Visual System Flashcards
(132 cards)
1
Q
What is the purpose of the parallel visual pathways?
A
• Parallel visual pathways- enable us to process colour, form and motion of objects in the visual world
2
Q
What is a general outline of the conscious vision optic pathway?
A
• Conscious vision optic pathway: Retina-> optic nerve-> decussation at optic chiasm-> lateral geniculate nucleus of the thalamus-> Primary visual cortex
o Cells of nasal retina decussate to contralateral side
o Cells in temporal half of the retina remain on ipsilateral side
3
Q
What is the optic disk?
A
• Optic disk- point where ganglion cell axons lave the eye to form the optic nerve
4
Q
What is eccentricity?
A
distance from centre of fovea
5
Q
Describe the dendritic trees of midget cells
A
o Midget cells-
Small dendritic trees
• As move away from fovea, dendritic tree size increases
6
Q
Describe the dendritic trees of parasol cells
A
Big dendritic trees
• As move away from fovea, dendritic tree size increases
7
Q
Compare the size of midget cells and parasol cells at any eccentricity
A
o At any eccentricity, midget cells are smaller than parasol cells
8
Q
How do ganglion cells cover the surface of the retina?
A
• Each ganglion cell type forms ordered mosaic which covers the surface of retina in most efficient way
o Any point in the retina is being covered by dendrites of at least one ganglion cell
9
Q
What is the purpose of wide-field ganglion cells?
A
o Chromatically selective (important for colour vision)
10
Q
What is an example of wide-field cells?
A
o Example of wild field cells are small bistratified cells
In general, these cells have low spatial density but large dendritic trees compared to midget and parasol cells
11
Q
What are the three main divisions of the lateral geniculate nucleus?
A
• Three main divisions-
o Top 4 layers- parvocellular
o Lower 2 layers- magnocellular
o Intermediate layers between parvocellular and magnocellular- koniocellular
12
Q
Compare the parvocellular lateral geniculate nucleus layer to the magnocellular lateral geniculate nucleus in terms of:
- Cell size
- Response time course and identity
- Input
- Function
- Receptive field size
- Type of contrast sensitivity
- What part of the visual image each layer is the best at detecting
A
Parvocellular layer:
- Small cells
- Sustained response
- Input from midget ganglion cells
- Responsible for high acuity vision
- On type response
- Small receptive fields
- Sensitivity at high contrast
- Better at detecting higher spatial frequencies
Magnocellular layer:
- Large cells
- Transient response
- Input from parasol ganglion cells
- Responsible for motion perception
- On type response
- Large receptive fields
- Sensitivity at low contrasts
- Better at detecting contrast sensitivity
13
Q
Compare the sizes of koniocells, parvocells and magnocells
A
koniocells are smaller than parvocells, which are smaller than magnocells
14
Q
Describe the response timecourse of parvocells
A
Response timecourse
• Responds quickly to stimulus changes
• Reaches maximum quickly but falls slowly
• Sustained type of response
15
Q
Describe the response timecourse of magnocells
A
Response timecourse
• Responds quickly to stimulus changes
• Reaches maximum quickly and falls quickly
• Transient type of response
16
Q
What cells do parvocells get input from?
A
Get input from midget ganglion cells
17
Q
What cells do magnocells get input from?
A
Get input from parasol ganglion cells
18
Q
What is the function of parvocells?
A
Responsible for high acuity vision
19
Q
What is the function for magnocells?
A
Responsible for motion perception
20
Q
What is the optic axis?
