Vision Flashcards
(158 cards)
1
Q
What is the sclera?
A
It is the tough outer covering of the eyeball.
2
Q
What is the conjunctiva?
A
It is the thin transparent membrane that covers the back surface of the eyelids and the eyeball.
3
Q
Name the two muscles that allow the iris to control how much light enters the eye.
A
Pupillary sphincter muscle.
Dilator muscle.
4
Q
What is the role of the ciliary muscle?
A
It alters the shape of the lens to focus light onto the retina.
4
Q
Does the lens get thicker or thinner to focus on nearby objects?
A
Thicker.
4
Q
Does the lens get thicker or thinner to focus on distant objects?
A
Thinner.
5
Q
What does the retina contain?
A
Blood vessels and photoreceptors.
5
Q
Name the two main types of photoreceptors.
A
Rods.
Cones.
6
Q
Which elements of vision do we perceive using rods?
A
Night and movement sensitivity.
Peripheral vision.
7
Q
Which elements of vision do we perceive using cones?
A
Sharp details.
Central vison.
Colour.
8
Q
Where in the retina is there a high density of cones?
A
The macula.
9
Q
Why is there a high density of cones in the macula?
A
To facilitate the high resolution and detail that we perceive within an image.
10
Q
Name the nine layers of cells and synapses that exist in the retina.
A
Pigment epithelium.
Photoreceptor layer.
Outer lining membrane.
Outer nuclear layer.
Outer plexiform layer.
Inner nuclear layer.
Inner plexiform layer.
Ganglion cell layer.
Nerve fibre layer.
11
Q
Which cells are located in the pigment epithelium?
A
Pigmented cuboidal cells.
12
Q
What are the two main functions of pigmented cuboidal cells?
A
They contain melanin which absorbs light not captured by the retina to protect the photoreceptors from damaging levels of light.
They provide glucose and essential ions to the photoreceptors.
13
Q
Are there more cones or rods in the retina?
A
Rods outnumber cones approximately 20:1 across most of the retina.
14
Q
Which region of the retina contains more cones than rods?
A
The fovea because it only contains cones.
15
Q
Which cells are located in the photoreceptor layer?
A
Rods and cones.
16
Q
What occurs in the outer plexiform layer?
A
Synaptic interaction between photoreceptors and horizontal and bipolar cells.
17
Q
Which cells are located in the inner nuclear layer?
A
Amacrine cells.
Horizontal cells.
Bipolar cells.
18
Q
What are the two main functions of amacrine cells?
A
Act as interneurons.
Modulate ganglion cell activity.
19
Q
What are the two main functions of horizontal cells?
A
Act as interneurons.
Process photoreceptor signalling.
20
Q
What is the main function of bipolar cells?
A
Process photoreceptor signalling.
21
Q
What occurs in the inner plexiform layer?
A
Synaptic interactions between bipolar, amacrine and ganglion cells.
22
Which cells are located in the ganglion cell layer?
Cell bodies of multipolar ganglion cells.
23
Which area of the retina has the greatest density of ganglion cells?
The centre of the fovea.
24
What are the two unique features of ganglion cells?
They are the only source of output from the retina.
They are the only retinal cells capable of firing action potentials.
25
Name the synaptic relay that visual information travels to upon leaving the optic nerve.
Dorsal lateral geniculate nucleus of the thalamus.
26
Where is visual information processed, interpreted and remembered?
In the cerebral cortex.
27
What is the fovea?
A depression in the centre of the macula.
28
What is special about the fovea?
It is the region of greatest visual acuity in the retina.
29
How is such a high density of cones packed into the fovea?
The diameter of the cone outer segments is decreased.
30
How do specific structures work to ensure that the fovea has high visual acuity?
Blood vessels and cells are diverted away from the fovea.
This limits the scattering of light before it hits the photoreceptors.
31
What is the foveola?
The centre of the fovea.
32
Name the three functional regions within a photoreceptor.
Outer segment.
Inner segment.
Synaptic terminal.
33
What is the role of the outer segment of photoreceptors?
Involved in phototransduction.
34
What is the structure of the outer segment of photoreceptors?
Consists of a stack of membranous discs.
Contains light absorbing photopigments.
35
How does the structure of the outer segment differ between rod and cone cells?
In rods, the membranous discs pinch off from the plasma membrane so they are free-floating.
In cones, the membranous discs remain attached to the plasma membrane.
36
How are photoreceptor outer segments renewed?
Discs are shed from the distal end of the outer segment.
Pigment epithelial cells remove these discarded discs by phagocytosis.
New discs are added to the proximal end of the outer segment.
37
Which organelles does the inner segment of photoreceptors contain?
Nucleus.
ER.
Golgi apparatus.
