Lectures 7 & 8 - Central Visual Pathways Flashcards
1
Q
In which central visual pathway structures are the receptive field properties of the retina inherited? What is this called?
A
Monocular cells (LGN, V1) and binocular cells (V1 and extrastriate) => Retinotopy
2
Q
What is the central visual pathway?
A
Retina => optic nerves => optic chiasm => optic tracts => LGN => optic radiations => primary visual cortex => extrastriate cortex => PPC or temporal lobe
3
Q
What is another name for the central visual pathway?
A
Retinofugal pathway
4
Q
What is each optic nerve composed of?
A
Afferents from only one eye
5
Q
What is each optic tract composed of?
A
Afferents from both eyes:
- Temporal retina of ipsilateral eye
- Nasal retina of contralateral eye
6
Q
Result of small lesion to optic nerve? 2 names
A
Monocular selective blindness = scotoma: partial loss of vision or a blind spot in an otherwise normal visual field
7
Q
Result of transection of optic nerve?
A
Monocular blindness
8
Q
Result of optic chiasm lesion?
A
Bitemporal blindness
9
Q
What is anopsia?
A
Specific blindness within the visual field
10
Q
What is the primary target of the optic tracts?
A
LGN (thalamus)
11
Q
What are 3 other targets of the optic tracts? Purpose?
A
- Hypothalamus: suprachiasmatic nucleus for daily biological rythms
- Midbrain: pretectal nucleus for constriction of the pupil and accommodation of the lens
- Midbrain: superior colliculus for movement of the head in response to light
12
Q
Are LGN cells monocular or binocular?
A
Monocular
13
Q
How are the cells organized in the LGN?
A
Layers according to which eye they receive input from and from what type of RGC
14
Q
How are receptive field characteristics of the retina inherited by the cells of the LGN?
A
Each cell only receives input from either M-type, P-type, or nonM-nonP-type RGCs
15
Q
How are the layers of the cells in the LGN numbered?
A
Most superficial layer is layer 1
16
Q
What do we call LGN cells that receive input from P-type RGCs?
A
Parvocellular
17
Q
What do we call LGN cells that receive input from M-type RGCs?
A
Magnocellular
18
Q
What do we call LGN cells that receive input from nonM-nonP-type RGCs?
A
Koniocellular
19
Q
Where are LGN cells that receive input from nonM-nonP-type RGCs located? From what eye do they receive input?
A
Ventral to each principal layer and receive input from the same eye as the overlying principal layer
20
Q
Where is the striate cortex located?
A
Occipital lobe (area 17)
21
Q
Describe the structure of the striate cortex.
A
Laminar structure with 6 principal cell layers (I-VI) and additional sub-layers
22
Q
Who has a larger striate cortex: primates or humans?
A
Primates
23
Q
What is the calcarine fissure?
A
Located on the medial surface of the occipital lobe and divides the visual cortex into two
24
Q
What is the major input layer of the striate cortex?
A
Layer IVC
25
From what cells does layer IVC of the striate cortex receive input?
1. Magnocellular LGN
| 2. Parvocellular LGN
26
From what cells do other layers of the striate cortex receive input?
Koniocellular LGN
27
How are cells in layer IVC of the striate cortex organized?
Organized into monocular dominance columns (each column receives input from the same eye)
28
How were monocular cell layers of the LGN and ocular dominance columns of the IVC layer of the striate cortex identified?
Radioactive tracing
29
Layer 1 of LGN:
1. Ipsilateral or contralateral visual input?
2. Type of RGC?
1. Contralaterial
| 2. Magnocellular
30
Layer 2 of LGN:
1. Ipsilateral or contralateral visual input?
2. Type of RGC input?
1. Ipsilateral
| 2. M-type
31
Layer 3 of LGN:
1. Ipsilateral or contralateral visual input?
2. Type of RGC?
1. Ipsilateral
| 2. P-type
32
Layer 4 of LGN:
1. Ipsilateral or contralateral visual input?
2. Type of RGC?
1. Contralateral
| 2. P-type
33
Layer 5 of LGN:
1. Ipsilateral or contralateral visual input?
2. Type of RGC?
1. Ipsilateral
| 2. P-type
34
Layer 6 of LGN:
1. Ipsilateral or contralateral visual input?
2. Type of RGC?
1. Contralateral
| 2. P-type
35
What is layer IV of the striate cortex divided into?
