Types , role and control of neural eye movements Flashcards
why do we have eye movements
to achieve clear and stable vision
so that the image is held stable on the fovea which allows for the best va
avoid oscillipsia whilst compensating for head movements
so that we stimulate both fovea simultaneously
binocular vision
steropisis
are eyes movement system is designed to move the two eyes together to look at the object of interest so that we get bincolar single vision and steroposis
what are the two types of eye movement
those that keep the images steady on the retina e.g. vestibular collar , and optokinetic and smooth pursuit systems , there are also eye movement systems that change the line of sight to a new object of interest and hold it on the fovea e.g. saccades and vergence
what does fixation entail
fixation i.e. keeping the eyes still is about holding the image steady on the fovea to achieve clear vision
because that is where there is the greatest density of photoreceptors
when the image moves from the central fovea you have a reduced va and oscillipsia (the image that we are seeing is not stable and it is actually wobbling)
we dont keep our eyes still all of the time there is a small amount of retinal motion called saccadic intrusions
when we slip from that slightly unstable motion to nystagmus that is when the oscillations are abnormal
that is when a another oscillation is happening and we possibly have nystagmus
so patients may report blur or oscillopisa or things moving around
describe saccades
conjugate movement i.e. the eyes are moving together
fast - ballsistic
they are very brief and accurate to place an object of interest on to the fovea
dont interfere with vision - saccadic suppression e.g. when you are crossing a road and a car is coming down the road you move your eyes quickly to your periphery so that you can see the car but you dont see your eyes making the movement
voluntary (different stimuli)
on command
rem occurring during sleep
fast phase of the okr and the vor
what are the features of saccades
latency (i.e. length of time before the saccade starts)
e.g. when has the object appeared and where have the eyes started to move
= 200m/s
velocity = 400- 700 degrees/sec
often reported as the peak velocity - how fast the eyes move when they are at there fastest
larger saccades have a higher peak velocity
what is the saccade mean sequence
the bigger the movement - i.e. the more they have too move the faster they will move to get there
describe the amplitude of saccades
amplitude referes to how far the eyes have to move to get from one target to another depends on the peak velocity
what are you looking at in saccades when you testing accuracy
you are looking at accuracy
speed - i.e. are they slow at initiating the saccade
latency
you are also testing it horizontally and vertically
hypometric = often in small saccades (they undershoot the target)
hypermetric = they overshoot the target
how do the eyes make a saccade
through a pulse and step process - this is the innervation that is ent to the eye muscles that gets them to a new position
there is a big pulse which makes the agonist muscle get to the new location
pulse gets the eyes to the new position - you dont want eyes to drift into the middle because you have lots of tissue in the orbit - the eyes are designed to be in the primary position so if you keep the eyes in the eccentric postion - they will drift back to the midline - so the step is the innervation that holds the eyes in there new position
what happens to the antagonist and agonist muscles during the pulse step process
pulse = high frequency burst of activity
which happens to the agonist muscle which generates the forces needed and created a vicious drag
during the step process theree is a higher level of tonic innervation to the agonist muscle which holds the eye in its new position
and there are orbital elastic restoring forces
the antagonist muscle receives a reciprocal innervaion of change (i.e. the antagonist muscle must relax aswell)
when a saccade is initiated how is its eccentric position maintained
there are a collection of neurone called the neuron integrators ( which are a distribution of neurone in the brainstem) - this helps to maintain eccentric gaze - neural integrator - integrates velocity and position to mathematically keep the eyes in that eccentric position of gaze
if you have a leak in your neural integrator you will not be able to maintain an eccentric position of gaze - the eyes will drift back to the middle and saccade
what happens if you have a leaky neural integrator
unable to hold eccentric gaze position
eyes drift back to the centre
saccade back to the eccentric position
gaze evoked nystagmus
what neurones are involved in saccades
burst neurons intikte the pulse (excitatory)
omnipause nuerons (inhibit the burst neruons)
where are the burst neurones responsible for horizontal saccades situated
paramedian pontine reticular formation
pprf (horizontal saccades
rostral interstitial nucleus of the medial longitudinal fasiculus (rimlf) - vertical saccades
what neurone are responsible for the holding the eyes in its eccentric position (step process)
the neurone that activate the tonic activity to maintain the eye In its eccentric position are in the nucleus prepostius hypoglossi for horiztonal saccades and for vertical saccades they are in the medial vestibular nucleus for horizontal gaze holding
interstitial nucleus of cajal is in the midbrain which is responsible for vertical and torsional gaze holding
what is accuracy of saccades dependent on
age and fatigues - old people and babies
what types of saccades can you have
you can have rudimentary reflexive saccades or higher level voluntary saccades the brain monitors accuracy and makes adjustments
e.g. for hypometric and hypermetric saccdes it will make an adjustment so that the eyes can reach the target
when you record saccades what do you record
you can record where the abnormlaities are in the saccadic system e.g.