Mitochondria.
Ribosomes.
38
What connects the photoreceptor inner segment to the outer segment?
Connecting cilium (CC).
39
What are the two main functions of the photoreceptor inner segment?
Protein synthesis.
Energy production.
40
What is the function of the photoreceptor synaptic terminal?
To make synaptic contact with other cells.
41
Why do rods capture more light than cones?
They contain more photosensitive pigment.
42
What issues can loss of rods lead to?
Night blindness.
Loss of peripheral vision.
43
What issues can loss of cones lead to?
Blindness.
44
Why are cones more sensitive to direct axial light rays?
Due to their conical shape.
45
How do photoreceptors respond to light?
Graded changes in membrane potential (NOT action potentials).
46
How does the response time differ between rods and cones?
Rods have a slow response time.
Cones have a fast response time.
47
Why does light have to go through the retina to the back to reach the photoreceptors?
The retina is inverted, so light has to pass through all the neuronal cell layers and blood vessels before reaching the photoreceptors.
48
Why are photoreceptors embedded within the pigment epithelium?
Pigment epithelium:
Provides blood flow, glucose and essential ions.
Helps degrade and replace the discs in the photoreceptor outer segment.
Regenerates photopigment in the discs.
49
What are the two main functions of Muller cells?
Support the function and survival of retinal neurons.
Transfer light through the inner retina by projecting the visual information forward from the photoreceptors to the bundle of optic nerves.
50
How are Muller cells directly in the path of light as it enters the retina?
They impinge onto photoreceptors.
51
Why are rods more sensitive to light than cones?
Multiple rods converge onto and activate one bipolar cell, whereas only one cone activates one bipolar cell.
52
Rods and cones transmit signals to bipolar and horizontal cells via which type of synapse?
Chemical synapse.
53
What is phototransduction?
The conversion of light energy into a graded change in membrane potential.
54
What is the RMP of photoreceptors?
-40mV.
55
What happens to the membrane potential of a photoreceptor when light impinges on the photopigment disc layer?
It is hyperpolarised.
56
What does cGMP stand for?
Cyclic guanosine monophosphate.
57
Which process does depolarisation of photoreceptors facilitate?
Calcium entry at the base of the inner segment to cause the consistent release of glutamate from the photoreceptor.
58
What is the role of sodium-potassium pumps in the inner segment of depolarised photoreceptors?
Potassium flows out across the inner segment through non-gated potassium channels.
Sodium-potassium pumps maintain the concentration balance between sodium and potassium.
59
What is the dark current?
The movement of positive charge (sodium influx) across the membrane.
60
What is the membrane potential of hyperpolarised photoreceptors?
-65mV.
61
How does light cause photoreceptors to hyperpolarise?
Light reduces cGMP levels.
Sodium channels close.
The membrane potential becomes more negative.
62
Name the photopigment found in rods.
Rhodopsin.
63
What type of receptor is rhodopsin?
G-protein coupled receptor.
64
What is the receptor protein of rhodopsin?
Opsin.
65
What is the prebound chemical agonist on rhodopsin?
Retinal.
66
What does photoreceptor hyperpolarisation lead to?
Reduction in calcium influx and glutamate release.
67
What is transducin?
G-protein in the membrane of the stacked discs in the rod outer segment.
68
Describe the conformational change that occurs in retinal due to light absorption.
Light breaks down the double bond in retinal, converting it from cis- to trans-retinal.
69
What type of process is phototransduction?
A biochemical cascade that facilitates signal amplification.
70
How does phototransduction facilitate signal amplification?
Each molecule of rhodopsin activates many G-proteins.
This activates more phosphodiesterase enzyme which breaks down more cGMP.
71
Why do we need signal amplification in vision?
So that we can detect even just a single photon of light energy.
72
Why is vision in bright light dependent on cones?
Prolonged light exposure causes rods to become saturated.
Levels of cGMP are reduced, leading to little or no response.
73
Why might one cone be activated at a different wavelength compared to another?
Each cone contains one of three opsins: blue, green or red.
Each opsin is activated at a different wavelength.
74
Which wavelength activates blue cones?
445nm (short).
75
Which wavelength activates green cones?
508nm (medium).
76
Which wavelength activates red cones?
565nm (long).
77
Which cones are least abundant in the retina?
Blue cones.
78
What does the ratio of green : red cones normally vary between?
Between 1:1 to 4:1.
79
What is scotopic vision?
The ability to see in low light conditions (night vision).
80
What is mesopic vision?
The ability to see in moderate light conditions (twilight vision).
81
What is photopic vision?
The ability to see in well-lit conditions (daytime vision).
82
Which elements of vision does bright light improve?
Colour vision.
Visual acuity.