A
o Optic axis-direction that the eye is pointing when it is fixated on an object in the real world
21
Q
What is the fovea? Describe structure and function
A
o Diameter less than a mm
o Point in which our visual acuity is highest
Over half the fibres in the optic nerve contain ascending signals from the fovea
Blood vessels are curled around the fovea but do not enter the fovea
o If lose function, become functionally blind as this is the area in which photoreceptors are most tightly packed
o Photoreceptor cell bodies are most dense at the centre of the fovea but post-receptor layers are most dense in the periphery
This is to reduce light scatter when light reaches fovea
Structural connectivity remains the same
22
Q
Describe the convergence at the fovea
A
o Each bipolar cell is driving by a single photoreceptor and ganglion cells are driven by a single bipolar cell (low convergence)
o Characterised by low post-receptoral convergence
23
Q
What is coverage?
A
• Coverage- the extent to which something is covered
24
Q
What is visual submodality?
A
• Visual submodality- Distinction of the visual modality
25
What is contrast sensitivity?
• Contrast sensitivity-Ability to distinguish an object from its background
26
What is spatial frequency?
• Spatial frequency-change in intensity as a function of space
27
What is low spatial frequency?
o Low spatial frequency: slow change in intensity as a function of space
28
At what spatial frequency do parvocells exhibit the higher amplitude at and why? (in doing so, describe its activity at other spatial frequencies
o Parvocells exhibit highest amplitude response at intermediate spatial frequency
o At low spatial frequency, off-surround antagonises the centre as it is also at a brighter phase of the grading- low response
o At intermediate spatial frequency, on-centre is activated and not antagonised by off-centre-high response
o At high spatial frequency, both on and off surround cancel each other out- no response
29
What is contrast modulation transfer function and when is it highest?
• Contrast modulation transfer function-how your sensitivity to contrast is modulated by spatial frequency
o Low frequencies, threshold is higher than at intermediate and higher frequency
30
What is the nyquist limit?
• Nyquist limit-the highest frequency that can be coded at a given sampling rate in order to be able to fully reconstruct the signal
31
What is the human nyquist limit for the visual system and what cell achieves this?
o At this limit (about 60 cycles/degree in humans), the parvocells are the ones activated
Parvocells can detect higher spatial frequencies than magnocells
32
How do on-centre cells respond to contrast?
• On-centre cells respond to positive contrast
o The greater the contrast between stimulus and background, the greater the response of on-centre cells
o Negative contrast is inhibitory for the on-centre cell
33
How do off-centre cells respond to contrast?
• Off-centre cells respond to negative contrast
| o The greater the negative contrast, the greater the response
34
What is Michelson contrast?
• Michelson contrast= (max phase-min phase)/(max+min)
35
Describe the relative response of magnocells and parvocells to increasing contrast
o Neuron activity increases linearly as contrast increases
o Magnocells show much greater contrast sensitivity than parvocells
At about 50% contrast magnocells response is saturated
• Good for low contrast
Parvocells can signal higher contrasts
• Good for high contrast
36
What is degree of spatial antagonism set by?
o Degree of spatial antagonism is set by the relative strength and relative size of centre and surround
37
What are the advantages of colour vision?
* Identifying objects in the world based on spectral reflectance
* Important linking feature to analyse visual scenes
* Colour signals used by organisms to attract/repel certain other organisms
38
What determines the colour that the object appears?
• White light can be broken up into a spectrum
• Objects appear the colour of the wavelengths that they reflect most strongly
o Each receptor sends its own level of activation to the brain
o Brain interprets relative activation of receptors as colour
39
What is the principle of univariance for colour vision?
• For a single receptor system, if the integral of photon catch by a receptor is identical then the message it sends to the brain will be identical
o Receptor is colour-blind and univariant-
The principle of univariance states that all absorbed photons generate the same response independently of wavelength (that is, any photon that is absorbed by photopigments generates the same response in the photoreceptor)
40
What is the trichromatic theory and who developed it?
• Trichromatic theory (Young and Helmholtz)-
o Different colour experiences are due to the differential activation of just 3 receptor types
Long wavelength- red
Medium wavelength-green
Short wavelength-blue
41
What is the trichromatic theory based on the observation of?
o Based on observation that could generate all possible colours if mix light of just three different wavelengths
42
What wavelength does the red cone absorb and what percentage of total cones does it makeup?
