⟶ more cGMP is synthesized
1. Ca2+ influx inhibits guanylyl cyclase (GC)
2. Less cGMP is remade
3. ∴ photoreceptors saturate more quickly
4. This leads to less light response
• This sets a regulatory negative feedback loop
o Ca2+ enters via cyclic nucleotide-gated channels
o Ca2+ then inhibits GC ⟶ ↓ cGMP ⟶ less Ca2+ entering the cell
o Ca2+ however continues to be extruded via Na+/Ca2+/K+ (NCX) exchanger
o ∴ Ca2+ concentration ↓ ⟶ relieving the inhibition of GC ⟶ more cGMP is synthesized
47
Define saturation
↑ stimulus intensity does not cause any more increase in receptor firing because the transduction pathways are limited
48
What is photoptic vision?
Vision in well lit conditions (uses cones)
49
What is scotopic vision?
Vision in poorly lit conditions (uses rods)
50
Where is scotopic sensitivity the highest?
In the parafoveal region (highest rod density)
51
What is mesopic vision?
Combination of photoptic and scotoptic (uses cones and rods)
52
Give two differences between rods and cones in terms of transduction and adaptation?
1. Cones are ~50x less sensitive, than rods, and cannot detect single photons.
2. Cone responses are much faster than those of rods
3. Rods saturate at lower intensities therefore are not used in photoptic conditions
2. Cone responses are much faster than those of rods
3. Rods saturate at lower intensities therefore are not used in photoptic conditions
53
Define colour vision:
Ability to distinguish objects based on their spectral reflectance, independent of their intensity
54
Define the principle of spectral univariance
Individual cone cell absorbs optimally at a particular wavelength, but cannot distinguish between two colours on its own
For example, a green cone might be activated by 10 green photons, but 100 red photons may also activate it.
For example, a green cone might be activated by 10 green photons, but 100 red photons may also activate it.
55
What type of colour system do humans have?
Trichromic
3 cone classes (Blue, green, red)
Colour = ratio of excitation of these cones
Can distinguish > 2 million colours
3 cone classes (Blue, green, red)
Colour = ratio of excitation of these cones
Can distinguish > 2 million colours
56
Define colour opponency:
Colour perception is based on the comparison of responses of the different types of cone receptors
Humans use double opponent cells
Humans use double opponent cells
57
Define colour constancy:
Perceived colours of an object remain the same irrespective of spectral composition of illuminating light
58
What is ratio of cone class excitation for white light?
Equal excitation of all classes
59
What cones absorb short/medium/long wavelengths preferentially?
Blue = short
Green = medium
Red = long
Green = medium
Red = long
60
Define protanopia
Red-dichromacy (no red cones)
61
Define deuteranopia
Green-dichromacy (no green cones)
62
Define tritanopia
No blue cones
63
Define deuteranomaly
Shifted spectral sensitivity of green cone →
green cone pigment is shifted towards yellow
green cone pigment is shifted towards yellow
64
Define protanomaly
Shifted spectral sensitivity of red cone (red cone pigment is shifted towards yellow)
65
Define tritanomaly
Congenital abnormality of the blue cones/mechanism. Very rare compared to red-green abnormalities
66
Define anomalous trichromacy
Inherited color vision deficiency - one of the three cone pigments is altered in its spectral sensitivity
67
Define natural polymorphism
Slightly shifted red cone
68
Define rod monochromat
Rods only
69
What are the 6 main types of cells of the retina?
1) Photoreceptors
2) Bipolar cells
3) Horizontal cells
4) Amacrine cells
5) Ganglion cells
6) Muller cells
2) Bipolar cells
3) Horizontal cells
4) Amacrine cells
5) Ganglion cells
6) Muller cells
70
Define the five layers of the retina
1) Outer nuclear layer
Photoreceptor cell bodies
2) Outer plexiform layer
Synapses between photoreceptors, bipolars and horizontal cell
3) Inner nuclear layer
Bipolar, horizontal and amacrine cell bodies
4) Inner plexiform layer
Synapses between bipolar, amacrine and ganglion cells
5) Ganglion cell layer
Cell bodies of ganglion cells.