83
Human vision is dependent on a linear process that is facilitated by what?
The ribbon synapse of rods.
84
What is the typical structure of a ribbon synapse?
One synaptic ribbon faces one postsynaptic density.
85
Why are ribbon synapses necessary?
They produce rapid synaptic transmission that facilitates the rapid adaptation required.
86
Why is rapid adaptation required in the visual system?
Light continually changes, so the synapses between retinal cells must rapidly convert graded light energy signals into action potentials.
87
Describe the three different pools of synaptic vesicles that are released at the ribbon synapse.
1. Pool is rapidly releasable on the synaptic bar near the membrane.
2. Pool is releasable on the synaptic bar but it is slightly further from the membrane.
3. Pool is a large filling pool in the cytoplasm around the synaptic bar.
88
Why is low order calcium dependency essential in the linear process of human vision?
Low level release of calcium is sufficient for release of NT at the ribbon synapse. This allows rods to relay small changes in light energy.
89
Light energy is transduced into an electrical signal that travels via the retinal ganglion cells which project up to where?
The dorsal lateral geniculate nucleus of the thalamus.
90
Where do lateral geniculate nucleus (LGN) neurons project their axons to?
To the primary visual cortex and then to the secondary visual cortex where they terminate.
91
How is visual information integrated with information from other pathways?
LGN axons fan out and travel through the temporal, parietal and occipital lobes.
92
Describe the two other key pathways that influence our sense of vision.
Some retinal ganglion cells project to the superior colliculus and/or suprachiasmatic nucleus.
93
What is the role of the superior colliculus?
Controls:
Reflexes.
Orientation of the head.
Eye movement.
94
What is the role of the suprachiasmatic nucleus?
Controls circadian rhythm.
95
What do the axons of the retinal ganglion cells form?
The optic nerves.
96
What occurs at the optic chiasm?
The optic nerves cross over (decussate) to form the optic tracts.
97
What is the purpose of crossing over?
The eye inverts the images that we see.
Crossing over of information from both visual fields ensures that none of the information is lost when it is processed in the visual cortex.
98
How many more neurons does the LGN contain compared to the retina?
10x.
99
What type of pathway goes from the retina to the LGN?
One-way efferent.
100
What type of pathway goes from the LGN to the cortex?
Two-way.
101
Describe the two-way pathway between the LGN and the cortex.
For every afferent connection projecting to the cortex, the LGN receives ten efferent connections from the cortex.
102
It is thought that the cortex may use the LGN as a selective filter. What does this mean?
The cortex may turn on certain neurons in the LGN that it is interested in, i.e. the brain is choosing what it attends to.
103
How is the LGN arranged?
Six layers made up of three different cell types.
104
Name the three cell types present in the LGN.
Parvocellular (P).
Magnocellular (M).
Koniocellular (K).
105
What is the importance of the P pathway?
Important in:
Fine detail.
Red-green colour vision.
106
What is the importance of the M pathway?
Important in:
Sensitivity to light, darkness and motion.
107
What are the four functions of the K pathway believed to be?
Complementary pathway to the primary visual cortex.
Sends signals directly to the extrastriate visual cortex.
Integrates retinal visual information with non-retinal input.
Plays an early role in binocular convergence.
108
The axons of which type of cells are projected to which regions of the LGN in the K pathway?
The axons of cells much larger than those in the P and M pathways project to very specific regions of the LGN.
109
Temporal and nasal retinal subfields project to alternate layers of the LGN from contralateral and ipsilateral eyes. What does this mean?
The right half of the LGN receives visual input about the left half of the visual field from EACH eye.
The left half of the LGN receives visual input about the right half of the visual field from EACH eye.
110
Where do the P and M pathways terminate?
In ipsilateral and contralateral stripes in V1.
111
Where is the visual cortex located?
In the occipital lobe.
112
Name the different areas of the visual cortex.
V1-V5.
113
What is the primary purpose of the visual cortex?
To receive, segment and integrate visual information.
114
What happens to visual information as it is passed through areas V1 to V5 of the visual cortex?
The level of processing and complexity of the visual information increases.
115
Axons from V1 project in two pathways to the V2 layer of the visual cortex. Describe these two pathways.
The dorsal pathway projects through the parietal lobe and into the frontal lobe.
The ventral pathway projects through the V4 layer into the inferior temporal cortex.
116
How are the synaptic connections from the photoreceptors to the bipolar cells through to the ganglion cells modulated?
Lateral sideways inhibition by horizontal cells and amacrine cells.
117
Describe OFF bipolar cells.
They respond to light off.
They depolarise in response to glutamate released from photoreceptors.
118
Describe ON bipolar cells.
They respond to light on.
They hyperpolarise in response to glutamate released from photoreceptors.