* Readily absorbs 564nm light
| * About 40% of total cones
43
What wavelength does the green cone absorb and what percentage of total cones does it makeup?
* Readily absorbs 534nm light
| * About 40% of total cones
44
What wavelength does the blue cone absorb and what percentage of total cones does it makeup?
* Readily absorbs 420nm light
| * About 10-25% of total cones
45
How does photoreceptor adaptation occur and what happens during photoreceptor adaptation?
o Photoreceptor adaptation-
When stare at colour, adapt to colour
• Excessive firing of particular receptors makes them reach threshold
When neutral colour is added, cone that has not been firing fires more than other cones, who have reached their firing threshold and hence don’t fire much anymore
• The excessive firing of the cones used when staring at the colour skews towards opponent colour
46
What does amino acid sequence in cones determine? Describe
• The amino acid sequence determines spectral tuning
• S cone is non-homologous to M and L cones
o M and L cones have a homologous structure (only 12 amino acid difference)
47
What determines isomerisation probability in cones?
• Isomerisation probability is determined by the local environment-> determined by local amino acid residues of opsin-encoding genes within region of protein
48
Describe how the different cones (short, medium, long wavelength) communicate to their respective ganglion cels
• Dendrites of bipolar neurons reach out across the retina to make contact with all cones of one type in range of their dendrites
o System for selective preservation of signals for one of the 3 cone types in the array
• Blue cone transmits signal to blue cone bipolar cell which transmits signal to on dendrites of small bistratified ganglion cell
o Off dendrite gets input from medium and long cones through diffuse bipolar cell
• Long wave/medium wave cone preserved through tight link between midget system and long wave/medium wave signals
o The density of bipolar and ganglion cells is so high that each cone can connect to an off-type and on-type midget type bipolar cell and in turn transmit that signal to an on-type and an off-type midget ganglion cell
• Different types of cells are excited by some wavelengths of light and inhibited by other specific wavelengths of light
• Cells have an opponent signature
49
Does change in overall density of stimulus change what wavelength cones responds to?
• Change in overall density of stimulus doesn’t change what wavelength cell responds to
50
Describe the evolutionary history of human colour vision and the implications of it
• All diurnal mammals showed dichromatic vision
o S cone gene- on chromosome 7
o L cone gene- on X chromosome
• At 50 million years ago, South American monkeys emerged allelic variations in the genes encoding L and M cone genes on the X chromosome
o Only females expressed trichromatic colour vision
• About 30 million years ago, there was a gene duplication event where both the L and M cone genes were situated on the same X chromosome-> means that every X chromosome encodes for both cones
o Males can show trichromatic colour vision
As there can be errors in transcription due to binding errors, the L or M cone gene can be untranscribed on that specific X chromosome
It is hence more likely that males will be colour vision deficient than females as females have a spare X chromosome
51
Describe the basis of the common inherited colour vision deficiency
* Colour vision deficiencies depend on lack of normal (3) cone number for humans
* As there can be errors in transcription due to binding errors, the L or M cone gene can be untranscribed on that specific X chromosome and one of these genes can be missed
* It is hence more likely that males will be colour vision deficient than females as females have a spare X chromosome
52
What is colour consistency?
• Colour constancy- the ability to maintain relatively uniform colour perception under different lighting conditions
o Means by which the average colour of illumination falling on a scene can be effectively measured or discovered
53
What is the supposed explanation for colour constancy?
• High level association cortex phenomenon
| o Transformation between cells that detect wavelength and cells that react to perceived colours
54
What information do signals that leave the retina contain?
• The signal that leaves the retina contains information concerning brightness, colour, movement, direction of movement and other features
55
Where do axons leaving the retina go to?
o 10% of retinal output goes to the hypothalamus (rhythms)
o 10% of midbrain, superior colliculus (some eye movements)
o 80% go to the layered lateral geniculate nucleus
56
From what cells does the lateral geniculate nucleus recieve input from, what is this input and what layers does this input project to?