in the initiation , accuracy , velocity , pulse , step or a pulse - step mismatch
what happens in the brain to get the signal to the eye muscles to make a saccade
there are 2 parallel pathways that converge In the brainstem one of the pathways comes from the frontal cortex and the other one comes from the parietal cortex
what are horizontal saccades in control by
horizontal saccades are under contralateral control if you want to make a saccade to the left it is the right frontal eye field and the posterior parietal cortex that are sending that signal to saccade to the left the pathway decussates at the midbrain probably at the level of the 3rd cranial nucleus
a saccade to the left is coordinated by which areas in the midbrain
right frontal eye fields and posterior parietal cortex
a saccade to the left is coordinated by which areas in the midbrain
left frontal eye fields and posterior parietal cortex
what are the two parallel pathways that converge in the brainstem responsible for
when the 2 parallel pathways converge - (the frontal cortex and the parietal cortex)
the frontal cortex pathway passes via the fef directly to the superior colliculus and also indirectly to the basal ganglia , its involved with self generated gaze changes to remembered anticipated or learned behaviour
the parietal cortex passes from the posterior parietal cortex to the superior colliculus
involved with shifting of visual attention to new targets that appear
what are frontal eye fields responsible for
frontal eye fields initiate visually guided purposive saccades
what is the dorso medial supplementary motor area responsible for
important in learned oculomotor behaviour
what is the dorso lateral frontal cortex responsible for
the dorso lateral frontal cortex is responsible for the programming of saccades and attention shifts to remembered targets
what is the role of the superior colliculus
contains a map to locate the eye to different areas of the visual field
how is the thalamus , the basal glaglia and the cerebellum important in making the pathway for saccades
the thalamus is involved in the programming of saccades
the basal ganglia is involved in thee initattion of voluntary saccades
and preventing unwanted reflexive saccades
and the cerebellum is involved in the control of saccadic accuracy and adaptive stuff(i.e. making adaptive changes if our eyes are not exactly where we need them to be)
what are the components involved in the saccadic pathway
there are areas in the frontal cortex and in the parietal Cortex , there is a pathway going to the superior colliculus and ending up in the pons - making connections with the cerebellum - ultimately the pawtahy ends up in the brainstem making a saccade because we want a saccadic pulse
describe the horziontal pathway for saccades
saccade begins in the left frontal cortex and the partial cortex
the pathway is sent to the superior colliculus - it then deecussates to the other side and ends up in the right pprf (Paramedian pontine reticular formation) - i.e. the pulse generator for horizontal saccades
if we want to saccade to the right we want our right lateral rectus and our left medial rectus to fire
if you were to saccade to the right horizontally where does the impulse come from
the signal ends up in the right pprf and that sends excitatory burst neurones to the right 6th cranial nerve nuclei which sends a pathway from the 6th cranial nerve nucleus to the right lateral rectus - right lateral rectus innervates the right cranial nerve , from the 6th nerve signals also go from the mlf to the ipsilateral 3rd cranial nerve nucleus which innervates the left medial rectus - these are the excitatory burst neurone making the right lateral rectus fire and the left medial rectus fire
when saccading horizontally to the right how are the antagonist muscles relaxing (i.e. how is the signal transmitted from the brain to the areas in the brainstem to ensure the eye muscles make the saccade)
from the right pprf - there are inhibitory burst neurones which inhibit the contralateral 6th nerve nucleus in the pons which makes the left lateral rectus relax and the right medial rectus relax which allows for a saccade to happen to the right
what happens if you have a lesion in the pontine paramedical reticular formation
you will have ipsilateral saccadic paresis (saccadic palsy)
ipsilateral saccadic paresis (saccadic palsy)
paresis of ipsilateral conjugate gaze (gaze palsy)
saccades/ pursuit/vestibular movements are affected
if a patient is unable to make a saccade to the right what type of lesion do they have
a lesion can be localised at a level in or around the right pprf
why is It important to test saccades in patients with 6th nerve palsies
if they have a ipsilateral gaze palsy - because the pprf is in close proximity to the nucleus of the 6th nerve
describe the vertical pathway of saccades
you need bilateral stimulation of the frontal eye fields
that signal ends up in the rostral interstitial nucleus of the MLF
through posterior commissure to the cranial nerve nuclei
it goes through the posterior commissure to the cranial nerve nuclei
what is the posterior commisure
a rounded band of white fibres crossing the middle line on the dorsal aspect of the rostral end of the cerebral aqueduct
connects the 2 cerebral hemispheres along the midline
describe the downgaze pathway for vertical saccades
