Flashcards
Eye movements
Cards in this deck (46)
- 1Role of brainstem (particularly midbrain and pons) as the "final common pathway" for eye movement controlFinal common pathway includes the CNIII, CNIV and CNIV nuclei of the ocular motor system - they are the outputs which drive the eye muscles for all eye movements of all types
* CNIII and CNIV nuclei are located in the midbrain while CNVI is located in the pons - 2Brainstem structures involved in producing the saccadic eye movement neural commandStructures involved are:
* Superior Colliculus
* CNIV nucleus
* rMLF - Vertical saccades
* PPRF - quick saccades
* CNVIII nucleus - 3key cortical regions associated with reflective and volitional (alternatively, exogenous and endogenous) saccades, including where different studies diverge about which areas contribute to waht1. Frontal cortex- influence top-down saccades (volitional)
2. Frontal eye fields - intentional saccade triggering (can be associated with anti than pro-saccades)
3. PPC (posterior parietal cortex) - reflexive saccades (inputs to SC - also contains visuospatial integration)
4. DLPFC (dorsolateral pre-frontal cortex) - prediction/spatial memory/reflexive saccade inhibition - antisaccade task
* damage to area shows increase % of antisaccade errors
5. Parietal cortex - know where to look (for anti-saccades) - 4What is smooth pursuit?Foveate and track slow moving object in visual space
* no delay in tracking despite motion processing and command generation presumably need - only for slow moving objects
* stimulus can be real or perceived (illusory) target motion - 5What is the current ideas about the neural pathway of smooth pursuit?Since smooth pursuit is essentially voluntary - required to actively attending to target
* FEF - key region for pursuit generation
* Overlap between saccade and pursuit pathways - where one falls behind, other can step in (also working together to keep object of interest on fovea)
* contain a number of different regions including attentional areas (e.g. frontal cortex), motion areas (e.g. area MT/MST), SC, basal ganglia and brain stem - 6How does smooth pursuit pathway interact with the saccadic system to maintain gaze on smoothly moving targetsThe saccadic and pursuit system work closely to maintain tracking smoothly moving object
* Where if pursuit lags behind - can have catch up saccades to keep tracking object - 7Why do humans need to have moveable eyes?Humans need moveable eyes as our eyes are frontward facing. While this allows us to have facilitated stereoscopic vision and higher visual acuity, it also limits the visual field. Therefore, our eyes are need to move to increase visual field.
- 8What are the different ways the eyes move?1. Saccades - able to make many small eye moves to perceive single, clear and sharp image of the world around us
2. Smooth pursuit - to track moving objects
3. Optokinetic response - slow eye movements generated in direction of moving stimulus and then a saccade is generated to re-centre eyes - response does not require a particular part of the moving environment, just to a huge movement in visual field
4. Vestibulo-ocular reflex - ocular response to body motion (mainly acceleration)
5. Vergence - when eyes move in opposite directions (works with accommodation of lens and constriction of pupils) - 9How do the various sort of eye movement each serve the cause of vision?1. saccades - obtain a clear and single view of the world - taking details, avoid danger
2. Smooth pursuit - used to track moving objects in visual space
3. Optokinetic response - maintain gaze during movement from visual field
4. Vestibulo-ocular reflex - maintain gaze during acceleration movements of head/body
5. vergence - allows for fixation to occur - 10Fovea (pros and cons)Pro: allows a high degree of visual acuity
cons: only occupies 5% = 1 degree of foveal region
* however, if fovea region is bigger - would require larger optic nerve and cortex - not possible. - 11Saccadic suppression* prevent from seeing our eyes moving from one place to another - shift of the world.
- 12Reflexive saccades* Peripheral retina contains fast-acting change sensitive cells that detect quick visual changes in the visual space.
* Saccadic system also responds to touch and sounds - 13Dorsal and Ventral Stream* serves action and perception respectively
* Ventral takes in move parvocellular input about fine details
* Dorsal takes in magnocellular due to faster speeds
* have multiple interconnections between them
* Koniocellular can contribution to both streams in motion detection, visual recognition and fear pathway - 14What are the demands imposed on ocular motor system by having frontally located eyes, foveae and binocular vision?Frontally located areas are not aligned with each other - thus eye muscles have to move around more than one cartesian axis, also with head movements.