Photoreceptor cell bodies
2) Outer plexiform layer
Synapses between photoreceptors, bipolars and horizontal cell
3) Inner nuclear layer
Bipolar, horizontal and amacrine cell bodies
4) Inner plexiform layer
Synapses between bipolar, amacrine and ganglion cells
5) Ganglion cell layer
Cell bodies of ganglion cells.
71
Fill in the blanks:
The direct pathway of information flow is from photoreceptors to ___________ to _______________.
All of these cells use ____________ as neurotransmitter.
_________ is mediated by horizontal cells (H) which are _________.
Amacrine cells (A) mediate a diversity of interactions in the inner retina, and use many different transmitters.
Photoreceptors synapse only with _________cells and __________ cells; ganglion cells receive input from _________and ___________cells.
The output of the retina is carried by the axons of _________cells, which together form the _________
The direct pathway of information flow is from photoreceptors to ___________ to _______________.
All of these cells use ____________ as neurotransmitter.
_________ is mediated by horizontal cells (H) which are _________.
Amacrine cells (A) mediate a diversity of interactions in the inner retina, and use many different transmitters.
Photoreceptors synapse only with _________cells and __________ cells; ganglion cells receive input from _________and ___________cells.
The output of the retina is carried by the axons of _________cells, which together form the _________
The direct pathway of information flow is from photoreceptors to bipolar cells to ganglion cells (G).
All of these cells use glutamate as neurotransmitter.
Lateral inhibition is mediated by horizontal cells (H) which are GABA-ergic.
Amacrine cells (A) mediate a diversity of interactions in the inner retina, and use many different transmitters.
Photoreceptors synapse only with bipolar cells and horizontal cells; ganglion cells receive input from bipolar and amacrine cells.
The output of the retina is carried by the axons of ganglion cells, which together form the optic nerve.
All of these cells use glutamate as neurotransmitter.
Lateral inhibition is mediated by horizontal cells (H) which are GABA-ergic.
Amacrine cells (A) mediate a diversity of interactions in the inner retina, and use many different transmitters.
Photoreceptors synapse only with bipolar cells and horizontal cells; ganglion cells receive input from bipolar and amacrine cells.
The output of the retina is carried by the axons of ganglion cells, which together form the optic nerve.
72
How many cones, rods + ganglionic cells are there in the retina?
Cones = 6 million
Rods = 120 million
Ganglionic cells = 1.5 million
There is massive convergence
This is the opposite in the fovea where there are more ganglionic cells cf. cones
Rods = 120 million
Ganglionic cells = 1.5 million
There is massive convergence
This is the opposite in the fovea where there are more ganglionic cells cf. cones
73
Define presynaptic ribbon
Modified presynaptic density characteristic of synapses that transmit graded signals
74
Define cone pedicle
- Large synaptic swelling end of cone terminals
- Allow divergence of the cone signal to numerous bipolar cells (up to 30)
- Form both invaginating and flat synaptic contacts
- Allow divergence of the cone signal to numerous bipolar cells (up to 30)
- Form both invaginating and flat synaptic contacts
75
Define rod spherule
- Rod synaptic terminals
- Smaller (than cones)
- No divergence
- Lots of convergence
- Smaller (than cones)
- No divergence
- Lots of convergence
76
Define receptive field
A visual cell's receptive field can be defined as the area on the retina from which its activity can be influenced by light
- Serves to define the position of a stimulus within the retina
- Size corresponds to the degree of convergence
- Serves to define the position of a stimulus within the retina
- Size corresponds to the degree of convergence
77
Define synaptic triad
Photoreceptor synapses with both horizontal and bipolar cells
78
Describe the structure of the receptive field
"- Retinal neurons exhibit centre-surround receptive fields, with a circular receptive field centre and a concentric surrounding annulus
- On-centre receptive field is excited by light in the central region and inhibited by illumination of the surrounding annulus
- Off-centre receptive field is inhibited by illumination of the centre and excited by the surround.
