38 - NeuroScience - Occulomotor System Flashcards
(75 cards)
1
Q
What are the differences between an eye and a camera?
A
Camera = Still, Eye = Moving Camera = 2D Image, Eye = 3D Image
2
Q
Eye Muscle Anatomy
A
3 Pairs of Muscles Meridians: Horizontal Vertical Vertical Twisty
3
Q
Types and Functions of Eye Movements
A
Stabilization
Depth
Foveation
4
Q
Brainstem Circuits
A
X
5
Q
Subcortical Circuits
A
X
6
Q
Cortical Control
A
X
7
Q
Pair of muscles that moves the eye along the horizontal meridian (towards nose or away from nose)
A
Lateral Rectus (Abducts) Medial Rectus (Adducts)
8
Q
Pair of muscles that moves the eye along the vertical meridian (up or down)
A
Superior Rectus (Elevation) Inferior Rectus (Depression)
9
Q
Pair of muscles that moves the eye along the vertical meridian while twisting
A
Superior Oblique (Depression & Intorsion) Inferior Oblique (Elevation & Extorsion)
10
Q
Intorsion
A
Top rotates towards the nose (Superior Oblique)
11
Q
Extorsion
A
Top rotates towards the ear (Inferior Oblique)
12
Q
Main Function - Stabilization
A
Stabilizes the visual world against large changes (moving head or moving world)
Primitive reflexes
Don’t require a fovea
Vestibulo-Ocular Reflex (VOR) Optokinetic Nystagmus (OKN)
13
Q
Main Function - Depth
A
Allow us to bring both eyes to focus at an appropriate distance
Vergence (slow)
14
Q
Main Function - Foveation
A
Specific to animals that have a fovea
Place the fovea on selected items of interest
Saccades (rapid rotations of the eyeball) Smooth pursuit (slow)
15
Q
Vestibulo-Ocular Reflex (VOR)
A
Follow large, full-field motion caused by HEAD MOVEMENTS (no visual input)
Quick
Habituates
Relatively little voluntary control
Mostly Mediated by subcortical pathways
16
Q
Optokinetic Nystagmus (OKN)
A
Follow large, full-field motion caused by EXTERNAL MOTION (visual input)
Quick
Does not habituate
Relatively little voluntary control
Mostly mediated by subcortical pathways
17
Q
Phases of Nystagmus (both VOR & OKN)
A
Quick Phase
Slow Phase
See-Saw Pattern, resets when it reaches the limit in the orbit
18
Q
VOR - Why does nystagmus in response to simple rotation of the head (no visual component) slowly habituate?
A
Vestibular signal is fairly transient
19
Q
OKN - When visual components are added, does nystagmus habituate?
A
No!!
20
Q
Vergence
A
Allows two eyes to simultaneously point at a single object.
Align eyes so that the single image hits the Foveal Region of each eye
21
Q
Converging
A
Eyes rotate inward
22
Q
Retinal Disparity
A
Difference between the displacements from the fovea that a single object exerts on each eye. Allows us to determine depth.
Encoded first in Primary Visual Cortex, used to compute distance relative to the center of gaze
Small range of disparities can be told apart. Anything outside of that small range (close to the object of focus) is actually seen in double. This double vision is filtered out of our perception.
23
Q
Strabismus
A
Ocular misalignment
24
Q
Types of Strabismus
A
Hypotropia
Hypertropia
Exotropia
Esotropia
25
Hypotropia
Eye Turns Down
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Hypertropia
Eye Turns Up
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Exotropia
Eye Turns Out
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Esotropia
Eye Turns In
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Effects of Strabismus
Images from each eye do not fuse with each other!!!!
Brain suppresses visual input from one of the eyes.
Monocular vision.
Some folks can alternate dominance of each eye. More commonly, one eye is favored.