Fovea region is small - required to make many small eye movements to have clear image - 15Anatomical structures - muscles, tendons, fat, nerves - carrying out movements?Muscles: 4 orthogonal rectus, 2 oblique
* Adduction - towards nose (medial rectus)
* Abduction - outwards (lateral rectus)
* Elevation - upwards (superior rectus)
* Depression - downwards (inferior rectus)
* Intorsion - in + down (superior oblique)
* Extorsion - up + out (inferior oblique)
Fat - protect eye form bony orbit
Nerves:
* CNIII - S, M, I rectus + inferior oblique
* CNIV - Superior oblique
* CNVI - Lateral rectus - 16What is a reference frame?Reference frame of the eye - sphere
- 17Which reference frame must be considered if we are to understand what the ocular motor system is doing?The reference frame we must consider that that we are projecting a cartesian world onto a sphere.
- 18What are Donder's and Listing's Law?Donder's Law: When position of line of fixation is given with respect to head, angle of torsion will have a perfectly definite value for that particular adjustment: which is independent not only of the volition of observer but of way the line of fixation arrived in position in question
Listing: Any eye position can be reached from primary position by single rotation of eye around axis lying in plane perpendicular to visual axis when in primary position. - 19What is the role of extra-ocular muscle pulleys in implementing Listing's Law and how does this (if true) reduce the demand on the brain for specifying eye movements?Pulleys change the functional origins of the EOMs - changes force of muscle to generate equal torsion
* constrain muscle movement to implement Listing's Law - 20Conditions that affect saccadesADHD, Schizophrenia and conditions associated with impulsive behaviour
* ability to suppress the unwanted saccade is impaired - error is corrected
Alzheimer's disease, Niemann-Pick Type C
* target is incorrectly looked at - error goes uncorrected
Parkinson's disease - number of impairments in generating endogenous saccades but reflexive saccades are normal early on - 21Complementary roles of Optokinetic and vestibulo-ocular system in gaze maintenance in face of self-motionVestibulo-ocular system - maintain stable gaze as we move in the environment
* senses translational and rotational motions around/along 3 axes - focus on rotation (linear acceleration)
* transient - responses to acceleration (at higher frequencies)
OKN - maintain gaze when environment moves
* steady state response - response to constant velocity
Where one fails to stabilise gaze, the other fills in - 22Function and anatomical basis of 3 neurone arc3 neurone arc - for conjugate eye to maintain gaze
1. Semicircular canals - 8th CN projects to vestibular nucleus
2. contralateral - project to abducens nucleus - 6th CN project to ocular muscle
3. Ipsilateral - project to oculomotor nucleus - 3rd CN projects to ocular muscles - 23Structure and function of indirect pathway which augments or suppresses the vestibulo-ocular reflexIndirect pathway contains:
1. semicircular canals - sense movement of fluid
2. Vestibular nucleus
3. PPH - indirect pathway - induce tonic innervation
4. 6th nucleus - integration of tonic and phasic innervation
5. phasic innervation is from vestibular
* allows for calibration of system - 24Basic OKN substrateLarge movement in visual field
- 25What are the ways in which the "more primitive" OK system has been augmented by smooth pursuit in humans and other primates?In humans - cortex is takes on bigger role in comparison to other primates where it and LGN inputs into NOT
* more smooth pursuit in comparison to OK
* in other animals - OKN arises solely from accessory optic system (AOS) - 26Distinctive features of vergence system* Non-conjugate eye movement
* Innervations to 3rd CN is bypassed - therefore damage to it will not affect convergence
* include cerebellar and parietal cortex
* unique to primates - 27How in daily life vergence is integrated with conjugate versional eye movementNormally, vergence is not done separately, always include versional eye movement as things up close can be of different distance and angle from previous far object
- 28The effect of pathological processes occurring at points throughout the ocular motor system, from extra-ocular muscles to higher cortical functionsExtraocular muscles:
* Fibrotic muscles - due to lack of innervation (congenital - lack of development of cranial nerves)
Neuromuscular junction: Myasthenia gravis - decrease transmission of ACh (result of post synaptic side)
Nerve:
* 3rd nerve palsy - result from diseases (type 2 diabetes) - LE overshoots to left due to compensation - using patch to force more use of RE - generate bigger step of innervation but firing rate is still limited
* cerebellar disease - step is not generated - gaze-evoked nystagmus, can be found in excessive alcohol consumption
Cerebellum:
* damage can cause inaccurate saccades (dysmetria)
* damage to omnipause neurones in pons - inadequately inhibiting the burst neurones - allows for unwanted saccades - found in Parkinson's and progressive supranuclear palsy
* unwanted saccades - due to higher order distractibility (e.g. Alzhiemers and ADHD) - 29How these disorders may affect visual function?* Congenital spinal muscular atrophy - atrophy of alpha motor neurons resulting in muscle wasting - lack of eye movements
* Graves ophthalmopathy/thyroid eye disease - attach ocular muscles by antibodies - expand periorbital fat + EOM - poor vision (can't close eyes) - impaired motility
* Eaton Lambert Syndrome - axon terminal are attached - hinder transmitter release - hinder eye movement
* Damage of PPRF and RIMLF - damage to saccades
* Spinocerebellar atrophy type 2 - lack of pulse step - saccades look like smooth pursuit - same if take anticonvulsant medication or sleepy - 30How eye movement abnormalities may be diagnostically important?* Bilaterally poor tracking - no localised impairment found (might be due to boredom, fatigue and drugs)
*Unilateral poor tracking - indicative of ipsilateral disease
*Certain disrupt to eye movement can be indicative of diseases (give hints of the underlying pathologies - 31Distinction between version and vergence eye movementVersion movements - eyes move in the same direction
Vergence eye movement - eyes move in opposite direction - 32Role of each type of eye movement in subserving vision...