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- On-centre receptive field is excited by light in the central region and inhibited by illumination of the surrounding annulus
- Off-centre receptive field is inhibited by illumination of the centre and excited by the surround.
79
Describe the structure of a receptive field in bipolar cells
"- Synaptic interactions at the cone pedicle establish an antagonistic, centre-surround receptive field structure in bipolar cells.
- Each cone contacts both ""on-centre"" and ""off-centre"" cells
OFF-CENTRE
- Off-centre bipolar cells hyperpolarize to a ""centre"" stimulus, but depolarize to a ""surround"".
- In the dark the cone continually releases glutamate which depolarizes the bipolar cell by opening cation channels (ionotropic glutamate receptors).
- When the cone is illuminated, it hyperpolarizes, transmitter release is reduced and the bipolar cell hyperpolarizes.
- The antagonistic surround is generated by lateral inhibition mediated by horizontal cells, which have broad dendritic fields, collecting inputs from a large number of cones, and which form inhibitory feedback synapses back onto the photoreceptors.
- Horizontal cells hyperpolarize in response to light and therefore when only the surround is stimulated the central photoreceptor (now not illuminated) will depolarise (as it receives less inhibition from horizontal cell feedback).
ON-CENTRE
- On-centre bipolar cells depolarise to centre stimulus via metabotropic glutamate receptor (mGluR6)
- This activates a G protein resulting in the closure of cation channels in response to photoreceptor transmitter release in darkness (a sign-inverting synapse).
- Cascade may operate via direct inhibition of the channel by the G protein α subunit.
- When light hyperpolarises the photoreceptor, thereby decreasing transmitter release, this inhibition is relieved and the channels open, so the on-centre bipolar cell depolarizes"
- Each cone contacts both ""on-centre"" and ""off-centre"" cells
OFF-CENTRE
- Off-centre bipolar cells hyperpolarize to a ""centre"" stimulus, but depolarize to a ""surround"".
- In the dark the cone continually releases glutamate which depolarizes the bipolar cell by opening cation channels (ionotropic glutamate receptors).
- When the cone is illuminated, it hyperpolarizes, transmitter release is reduced and the bipolar cell hyperpolarizes.
- The antagonistic surround is generated by lateral inhibition mediated by horizontal cells, which have broad dendritic fields, collecting inputs from a large number of cones, and which form inhibitory feedback synapses back onto the photoreceptors.
- Horizontal cells hyperpolarize in response to light and therefore when only the surround is stimulated the central photoreceptor (now not illuminated) will depolarise (as it receives less inhibition from horizontal cell feedback).
ON-CENTRE
- On-centre bipolar cells depolarise to centre stimulus via metabotropic glutamate receptor (mGluR6)
- This activates a G protein resulting in the closure of cation channels in response to photoreceptor transmitter release in darkness (a sign-inverting synapse).
- Cascade may operate via direct inhibition of the channel by the G protein α subunit.
- When light hyperpolarises the photoreceptor, thereby decreasing transmitter release, this inhibition is relieved and the channels open, so the on-centre bipolar cell depolarizes"
80
What are the two classes of bipolar cell that synapse with cones?
1) Midget bipolar cell
Majority of bipolar cells in primates
- Receive input from just one cone
- Small receptive fields.
- Signal the finest spatial detail
- Signals are also colour-specific
1) Diffuse bipolar cell
- Receive input from 10-15 cones
- Larger receptive fields
- Response is more sensitive
- Spatial detail = lost
- Colour detail = lost
Majority of bipolar cells in primates
- Receive input from just one cone
- Small receptive fields.