Input from midget cells-
• Carry information about colour and shape
• Project to the parvocellular layers
Input from parasol cells-
• Carry information about movement and brightness
• Project to the magnocellular layers
57
From what visual field does the visual cortex receive information from?
o Visual cortex will receive information from contralateral visual field
58
When do retinal ganglion axons become myelinated?
• The optic nerve within the eye is composed of axons that arise from retinal ganglion cells
o In most mammals, most of these axons only become myelinated when they are bundled together at the level of the optic disk to form the optic nerve
59
Is the visual field covered by both eyes entirely separate or entirely overlapping? Why?
• The visual fields covered by both eyes overlap extensively, which is essential for depth perception
o However, the visual fields are not identical, as some visual fields can only be seen by one eye because they are too far in the periphery to be seen by the contralateral eye
60
What is the binocular visual field?
o The section of the visual field that can be seen by both eyes is called the binocular visual field
61
What are the three major steps to the conscious visual pathway?
* Partial decussation of the optic nerve at the optic chiasm
* Retinofugal projection to the lateral geniculate nucleus: segregation of inputs according to eye of origin
* Lateral geniculate projects to the primary visual cortex and terminate in layer 4 (the stria of Gennari)
62
Describe the partial decussation of the optic nerve and why this occurs
• Partial decussation of the optic nerve
o Nasal retina of the eye looks at the ipsilateral visual field
Fibres decussate at the optic chiasm
o Temporal retina of the eye looks at the contralateral visual field
Fibres remain ipsilateral at the optic chiasm
63
What layers of the lateral geniculate nucleus do inputs from ipsilateral fibres terminate in?
o Inputs from ipsilateral fibres project to layers 2, 3 and 5
64
What layers of the lateral geniculate nucleus do inputs from contralateral fibres terminate in?
o Inputs from the contralateral fibres project to layers 1,4 and 6
65
Do neurons in the lateral geniculate nucleus possess different or similar receptive field properties to the neurons in the retina?
o Neurons in the lateral geniculate nucleus possess very similar receptive field properties to the neurons in the retina
They have concentrically and antagonistically organised receptive fields
66
Where is the primary visual cortex located?
o The primary visual cortex (V1, Brodmann area 17, striate cortex) is located in the occipital cortex surrounding the calcarine sulcus
67
How many layers does the primary visual cortex have?
Organised in 6 main (9 real) layers
68
What layer of the primary visual cortex do lateral geniculate inputs from magnocellular cells target?
o Lateral geniculate inputs from magnocellular cells target layer IVCalpha of the primary visual cortex
69
What layer of the primary visual cortex do lateral geniculate inputs from parvocellular cells target?
o Lateral geniculate inputs from parvocellular layers target layer IVCbeta of the primary visual cortex
70
What layer of the primary visual cortex do lateral geniculate inputs from koniocellular cells target? What is the functional consequence of this?
o Lateral geniculate inputs from koniocellular layers go to layers II and III of the primary visual cortex and target the blobs for colour analysis
71
How were primary visual cortex blobs found?
revealed by cytochrome oxidase staining
72
Describe the properties of cells found in layer IVCalpha of the primary visual cortex
• Simple cells have elongated receptive fields and their preferred stimulus is an elongated bar of a particular orientation
o Simple cells have an on centre and an off surround
• These are mostly monocular
73
To what layer do neurons from the primary visual cortex layer IVCalpha project to and what is the functional consequence of this? What is the name of this channel?
