Eye movements Flashcards
Where do most of the optic tract fibers go?
To the LG nucleus in the thalamus,
Other than the LG nucleus, where else do the optic tract fibers go?
Superior colliculus, very automatic and short - a reflex!
What does the dorsal stream do? and where does it end?
Identifies where the object is in space and ends in the parietal cortex
What does the ventral stream do?and where does it end?
The “what pathway” - to temporal cortex for object identification
what are the two vertical gaze centers? midbrain!
rostral interstitial nucleus of the MLF (riMLF)
The interstitial nculeus of cajal (INC) (Neural integrator)
what does the superior colliculus do?
Reflex saccades
What two structures relating to visual fields does the pons hold?
- 6th nerve nucleus
2. Horizontal gaze center - PPRF (paramedian pontine reticular formation)
What does the border between the pons and medulla include?
- Vestibular nuclei
2. Nucleus prepositus hypoglossi - (neural integrator)
What does the NPH do?
Holds the eye on a target
What does the PPRF do?
tells the 6th nerve nucleus what to do!
What 3 things are present in the midbrain which relate to eye movement?
- 3rd CN
- riMLF
- 4th CN
What is the function of the VOR?
To keep the eyes still in space by rotating eyes in opposite direction to the head
What 4 things happen when you rotate the head to the right?
- activate the right vestibular nucleus
- Activate 6th nerve nucleus
- contraction of the left LR
- activation of right CN3 and right MR
What is the phasic response in the VOR?
- Occurs in the vestibular afferents
- senses how fast you rotate
- PULSE - higher frequency
To maintain eyes fixated on the left with a right head rotation what needs to happen?
- “step” phase - lower frequency
- Indirect pathway through NPH (nucleus prepositus hypoglossi)
(constant firing of neurons)
What does the NPH do?
Converts the pulse or phasic activity into a tonic activity
responsible for horizontal movements
What is the Pulse or phasic activity from the canal proportional to?
Eye velocity
What is the tonic activity proportional to?
Eye position
Which nucleus in the midbrain acts as the integrator for vertical movements?
Interstitial nucleus of cajal
After how long does the VOR fade/habituate?
30 seconds
How does the visual system compensate for the fading VOR?
The optokinetic reflex
What is the OKR?
Combination of slow pursuit and saccade eye movements -
slow movement in direction of stimulus followed by fast, corrected saccades
- needed when there is sustained slow movement - or large object moving across visual field
How fast are saccades?
900 degrees per second - can change direction/position but not this velocity!!!
How are saccades generated?
- Phasic burst of AP originating in the PPRF
What does the right PPRF do?
generates conjugate ipsilateral rotations
How does the SHORT pathway via the SUPERIOR COLLICULUS respond to flashing/moving stimulus in peripheral retina?
Contralateral INVOLUNTARY reflex saccade via the PPRF in the pons
What stimuli does the SC receive?
- Auditory - loud sound
- Somatosensory - spinal cord - tap on shoulder
- flashing/moving stimulus
What gives the command when we want to make a voluntary saccade?
Parietal eye field - where
Frontal eye field - make the move
Describe the path from the left FEF….to generate a SACCADE - (think PPRF) (move eye to right)
- project to left SC
- cross to contralateral PPRF
- to right abducens (abduct right eye)
- to LEFT MLF
- to LEFT CN3
- —-> contralateral saccades
What neurons prevent unwanted saccades?
Inhibitory pause neurons
Follow the path from the visual cortex to produce a saccade…. describe the contribution of the rest of the cortex in this.
- Visual cortex
- PEF
- PRE frontal eye field (dorsolateral prefrontal cortex - memories of saccades)
+ Supplementary eye field - FEF
- SC
- PPRF
What is the PEF involved in?
Position of the object identification
What does smooth pursuit do?
Matches Target velocity to eye velocity
What does the VOR match?
Matches head velocity to eye velocity
What do saccades match?
Eye position to target position
To generate rightward pursuit….. describe the path from the right Parietotemporal junction
- Right parietotemporal junction provides velocity info to
- Right FEF
- Ipsilateral rightward pursuit
- to RIGHT pons
- left cerebellum/left vestibular
- left brainstem and crosses to right abducens
RIGHT FEF - RIGHT ABDUCENS
Why are vergence movements slower than saccades?
ciliary muscles for accommodation are slow
Which reflex are vergence movements related to?
Accommodation reflex
Saccades VOR have Contralateral effects… but what has ipsilateral effects…
Smooth pursuit
Eyes typically deviate towards…
SAME side of the lesion - AWAY from hemiparetic side
Pontine lesions result in…
Eyes deviating AWAY from the lesion (towards the hemiparesis)
A right cerebral cortical lesion will result in…
- Right frontal lobe damage
- Left hemiparesis
- gaze to the right (unopposed left FEF)
A lesion in the right pons will result in
- Left hemiparesis
2. Leftward gaze
If you have someone with horizontal gaze palsy (cant look to left)… what are two possible reasons for that?
- lesion could be in right FEF
2. or Left pons
what happens with a left MLF lesion
- when you look right, left eye cannot adduct
- Nystagmus in the abducting eye
- Convergence is intact
What is one reason for upgaze palsy?
Dorsal midbrain lesion
In nystagmus if the fast phase is towards the left…..
it is left horizontal nystagmus
3 symptoms of nystagmus
- Blurred vision
- Oscillopsia
- Unsteady gait
What do aminoglycosides - antibiotics do to VOR?
toxic to inner ear - knock out VOR - cant recognize faces - have to keep eyes perfectly still.
Which responds faster the VOR or Pursuit?
VOR responds faster than pursuit
Saccades also have a phasic burst and a tonic phase
Direct path mediates phasic command - PULSE
The tonic phase is still mediated by the NPH to hold eyes in position - STEP
What supplementary parts of the cortex are involved in saccades
goes to PEF - tracks where, FEF tells brainstem to move the eye
- supplementary eye field (learned sequences of saccades)
- Dorsolateral prefrontal cortex (memories of objects/saccades and decisions, delay a saccade or other)
What does the parieto-temporal junction provide?
VELOCITY information - remember pursuit
Nystagmus lesions often occur …
- Inner ear
- Brainstem
- Cerebellum