- Signal the finest spatial detail
- Signals are also colour-specific
1) Diffuse bipolar cell
- Receive input from 10-15 cones
- Larger receptive fields
- Response is more sensitive
- Spatial detail = lost
- Colour detail = lost
81
What is the nature of bipolar to ganglionic synapses?
Excitatory
(therefore ganglionic cells have same receptive field as bipolar cells)
(therefore ganglionic cells have same receptive field as bipolar cells)
82
What cells modify bipolar to ganglionic synapses via lateral interactions?
Amacrine cells
83
What are the two types of ganglionic cells?
1) Parvocellular
- a.k.a midget
-80% of ganglionic
- Sustained response
- Slow response
- High acuity
- Small fields
- Colour + form
2) Magnocellular
- a.k.a parasol
- 10% of ganglionic
- Transient response
- Fast response
- More sensitive
- Low acuity
- Larger field
- Speclialised for motion
- a.k.a midget
-80% of ganglionic
- Sustained response
- Slow response
- High acuity
- Small fields
- Colour + form
2) Magnocellular
- a.k.a parasol
- 10% of ganglionic
- Transient response
- Fast response
- More sensitive
- Low acuity
- Larger field
- Speclialised for motion
84
Fill in blanks:
The extra sensitivity of the rod pathway sacrifices __________ due to convergence of rod signals (_____________). Colour vision is also lost in the scotopic intensity range, and the overall spectral sensitivity is shifted from ___________ (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of ___________ (______________).
The extra sensitivity of the rod pathway sacrifices __________ due to convergence of rod signals (_____________). Colour vision is also lost in the scotopic intensity range, and the overall spectral sensitivity is shifted from ___________ (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of ___________ (______________).
The extra sensitivity of the rod pathway sacrifices spatial resolution due to convergence of rod signals (rod pooling). Colour vision is also lost in the scotopic intensity range, and the overall spectral sensitivity is shifted from ~560 nm (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of ~500 nm (Purkinje shift).
85
What is the rod pathway?
Rods → Rod bipolar cells (all 'on') → AII amacrine cells → 'on' cone bipolar cells → 'on' ganglion cells
86
Describe rod processing under twilight
- At slightly higher intensities (twilight) an alternative simpler route involves electrical synapses from rod spherules directly to cone pedicles.
- Rod signals hijack cone pathway at mesopic intensities when both rods and cones function.
- Rod signals hijack cone pathway at mesopic intensities when both rods and cones function.
87
What are the projections from the retina?
1) Lateral geniculate nucleus (main one)
2) Pretectum (pupillary reflexes)
3) Suprachiasmatic nucleus (circadian rhythm)
4) Superior colliculus (eye movement)
2) Pretectum (pupillary reflexes)
3) Suprachiasmatic nucleus (circadian rhythm)
4) Superior colliculus (eye movement)
88
What are the layers of the lateral geniculate nucleus?
1) 4 parvocellular layers
- Ventral
- 6,5,4+3
2) 2 magnocellular layers
- Dorsal
- 2+1
3) Koniocellular layers
- In between above layers (for colour)
- Ventral
- 6,5,4+3
2) 2 magnocellular layers
- Dorsal
- 2+1
3) Koniocellular layers
- In between above layers (for colour)
89
How thick is the grey matter of the visual cortex?
2mm
90
How many layers does the visual cortex have?
6
91
What happens at each layer?