Neurons from layer IVCalpha project to layer IVB (M channel for motion detection)
74
Describe the properties of neurons in layer IVB of the primary visual cortex
• Neurons in layer IVB have elongated receptive fields and are responsive to bars
• Neurons in layer IVB are direction selective (sensitive to direction of the bar)
o When bar moves from left to right- neuron fires action potentials
o When bar moves from right to left-neuron produces no change in action potentials
• Neurons in layer IVB are binocular and can be triggered by both eyes
75
Describe the properties of neurons in layer IVCbeta of V1 (primary visual cortex)
Neurons have small, circular and monocular receptive fields
76
To what layers do neurons from layer IVCbeta of the primary visual cortex project to? What is the functional consequence of this?
P channel interblob for shape analysis
• Neurons from layer IVCbeta project to the interblob area of layers II and III
Blobs for colour analysis
• Neurons from layer IVCbeta project to the blobs in layer II and III of the primary visual cortex
77
What are the properties of neurons in the interblob area of layers II and III of the primary visual cortex
o These neurons are complex cells with elongated receptive fields that respond to bars and the orientation of bars, but don’t have a centre-surround area
They have a binocular response and are wavelength insensitive
78
What are the properties of neurons in the blob area of layers II and III of the primary visual cortex?
o Neurons in the blobs of layer II and III have a small and circular receptive field
Display colour opponent responses
Wavelength sensitive and monocular responses
79
How are the inputs from the right and left eyes mapped in the visual cortex?
o Right and left eyes input to adjacent cortical columns
| Inputs from each eye are separated in alternating columns
80
Describe the layered structure of the visual cortex
• Most superficial layer is layer I whilst the deepest layer is layer VI
o Layer IV can be divided into layer IVA, IVB and IVC
Layer IVC can be divided into alpha and beta layers
81
What are the most common receptive fields in V1 and what are they sensitive to?
o V1 receptive fields are elongated and rectangular, and are extremely sensitive to direction of bars of light: different cells have different orientation preferences
82
Describe the orientation-specific arrangement of cells in V1
Orientation preference varies across primary visual cortex areas
• All different orientations were covered in a distance of 1 millimetre
• Cells preferring different orientations are found in columns
o Cells within a column have the same orientation preference to each other
o Cells within a row have different orientation preferences to each other
83
Describe the stimulus activation of elongated receptive field cells in V1
Stimulus activation of the neuron-
• Bars that are not in a preferred orientation will produce no firing rate for the neuron
• If the stimulus is in an angle close to the preferred orientation, a relatively weak response will be produced
• If the bar stimulus is in the preferred orientation, it will elicit a strong response
84
Describe the Hubel and Wiesel model of how V1 elongated receptive fields are built from retinal circular receptive fields
o Hubel and Wiesel model of elongated receptive field
If V1 cortical neurons receive converging input from three or more lateral geniculate nucleus neurons whose receptive fields are aligned on a line in the retina
85
Do ocular dominance and orientation preference map on the same or different patch of cortex?
• Ocular dominance and orientation preference maps from same patch of cortex
86
How is information segregated in the visual cortex (what is the location of the two streams of information)
o Visual information will flow from V1 to different extrastriate cortex via two streams
Ventral stream-analysis of object form (what an object is)
Dorsal stream- analysis of object location (where an object is)
87
Describe the inputs and outputs of the ventral stream of visual information, and what happens when you damage this stream
• Receives mostly parvocellular and some magnocellular inputs
• Projects to extrastriate cortex (V2 and V4) and to inferior temporal cortex
o Damage produces difficulty in object perception
88
Describe the inputs and outputs of the dorsal stream of visual information, and what happens when you damage this stream
Dorsal stream- analysis of object location (where an object is)
• Receives mostly magnocellular input
• Projects to post parietal cortex via V2, V3, V3A, MT (V5), MST
• Damage can produce
o Problems in grasping an object
o Perception of motion problems
89
How does selectivity of cells increase along the visual information flow? Provide examples
o After V1 there is segregation of processing-
Selectivity of cells increases across the visual information flow: specialisation of functional properties along the route from V1 to V4 and MST
• MT- most cells are direction selective
• V4- most cells are responsive to wavelength and colour
V4 seems specialised for colour whilst area MT seems more specialised for motion
Intertemporal cortex (area IT) seems to be specialised for face recognition
90
What implies serial processing in visual areas of the cortex?
o Different visual areas are arranged in a hierarchical manner
Implies serial processing
91
What is the fovea and the role of the fovea, as well as how it achieveshat role?