for downward pathways Impulse needs to go to the 3rd and 4th nuclei that innervate thee inferior rectus and the superior oblique
describe the pawthay for up gaze saccades
pathway needs to go the 3rd narwal nerve nucleus to innervate the superior rectus and the inferior oblique
where do the neurone originate and transmit to that ar responsible for vertical saccades
the excitatory burst neruons originate in the riMLF
motor neurons in the 3rd and 4th nuclei
in the interistial nucleus of cajal
upward EBN projects bilaterally
downward EBN
projects ipsilaterally
describe the pathway for vertical saccades
begins in the midbrain looking at the rIMLF
and the INC to make a upward scare there are excitatory burst neurones sent to the 3rd and 4th cranial nuclir- they innervate the inferior oblique and the superior rectus the INC will get the contralateral superior oblique to relax and the ipsilateral inferior rectus to relax
what happens if you have a lesion in the riMLF
it may have a mild effect on vertical saccades
you will be unable to make ipsilateral torsional saccades
if you have bilateral lesions of the rIMLF you will be unable to make vertical and torsional saccades
describe the smooth pursuit movement
smooth tracking movement (object of interest must be kept on the fovea
to ensure that the object of interest is maintained on/near the fovea
velocity has to match the velocity of the target
stimulus = movement off the fovea
latency = 80- 120ms
can be affected by drugs and age
what is the role of the brain during smooth pursuit movements
during smooth pursuit movements the brain uses vision to monitor the performance of the smooth pursuit and it ig notes movement of background the brain can adjust smooth pursuits and the cerebellum is important in adaptation- if the eyes are going to slow/fast and aren’t on the target the change in smooth pursuit can occur to ensure the eye is looking directly at the target
what are the pathways hypothesised for smooth pursuits called
2 functional divisions of the visual system - they are detecting moving stimuli or what is happening to the object of interest (i.e. the features) in that
vision of moving system = the magnocellular pathway - has retinal ganglion cells - m type and magnoceullaur layers of the LGN
4c alpha of primary visual cortex
parvocellular pathway is involved in feature analysis
contains retinal ganglion cells (p type)
and parvocellular layers of the LGN
layer 4c (beta) of the primary visual cortex
from the visual cortex what areas are important for the response of moving visual stimuli
from the visual cortex the areas that are important the areas are in the striate cortex which projects to the middle temporal visual area - and the middle temporal visual area projects to the medial temporal visual area
what cells in the visual cortex are responsible for smooth pursuits
in the primary visual cortex, v1 brodmann area 17 , striate cortex
cells respond to moving visual stimuli
striate cortex - projects to the middle temporal visual area , which is important in the processing of moving visual stimuli
the mt projects to the medial superior and temporal visual area (MST) which is important for head and eye movement , the mt , most and posterior parietal cortex have connections with frontal eye fields and then the supplementary eye field in the frontal lobe
describe the smooth pursuit pathway
descending pathways (originates in the parties - temporal occipital cortex)
the pathway gets to different nuclei in the pons - e.g. the dorsolateral pontine nuclei and the nucleus reticular tegmenttai points from pontine nuclei tp cerebellum
from cerebral,lum to pontine nuclei (adaptation happening - adaptive feedback)
from the pontine nuclei to the 3rd , 4th and 6th cranial nerve nuclei
describe the simplified pathway of smooth pursuits
detect that something is happening e.g retinal image motion - this goes to the lateral geniculate nucleus which then goes to the primary visual cortex from the primary visual cortex there is then a pathway that goes to the extra striate areas which sends signals to areas in the pons but also to the frontal lobe - you have adaptive feedback from the cerebellum and then the pathway ends up in the cranial nerve nuclei in the pons to make the Smoot pursuit
what abnormalities occur in smooth purists
abnormalities in initiation , gain and symmetry
unilateral lesions can cause a ipsilateral defect of smooth pursuit
what is vergence
visual axes of the image object - interest falls on both fovea smulatenously
disjugate movement - i.e. the eyes are moving in an opposite direction e.g. convergence , divergence , vertical vergence and cyclovergence
when vergence fails the patient will report diplopia and confusion - when vergence fails patients= symptomatic
what is vergence driven by
retinal image disparity and retinal image blur
disparity = a stimulus for fusional vergence movements and blur is a stimulus for accommodative vergence movements
accomodation of lens
pupil constrictiion
describe the speed of vergence
vergence is a slow movement - we have to encourage the patient to do it
blur driven vergence = 200m/s reaction time
disparity driven vergence= 160m/s reaction time
3/12old neonatal misalignments
45 yrs old (slower vergence)
describe vertical vergences