119
Name the two components of the receptive field of a cell.
The receptive field centre.
The receptive field surround.
120
What is the receptive field centre?
A circular area of retina providing direct photoreceptor input.
121
What is the receptive field surround?
A surrounding area providing input via horizontal cells (the indirect pathway).
122
How does light affect an ON centre/OFF surround bipolar cell?
Light hitting the centre increases the cell's response.
Light hitting the surround inhibits the cell's response.
123
How does light affect an OFF centre/ON surround bipolar cell?
Light hitting the centre inhibits the cell's response.
Light hitting the surround increases the cell's response.
124
What are the two types of ganglion cells in terms of light response?
ON-centre and OFF-centre.
125
What are the two major types of ganglion cells?
Smaller P-type ganglion cell.
Much larger M-type ganglion cell.
126
Which are more prevalent: P-type or M-type ganglion cells?
90% = P-type.
5% = M-type.
127
Describe the properties of M-type ganglion cells.
Larger receptive fields.
Conduct action potentials more rapidly in the optic nerve.
Respond to light with a transient burst of action potentials.
More sensitive to low-contrast stimuli.
128
Describe the properties of P-type ganglion cells.
Smaller receptive fields.
Conduct action potentials more slowly in the optic nerve.
Continue to respond to light for as long as they are stimulated.
129
What role are M-type ganglion cells thought to have?
Detection of stimulus movement.
130
What role are P-type ganglion cells thought to have?
Sensitivity to shape and fine detail.
130
Describe the ON-OFF response that occurs when light is shone onto the retina.
Light activates the ON bipolar cell.
This activates the centre of the ON-centre ganglion cell.
We see an ON response with an antagonistic OFF response in the OFF-centre or receptive field surround cell.
131
Describe the ON-OFF response that occurs when a dark spot hits the centre of an ON-centre ganglion cell.
The centre is darker than the light in the receptive surround field.
There is no ON response in the ON-centre ganglion cell.
There is a response in the OFF-centre ganglion cell.
131
Describe the ON-OFF response that occurs when the whole visual field is illuminated.
There is a drop in response firing rate in the ON-centre cell.
This is due to inhibition from the receptive field surround.
132
How do horizontal cells contribute to a constant level of negative feedback?
They hyperpolarise input into the cone.
133
Describe the action of horizontal cells under normal conditions.
They are being driven by the constant release of glutamate from the cone, and they feedback and inhibit the presynaptic terminal of that cone.
134
Describe the action of horizontal cells when light hits the centre field of the cone.
The cone cells hyperpolarise which reduces the glutamate release resulting in a response.
135
Which type of cell are horizontal cells?
GABAergic.
136
Define vision.
Retinal input that the brain processes based on observation and experience.
137
How does the brain use experience in terms of vision?
The brain processes visual information and then decides on brightness, contrast, edges, forms, etc. based on what it expects to happen rather than on specific retinal patterns of illumination.
138
What are the three levels of visual perception?
Low, mid and high.
139
Where does low-level visual processing occur?
In the retina.
140
Where does mid-level visual processing occur?
V1 of the visual cortex.
141
Where does high-level visual processing occur?
V2, V3 and V4 of the visual cortex.
142
What is categorical linking?
Using context to determine what category the image we see belongs to.
143
What is associative linking?
Prior experience or memories are linked to an image.
144
What is emotional valence?
The emotions that we attach to an image are influenced by prior experience.
145
Which pathway is involved in object recognition?
The ventral stream.
146
Describe the layout of the ventral stream.
Information from the retina projects to V1.
V1 projects to V2, V3 and V4.
V3 projects to the prefrontal cortex (PF).
V4 projects to the temporal-occipital cortex (TEO) and the inferior temporal cortex (IT).
147
Where are the ventral and dorsal streams integrated?
In the prefrontal cortex.
148
Name the five regions that are key to object recognition, and explain why these regions are key to object recognition.
Temporal lobe.
Para-hippocampal regions.
Peri-rhinal cortex.
Entorhinal cortex.
Hippocampus.
These regions mediate memory and recognition.
149
Which concept underlies object recognition?
Perceptual constancy.
150
Define perceptual constancy.
Things that do not change, such as colours or spatial relationships, are represented independently of distance or movement.
151
What is the importance of perceptual constancy?
It allows you to recognise objects or people in all different types of orientations or environments.
152
How does shining a light onto the centre of the receptive field impact ganglion cell function?
It causes AP firing of the ON-centre ganglion cell with inhibited spiking in the OFF-centre bipolar cell.
153
How does a shadow over the centre of the receptive field impact ganglion cell function?
It causes depolarisation of the OFF-centre bipolar cell resulting in AP firing of the OFF-centre ganglion cell.