• Retina
o Fovea contains the highest density of cones connected in a very selective manner
High spatial resolution and high visual acuity
• Limited to 1 degree of the visual field
Low convergence
92
What is the concept of retinal magnification and how is this achieved?
o Visual information arising from the fovea is highly represented in other visual areas- big areas devoted to foveal information
Lateral geniculate nucleus and primary visual cortex are retinotopically organised
Foveal information only occupies 1% of the retinal surface but 20% of the lateral geniculate nucleus and primary visual cortex
• Retinal magnification
93
What moves the eye?
Extra-ocular muscles
94
Are eye movements voluntary or involuntary?
Voluntary and involuntary
95
What are the antagonist extra-ocular muscle pairs and what general movement are they responsible for?
```
Horizontal movement:
• Medial rectus
• Lateral rectus
Mainly vertical movement:
• Superior rectus
• Inferior rectus
Torsion movement
• Superior oblique
• Inferior oblique
```
96
What is adduction?
o Adduction- moves towards the midline
97
What is abduction?
o Abduction- moves away from the midline
98
At what angle does the lateral rectus insert itself into the eye and what is the consequence of this?
o Inserts with an angle of 90 degrees- movement occurs in only one plane
99
At what angle does the superior rectus insert itself into the eye and what is the consequence of this?
o Inserts to the eye with angle of 23 degrees- complex movement
100
At what angle does the superior oblique insert itself into the eye and what is the consequence of this?
o Passes through the trochlea and inserts into the eye at an angle of approximately 50 degrees-complex movements
101
How does the lateral rectus move the eye, what is its primary action, secondary action and tertiary action and by what cranial nerve is it controlled by?
```
Action: Moves eye laterally
Primary action: Abduction
Secondary action: None
Tertiary action: None
Controlling cranial nerve: Abducens (CNVI)
```
102
How does the medial rectus move the eye, what is its primary action, secondary action and tertiary action and by what cranial nerve is it controlled by?
```
Action: Moves eye medially
Primary action: Adduction
Secondary action: None
Tertiary action: None
Controlling cranial nerve: Oculomotor (CNIII)
```
103
How does the superior rectus move the eye, what is its primary action, secondary action and tertiary action and by what cranial nerve is it controlled by?
```
Action: Elevates eye
Primary action: Elevation
Secondary action: In(cyclo)torsion
Tertiary action: Adduction
Controlling cranial nerve: Oculomotor (CNIII)
```
104
How does the inferior rectus move the eye, what is its primary action, secondary action and tertiary action and by what cranial nerve is it controlled by?
```
Action: Depresses eye
Primary action: Depression
Secondary action: Ex(cyclo)torsion
Tertiary action: Adduction
Controlling cranial nerve: Oculomotor (CNIII)
```
105
How does the inferior oblique move the eye, what is its primary action, secondary action and tertiary action and by what cranial nerve is it controlled by?
Action: Elevates eye and turns it laterally
Primary action: Excyclotorsion
Secondary action: Elevation
Tertiary action: Abduction
Controlling cranial nerve: Oculomotor (CNIII)
106
How does the superior oblique move the eye, what is its primary action, secondary action and tertiary action and by what cranial nerve is it controlled by?
Action: Depresses eye and turns it laterally
Primary action: Incyclotorsion
Secondary action: Depression
Tertiary action: Abduction
Controlling cranial nerve: Trochlear (CNIV)
107
What are the different types of eye movements and their primary prupose?