vertical prism fusion range improves with practice , - however they are slower and a lower range of amplitude than a horizontal movement extended in longstanding vertical deviations e.g. in patients with TED
describe the vergence pathway
visual signals (e.g. blur and retinal disparity)
visual signals from the
occipital cortex goes to the mesencephalic reticular formation (midbrain) - that projects to the 3rd cn nucleus - different cells respond to different types of movement
pro[osed tha other cells are involved in vergence e.g mergence neural integrator, vergence tonic and burst cells
abnormalities of conergence
erg, convergence insufficiency / paralysis
divergence paralysis
spasm of near reflex
what is the vestibular ocular reflex
relates to keeping vision steady and gaze steady even though the head is moving
dolls head movement is when the eyes are moved I n an opposite direction to the head movement but at the same speed e..g head movement to the left , eye movement to th =e right
fast movement with a short latency
use vision to monitor the accuracy of the movement
cars indendeptly of visually mediated eye movements
how does the brain detect that the head is moving
vestibular system - inner ear
hair cells and fluids in the ear that detect movement difff structures in the ear detect head movement and send neural impulses to the vestibular nuclei
what structures in the ear detect head movements
peripheral vestibular apparatus
linear movements. transitional vor
otolith organs (utricle and scull)
rotational movements
angular vor
3 semicircular canals on the inner ear
specialised hair cells
convert mechanical forces into neural impulses - inputs are sent to the vestibular nuclei
what are the 2 pathways for the vestibular ocular pathway
the pathway goes via the cerebellopontine angle and ends up in the vestibular nuclei which then project to the
3rd, 4th and 6th nuclei , the different canals in the head detect different types of head movement and initiate different types of eye movement - depending on where that eye movement comes from determines which signals get a movement
which part of the ear is responsible for upward and torsional eye movements
anterior semicircular canal (Ipsilateral superior rectus and contralteeral inferior oblique)
posterior semiccircular canal is responsible for downward and torsional eye movements (ipsilateral superior oblique an d contralateral inferior rectus’s)
lateral semicircular canal is reesponsbiblee for horizontal eye movement s 9ipsilateral medial rectus and contralateral lateral rectus
what other parts of the brain are responsible for the vestibular ocular pathway
vestibular. nuclei also receive projections from other brainstem nuclei , cerebellum and cerebral hemispheres
central role in generating compensatory eye movements (head movement) adaptation
what ar symptoms of vor abnormalities
disorders of vor can cause changes in gain , are and or balance
common signs / symptoms
disequilibrium
unsteadiness
vertigo
nystagmus
blurred vision
oscillopsia(particularly with head movement)
what are features of vestibular nystagmus
it will only be present in the dark where there are no visual stimulus
peripheral and central vestibular lesions can result in vestibular nystagmus- test with and without visual fixation -
peripheral vestibular lesions - mixed vertical and torsional nystagmus
central vestibular lesions (purely vertical or purely torsional nystagmus)
what is caloric stimulation
irrigating the ear
with different temperatures f water to try and initiate different eye movements used to try and compare the response to different tempretures of water using it to investigate acoustic nerves
cold water in ear - eyes beat in opposite direction
warm water eye beat in the same direction
what is an optokinetixc response
slow following. movement (pursuit) and a fast re- fixation (saccade) in the opposite direction
only present if afferent visual pathway to cortex and connections to brainstem ocular motor system are intact
when does a optokinetic response occur
occurs during sustained self rotation/sustained movement of the visual envrioment (own drum)/ full field stimulus
persists after stimulus has ceased
optokinetic after nystagmus
what is the ateiology of latent nystagmus
accessory optic pathway
what is the pathway of the optokinetic response
pathway unkown
important areas are MT OR MST
what is a vestibular ocular reflex
holds images of the seen world steady on the retina during head rotation
direction = conjugate and the control is vestibular and the speed is slow
what are optokinetic eye movements
holds images steady on the retina during sustained head rotation direction = conjugate control is supra nuclear and the speed is slow
what are smooth pursuit eye movements
holds images of a moving target on the fovea , direction = conjugate control = supranuclrar and the speed = slow
what are saccadic eye movements
brings objects of interest onto the fovea , the direction= conjugate and the control = supranuclrar and the speed = fast
what are vergence eye movements
when the eyes move in opposite directions so that the images of a single object are placed on both fovea
it is a disjugate movement and the control is supranuclear and the speed is slow
what are nystagmus quick phase movements
directs the fovea towards the oncoming visual scene during self rotation
resets the eyes during prolonged rotation
conjugate movement
control is supranuclear
speed = fast