1 -
2 - Output to higher visual centres
3 - Output to higher visual centres
4 - Fibres from LGN (magno → 4Cα / parvo → 4Cβ)
5 - Output to deep brain structures (superior colliculus)
6 - Projections back to thalamus
2 - Output to higher visual centres
3 - Output to higher visual centres
4 - Fibres from LGN (magno → 4Cα / parvo → 4Cβ)
5 - Output to deep brain structures (superior colliculus)
6 - Projections back to thalamus
92
Define orientation tuned:
Receptive fields respond optimally to bars/edges of specific orientation
93
Define the receptive field of simple cells:
- Respond to edges of specific orientation
- Specific well defined position
- Layers 4+6
- May have inhibitory flanks
- Specific well defined position
- Layers 4+6
- May have inhibitory flanks
94
Define the receptive field of complex cells:
- Respond to bars/edges of specific orientation
- Position is not as critical
- Layers 2,3+5
- Directionally sensitive (better for movement)
- Position is not as critical
- Layers 2,3+5
- Directionally sensitive (better for movement)
95
What is end-inhibition (end-stopping)?
Exhibited by some simple and complex cells in V1.
Respond to specific bar length and output is inhibited if bar exceeds a critical length.
Respond to specific bar length and output is inhibited if bar exceeds a critical length.
96
What are orientation columns?
Cells of same orientation are aligned in one column
97
What are ocular dominance columns?
Alternating slabs in layer 4 of V1. Receive info one eye then the other.
98
What are disparity detectors?
Neurons that detect objects in a different depth to the plane of fixation. Used for stereopsis
99
Define stereopsis:
Binocular judgement of depth
100
Define the receptive field of cytochrome oxidase blobs:
No orientation tuning
Centre-surround
Process colour
Centre-surround
Process colour
101
What are hypercolumns?
Represents a region of the visual field.
Contains:
1) All orientation columns
2) Left and right ocular dominance columns
3) Colour analysis blobs
Contains:
1) All orientation columns
2) Left and right ocular dominance columns
3) Colour analysis blobs
102
What is ambylopia?
Also known as lazy eye. Eye appears normal however vision is impaired due to a wiring defect occurring in early life. Needs to be corrected <2yrs old
103
What causes ambylopia?
1) Strabismus
2) Astigmatism
2) Astigmatism
104
Define astigmatism:
Refractive error due to abnormally curved cornea
105
"Draw the ventral ""what"" pathway:"
1) COLOUR
Parvocellular → blob → thin stripe → V4 → Infratemporal cortex
2) FINE FORM AND EDGES
Parvocellular (some magno)→ interblob → interstripe → V4 → Infratemporal cortex
Parvocellular → blob → thin stripe → V4 → Infratemporal cortex
2) FINE FORM AND EDGES
Parvocellular (some magno)→ interblob → interstripe → V4 → Infratemporal cortex
106
Define acromatopsia:
Cortical colour blindness
107
Define prosopagnosia:
Inability to recognise faces
108
"Draw the ventral ""where"" pathway:"
Magnocellular → 4Cα → 4B → thick stripe → V5 (MT) + V5a (MST) → Posterior parietal cortex
109
What can occur with a lesion in the dorsal 'where' pathway?
Inability to see movements (akinetopsia)
110
What is the formula for webbers law?
∆I/I = constant
(with brighter light rod response is less sensitive and faster
(with brighter light rod response is less sensitive and faster
111
Define light adaptation:
As steady light intensity increases, the sensitivity of photoreceptors decreases to avoid saturation and to allow operation over a wide range of intensities
112
Define bleaching adaptation:
Profound reduced sensitivity induced by bright light. Recovers slowly with dark adaptation. Due to rhodopsin bleaching
113
Define bleaching sensitization:
Prolonged depression of phototransduction sensitivity exhibited by rods after their exposure to bright light
114
Define dark adaptation:
Dark adaptation is the process by which sensitivity is recovered following bleaching adaptation. Occurs in two stages:
1) Cones recover first
2) Rod cells take longer
1) Cones recover first
2) Rod cells take longer
115
What is the rod-cone break?
During dark adaptation, cones recover before rods
116
Define critical fusion frequency:
At frequencies above this, a flickering light is perceived as steady (critical fusion frequency rises as light intensity rises)
117
Define post-bleach noise:
Increase in spontaneous quantal events following bleaching