• Gaze alignment (keep the fovea on a visual target)
o Saccades
o Smooth pursuit
o Vergence
• Gaze stabilisation (stabilize the eye when the head moves)
o Vestibulo-ocular reflex
o Optokinetic reflex (OKR)
108
What is a saccade?
Sacccade- A rapid, ballistic conjugate eye movements to bring new objects of interest onto the fovea
109
Are saccades voluntary or involuntary?
• Eye movements towards a stimulus are voluntary or involuntary (sounds, tactile stimuli, verbal commands, memories of space)
110
How big are saccades?
• Most saccades are 15 degrees in size
111
Are saccades always accurate?
• Saccades can land on object accurately or overshoot/undershoot in which case a small corrective saccade is required
112
What is the velocity of saccades?
• Can have a velocity of 700 degrees/second
113
What is conjugate eye movement?
Conjugate- both eyes move in the same direction
114
What is saccadic suppression?
Saccadic suppression-healthy humans do not perceive the motion of this saccade
115
Describe the saccade refractory period
After a saccade, there is a short refractory period of about 150 ms
• Limits number of saccades that can be done in a given period
116
How long do fixation periods last?
200-300 ms
117
Describe Yabus 1960 experiment and findings on saccades and fixation
• Fovea fixation
o Looked at saccades performed when a human face is displayed
o Humans tend to fixate on nose, eyes and mouth which are vital in human communication
Fixation periods lasted from 200-300 ms and movements between fixation points were very rapid
• Saccades for inspecting and planning
o When people look at a painting/scene, people mostly focus on faces
118
Describe the neuroanatomy of saccades
• Cell bodies of neurons that send axons to CN III, IV and VI are located in the midbrain and the pons (output)
o Midbrain and pons receive inputs from the superior colliculus, frontal eye fields (which are under the influence of the supplementary eye fields and posterior parietal cortex)
119
What are smooth pursuits eye movement?
Conjugate movements of eye in order to hold the image of a moving target on the fovea
120
Are smooth pursuits eye movement voluntary or involuntary?
Involuntary (requires a moving visual stimulus)
121
Are smooth pursuits eye movement ballistic or not ballistic?
Not ballistic
122
Describe the activation of muscles in smooth pursuit eye movement
Both agonist and antagonist muscles are activated
123
What is the velocity of smooth pursuit eye movement?
Velocity of 40-50 degrees/second
124
What is the neural pathway of smooth pursuit eye movement?
Neural control of smooth pursuit movement
| • Striate cortex -> MT and MST-> pontine nuclei-> cerebellum-> brainstem
125
What is are vergence movements?
Disconjugate eye movements to adjust for different viewing distances
126
What are disconjugate eye movements?
Disconjugate- both eyes move in opposite directions
127
What are convergent eye movements and what muscles do they engage?
Convergence- both eyes move towards midline, bilateral contraction of medial recti
128
What are divergent eye movements and what muscles do they engage?
Divergence- both eyes move away from midline, bilateral contraction of lateral recti
129
What is the purpose of the vestibulo-ocular reflex and how does it achieve this purpose?
o Vestibulo-ocular reflex
Compensates for rapid head movements
• Not sensitive to slow head movements
Use vestibular input to hold images stable on the retina during brief or rapid head rotation
• Eyes move in opposite direction to head movement
130
What is the purpose of the optokinetic reflex and how does it achieve this purpose?
Eyes move to reduce image motion
Sensitive to slow visual motion
Use visual input to hold images stable on the retina during sustained or slow head rotation
• Repetitive manner- slow movement followed by a fast movement and etc.
o Slow phase is like a smooth pursuit movement
o Fast phase is the return of the eye to the centre of gaze
131
In the LGN, describe which colour vision the parvocellular layer is responsible for
Carries signals for red-green colour vision and for high acuity spatial vision
132
In the LGN, describe which colour vision of the koniocellular layer is responsible for
Adifferent to achromatic changes but strongly respond to short wave (B) sensitivity cone modulation – strong colour selectivity