Relatively normal vision, but LACK OF DEPTH PERCEPTION
30
Amblyopia
No depth perception
Strabismic or Refractive
2 - 3% of the population
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Deficits:
Driving
Walking
Manual Dexterity
Reading
Visual Function
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31
Refractive Ambylopia
Two eyes have very different refractive errors, and one eye becomes suppressed
32
Traditional Theory on Amblyopia
"Critical Period"
Depth perception can only be learnt if there is normal visual input EARLY in life (6 - 12 months of age)
33
More Modern Theory on Amblyopia
Improvements can happen at any point, but just with LOTS of practice. This theory is undergoing a lot of research
Sue Barry gained depth perception at 48 yo
https://www.youtube.com/watch?v=XCCtphdXhq8
34
Saccades
Rapidly moves the fovea to a new position
Target moves
Brief extreme burst of eye velocity
Eye snaps to a new position very rapidly
Referred to as a "step" in eye position
Relies on "position errors"
"Muscle Burst"
Vision is suppressed during saccade (no visual input for 10 - 50 ms)
35
Position Errors
The distance of the target from the current center of gaze
36
Smooth Pursuit
Matches eye velocity to target velocity
Target moves
Eye moves initially in the direction of the VELOCITY of the target, not necessarily the new position.
Kicks in earlier than the position system (Saccades).
When the position system has caught up, the eye tracks the target smoothly
Relies on "velocity errors"
Slow
Foveal vision is not suppressed
37
Velocity Errors
The velocity of a target relative to the retina
| Sometimes called "retinal slip"
38
Stabilization Pathways
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Vestibular/Full Field
Rapid
Reflexive
Don't require much cognitive control
Mostly subcortical pathways
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39
Depth and Foveation Pathways
Small Stimuli
Requires selection (which target am I focusing on?)
Engages cortical areas like whoa
40
Lowest Level Pathway
Brainstem/Oculomotor Circuitry
Through cranial nerves
Directly drives ocular muscles
41
Middle Level Pathway
Basal Ganglia
Caudate
Substantia Nigra
Superior Colliculus
Projects down and drives the Brainstem nuclei
42
Highest Level Pathway
Cortical areas in the parietal and frontal lobe
43
Principles of Ocular Engineering
Inter-ocular coordination: Conjugate and disconjugate movements
Separate neural signals for:
High velocity (saccadic "burst")
Low velocity (smooth pursuit, VOR, OKN, vergence)
Position
Calibration by the cerebellum
44
Inter-Ocular Coordination
Brainstem/Oculomotor Pathway
| Neurons innervating EOM live in Nuclei III IV & VI in the brainstem, send axons through cranial nerves
45
Horizontal Conjugate Version (looking left or right with BOTH eyes) - Pathway
Ipsilateral Abducens Nucleus (Lateral Rectus)
Contralateral Oculomotor Nucleus (Medial Rectus)
Medial Longitudinal Fasciculus connects the two
46
Medial Longitudinal Fasciculus
Coordinates ipsilateral Abducens nculeus to contralateral Oculomotor nucleus for horizontal conjugate version
Susceptible to stroke or MS, leading to Internuclear Ophthalmoplegia
Vergence is STILL INTACT
47
Internuclear Ophthalmoplegia
Failure of horizontal conjugate version (fast or slow)
48
Position & Velocity Signals
Intense bursts of nerve activity corresponding with saccade
Firing rate scales with saccade amplitude
Peak velocity of eye increases with the size of eye movement
Tonic firing maintained after the burst
Firing rate scales with position of the eye
49
Saccade "Burst" Velocity - Pathway
Frontal Eye Field (SOME excitation) + Omnipause Neurons (STRONG inhibition)
Superior Colliculus
Paramedian Pontine Reticular Formation ("burst generator")
Abducens Nucleus
Lateral Rectus & Contralateral Medial Rectus (Via MLF, then Occulomotor Nucleus)
50
Omnipause Neurons
Housed in the Dorsal Raphe
Provide strong inhibition to the "burst neurons"
Active continuously when eye is still
Pause activity when the eye is in motion
It's a STRONG brake within the brainstem on the saccadic system
51
Slow Eye Velocity - Pathway
Semicircular Canals, Subcortical Input (OKM) & Cortical Input (speed)
Medial Vestibular Nuclei
Bilateral input to Abducens Nucleus
Lateral Rectus & Contralateral Medial Rectus (Via MLF, then Occulomotor Nucleus)
52
Signals of eye position
Specify stationary position in the orbit
Nucleus Prepositus Hypoglossi
Projects bilaterally to Medial Vestibular Nucleus & Abducens Nucleus
53
Calibration
Adjust neural signal to compensate for muscular weakness (greater neural signal necessary for same saccadic movement)
Adjust for curent position of the eye, elastic restoring forces in the orbit (greater signal required to move the eye to a more eccentric orbital position because of the elastic restoring force
Calibration provided by the Cerebellum
54
Cerebellar Degeneration - Effects on Calibration
Eye movements are too small (if the eye is moving further deviated)
Eye movements are too large (if the eye is moving towards the center)
55
Saccadic Adaptation
Tests how subjects compensate for visual errors.