Layers a, b, c alpha, c beta
36
Where do magnocellular LGN neurons project?
Layer IV c alpha of striate cortex
37
Where do parvocellular LGN neurons project?
Layer IV c beta of striate cortex
38
Where do koniocellular LGN neurons project?
Layers II and III of striate cortex
39
How many layers in the striate cortex?
6
40
What do the cells of the striate cortex that receive input from kionocellular LGN neurons form?
Monocular cells organized in blobs
41
Describe the binocular cells of the striate cortex. Where are they located?
Cells that receive converging output from monocular cells in layer IV c ocular dominance columns (alpha and beta) located in interblob regions
42
Describe the receptive field characteristics of binocular cells of the striate cortex.
Orientation selective cells, meaning they respond maximally to a bar of light at a specific orientation in their receptive field
43
What are the 2 types of binocular striate cortex cells?
1. M-type
| 2. P-type
44
Can a binocular striate cortex cell receive input from both magnocellular and parvocellular cells of layer IV?
NOPE
45
Which layers of the striate cortex have interblob regions?
All except IVC
46
Describe direction selective cells of the striate cortex.
Subset of M-type orientation selective cells that are ALSO direction sensitive, meaning they respond they respond maximally to a bar of light moving in a direction perpendicular to the preferred orientation
47
What are the 3 central visual pathways originating at the RGCs of the retina? What can be said about how they relate to each other?
1. Magnocellular pathway
2. Blob pathway
3. Parvo-interblob pathway
They are parallel (but not perfectly)
48
What is another name for the blob pathway?
Koniocellular
49
Function of magnocellular pathway?
Analysis of motion
50
Function of blob pathway?
Analysis of color
51
Function of parvo-interblob pathway?
Analysis of shape
52
Which central visual pathway(s) constitute the ventral stream?
1. Blob pathway
| 2. Parvo-interblob pathway
53
Which central visual pathway(s) constitute the dorsal stream?
Magnocellular pathway
54
Describe the magnocellular pathway.
M-type RGCs in retina => magnocellular LGN cells in layers 1 and 2 => monocular cells in layer IV C alpha of striate cortex => Direction selective binocular cells AND unicular cells of the striate layers outside IVC => extrastriate cortical areas
55
Describe the blob pathway.
nonM-nonP type RGCs in retina => koniocellular LGN cells in between layers of LGN + layers IV C alpha and beta of striate=> monocular cells in blobs of striate cortex outside IVC => extrastriate cortical areas
56
Describe the parvo-interblob pathway.
P-type RGCs in retina => parvocellular LGN cells in layers 3-6 => monocular cells in layer IV C beta of striate cortex => orientation selective binocular cells AND unicular cells of the striate layers outside IVC => extrastriate cortical areas
57
Where is the extrastriate cortex?
Occipital lobe
58
Describe the 2 extrastriate pathways.
Extrastriate cortex => dorsal "where" pathway to posterior parietal cortex OR ventral "what" pathway to inferior temporal lobe
59
Function of dorsal pathway?
Analysis of visual motion and visual control of motor action
60
Function of ventral pathway?
Object recognition
61
What are the receptive field characteristics of the dorsal extrastriate pathway?
1. Direction selective cells
2. Radial motion cells
3. Circular motion cells
62
What are the receptive field characteristics of the ventral extrastriate pathway?
1. Color selective cells
| 2. Orientation selective cells
63
What are the symptoms of damages to the dorsal extrastriate stream?
Deficits in:
1. Spatial orientation
2. Motion detection
3. Guidance of visual tracking eye movements
64
What are the symptoms of damages to the ventral extrastriate stream?
Deficits in:
1. Complex visual perception tasks
2. Attention
3. Learning/memory
65
What 2 particular conditions can result from damage to the ventral extrastriate stream?
1. Cerebral achromatopsia
| 2. Prosopagnosia
66
What is cerebral achromatopsia?
Type of color-blindness caused by damage to the ventral extrastriate stream, rather than abnormalities in the cells of the eye's retina.
67
What is prosopagnosia?
Damage to fusiform face area in the inferior temporal lobe leading to face blindness
68
What kind of neurons are found in the striate cortex?