- 33Detail at vergence eye movement system* Eye moves in opposite direction
Stimulus for vergence:
* retinal disparity - non-corresponding retinal locations
* fusion reflex
* accommodation - 34Fusion reflex and other drivers of vergenceFusion reflex - make double images single (desire to fuse the disparate retinal image elicits con/divergent eye movement)
Accommodative vergence - when eye looks at near target
Proximal vergence - response to an awareness of near - can initiate vergence
Dark vergence - eye adopt baseline vergence angle that varies between people (40cm - 1m) - 35integration of version and vergence eye movement to facilitate accurate positioning of gaze and the facilitation of stereopsisIn real life, require both vergence and version eye movements
* does this by combining saccades with vergence movements - act of convergence can lead to asymmetric saccades between the two eyes as well as speed up vergence
* suggests that PPRF and PPH contains monocular unit that encodes demands for monocular eye movements - 36Hering's Law* used to explain conjugacy of eye movement in saccades
* Law suggests that conjugacy is due to innate connections in which eye muscles involved in eye movement are innervated equally. - 37What are vergence movements made ofMade up of a phasic and tonic component
* initial pulse firing followed by topic step to keep eyes in converged position - 38Accommodative - vergence control pathwayVergence and accommodation are controlled by their own negative feedback loops.
* output of accommodation controller is sent to vergence subsystem
* both system feed into each other - 39Vergence pathwayBoth inputs from each eye should be conjugate and vergence inputs to abducens nucleus but recent studies suggest that the inputs are monocular ipsilateral and contralateral eye movement inputs
- 40Ductions of each eye individually as well as gaze in cardinal direction1. Double H test - examine ductions-conjugate motion of the eyes in all direction - examine cardinal position of gaze as well as duction of individual eye by occluding one eye
2. Cover test - detect phoria and tropia - 41What implication are of failure to move eyes or of misalignment?* can lead to double vision
- 42What saccades/smooth pursuit/vergence can be seen or can't be seen clinically?Saccade testing - ask Px to fixate on target then fixate on another
* can observe overshooting or nystagmus
* saccades can be fast or slow
Smooth pursuit assessment - unlikely that Px will complain about unable to make smooth pursuit - will still have single vision
* observe for jerkiness while Px follows target slowly
* absent of pursuit is easy to see but subtile deficits are not readily apparent
Vergence testing: ability to converge eyes when target is brought closer - 43Assessment of vestibular functions - head thrust test + slower doll's head testing of VOR enhancement and suppressionVOR can be assessed by head thrust (quick rotations around one axis)
* deficit only in one direction
* eyes should not move with head
Suppression and enhancement
* imagined targets - moving with or is stable with us - can be affected in both directions
* In some cerebellar diseases VOR gain can be stuck at 1.0 - no suppression - 44Superior rectus palsyDuring fixation of good eye- bad eye is downwards
Fixation of paretic eye - innervation to fixate eye drives good eye upwards - 45Partial 3rd nerve palsy* creates complex deficits involving pupil and lid as well as the involved eye
* can be subtle in diabetes and other catastrophes
* affected eye doesn't adduct, elevate or depress but can abduct (lateral rectus) - 46Bilateral Internuclear Ophthalmoplegia* caused by demyelinating lesion of MLF bilaterally
* Adduction is affected but is still able to do convergence as it is mediated outside 3rd nerve nucleus so it doesn't go through MLF
* nystagmus can be present upon trying to adduct - suggest that brain is trying to move paretic eye by producing bigger movement in non-paretic eye
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- Postgraduate Visual Functions - Optometry (Visual Functions - OD1) Flashcards on Eye movements , created by Kim Vu on 28/05/2018.
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