Aim for 21 degrees, and the computer tricks you by moving to 15 degrees.
You adapt by aiming for 15 degrees in the future.
Can't do it with cerebellar degeneration
56
Cognitive Control
"Scan Path" depends critically on what subjects are trying to do. Ask about riches, they look at the furniture. Ask about age, they look at the face. Duhhhhh
Eye precedes hand, leaves before hand is done.
57
Visuomotor transformations when selection is made
SNpr typically inhibits Superior Colliculus
When a saccade is needed, Caudate Nucleus inhibits SNpr for a "pause," disinhibiting Superior Colliculus
58
Frontal Eye Field
Projects to Caudate Nucleus
Projects to Superior Colliculus
Directly projects to brainstem
Closely related to final eye movement
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Parietal Eye Field
Posterior Parietal Cortex
Projects to Caudate Nucleus
Projects to Superior Colliculus
60
Supplementary Eye Field
Higher level structure
Poorly understood
Has to do with eye movements
Activity is more cognitive and task-dependent
May provide cognitive control to the Frontal Eye Field
61
Frontal Eye Field Visual Neuron
Visual Neurons - Majority of Neurons in the FEF
Visual Neurons respond to the appearance of a salient object within its receptive field
62
Attentional Enhancement
Visual Neuron firing is STRONGER if the subject is planning to make an eye movement to that stimulus. The subject is USING the visual stimulus to plan a movement, so there is a greater signal.
No temporal relationship to saccade. Visual Neuron firing is over by the time the eye moves.
No Visual Neuron firing in the dark, even in the presence of saccades.
63
Frontal Eye Field Movement Neuron
Movement Neurons respond VERY weakly to the appearance of a salient object within its receptive field.
When the subject plans and executes an eye movement, the neuron gears up, then fires really hard to execute the saccade.
These neurons respond to a saccade in the dark.
64
Some neurons do both.
They are prevalent in both FEF and PEF
65
Where are Movement Neurons found?
Exclusively in the FEF
66
Parietal Eye Field's Goal
Selecting targets from the visual world.
67
Frontal Eye Field's Role
Make the final decision about whether or not to move to that target.
68
Parietal Eye Field damage leads to
Parietal Neglect
| Impaired attentional selection in visual space
69
Parietal Neglect
Patients lose awareness of visual stimulae that are contralateral to the parietal lesion (often in a hemifield)
Not blindness. Completely unaware that there is something there.
70
Frontal Eye Field Damage
Impairs attentional selection
| Impairs voluntary saccades
71
Combined Collicular & FEF lesions (in monkeys)
No saccades what so ever
72
Frontal lesions
Can't make antisaccades.
Can look AT something all right, following visual input.
Can't look AWAY from something, executing a movement of the EOMs without visual guidance.
73
The Antisaccade Task
Have patient stare straight ahead
Flash a stimulus on one side and tell the patient to look AWAY from it.
This voluntary ability is severely impaired with frontal damage
74
Supranuclear Control of Pursuit
Velocity Errors (movements of small targets across the retina)
Striate Cortex
Projects to Middle Temporal (MT) and Middle Superior Temporal (MST), which contain movement-sensitive neurons.
Project to brainstem nuclei (Nucleus Reticularis Tegmenit Pontis)
Provide command for slow eye velocity
OR
Project to FEF (adjacent to representation of rapid saccadic eye movement) deep in the suclus. Important for INITIATING smooth pursuit
75
Deficits in Smooth Pursuit - Origins
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Cerebellar Disease
Brainstem Disease
Parietotemporal Lesions
Frontal Lesions
Clinical diseases with an attentional deficit (Alzheimer's or any frontal dementia, schizophrenia)
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