Pyramidal, except for the ones in layer IV
69
What 2 factors is the initial development of the organized visual system dependent on?
1. Prenatal chemical signals (e.g. ephrins)
| 2. Prenatal spontaneous activity with Hebbian plasticity
70
What is the maintenance of the organized visual system dependent on?
Normal visual experience during the postnatal sensitive period to trigger experience (activity) dependent plasticity and compare relative activity levels between eyes
71
What is Hebbian plasticity?
Cells that wire together must first fire together = only synapses in which depolarization is concurrent in the presynaptic and postsynaptic element are maintained
72
What kind of receptors mediate Hebbian plasticity? Describe each.
Ionotropic glutamate receptors:
1. AMPA receptors: Na+ permeable
2. NMDA receptors: Ca++ and Na+ permeable with a unique Mg++ block removed by AMPA mediated depolarization
73
What is LTP?
Calcium influx through NMDA channels opened by AMPA mediated depolarization causes activation of CamK II to mediate AMPA receptor insertion to make the postsynaptic membrane more sensitive to glutamate
74
What is another name for LTP?
AMPAfication
75
How is LTP induced?
High frequency stimulation
76
Which neurons are more susceptible to LTP: neurons with only NMDA receptors or neurons with both NMDA and AMPA receptors?
Neurons with both NMDA and AMPA receptors
77
What are the 3 characteristics of LTP? Explain each.
1. State dependent: state of postsynaptic membrane potential determines whether or not LTP will occur (it'll only occur if postsynaptic depolarization occurs within 100 ms of presynaptic depolarization)
2. Specific: when LTP is induced at active synapses it does not occur at inactive synapses at the same cell (inactive synapses are subject to LTD)
3. Associative: if a synapse is weakly active at the same time another synapse onto the same neuron undergoes LTP, both synapses will undergo LTP
78
What is synaptic weakening?
LTD: lack of correlated pre and postsynaptic activity => low NMDA mediated Ca++ influx => removal of AMPA receptors => synapse silencing and depression => removal during development
79
What is LTD during development called?
Neural pruning
80
What is LTD modulated by?
Calcineurin and protein phosphatase 1
81
What is calcineurin?
Calcium and calmodulin dependent serine/threonine protein phosphatase
82
LTD Ca++ rise?
Small
83
LTP Ca++ rise?
Large
84
Are NMDARs activated in both LTP and LTD?
YUP
85
What is the sensitive (critical) period in cats?
3rd week to 3 months
86
What is the sensitive (critical) period in monkeys?
0 to 6 months
87
What is the sensitive (critical) period in humans?
0 to 1 year (most critical) but extends to 5-10 years old
88
What are the effects of monocular deprivation postnatally?
- The eye not receiving stimulation will be less represented in the cortex (shrunken ocular dominance column)
- The eye receiving stimulation will be more widely represented in cortex (expanded ocular dominance column): basically compensation
89
What are the 2 forms of activity dependent plasticity?
1. Hebbian plasticity
| 2. Experience dependent plasticity
90
What do bright lines on images about ocular deprivation mean?
Cells received signal (aka tracer)
91
Does the normal central visual pathway have about the same allocation of neurons for both eyes?
YUP
92
What could lead to monocular deprivation?
Monocular congenital cataract
93
What happens if ocular deprivation extends past the critical developmental period?
Alterations to the visual system will
| be permanent
94
Effect of binocular deprivation?
No relative changes to ocular dominance columns because the visual system compares relative activity in each eye, BUT shrinking of all layers due to decrease innervation
95
Do binocular cells of V1 receive input for kionocellular LGN?
NOPE
96
Does the blob pathway have any binocular cells
NOPE
97
Do all striate cortex layer other than 4C have interblob regions?
YUP
98
From which 3 places do striate blobs receive input?
1. Koniocellular LGN cells (layers II and III)
| 2. Layers IV C alpha and beta of striate
99
Which cells are hyperactive in acromegaly/gigantism? What are the 2 types? Describe each.
Acidophil cells of the anterior pituitary:
1. Somatotrophs secrete somatotropin/GH
2. Mammotrophs secrete prolactin
100
Result of transection of optic tract?
Homonymous hemianopsia
101
Result of transection of optic radiations?
Left homonymous hemianopsia
