HNS45 Eye Movement Flashcards
Eye movements
- Complex and interesting behaviour does NOT rely on higher-order cognitive processing (like language)
- Visual perception
- Complex cognitive processes such as reading, thinking
- Interaction between sensory-motor systems
3 layers of eyeball
- Outer
- Sclera
- Cornea - Middle
- Choroid
- Ciliary body
- Iris - Inner
- Retina
Anterior chamber and Posterior chamber
Anterior chamber: Aqueous humour
Posterior chamber: Vitreous humour
Formation and drainage of aqueous humour
Aqueous humour is formed by capillary network in Ciliary body
—> drains into Schlemm’s canal (Trabecular meshwork)
—> enters blood
Importance of eye movements
- Achieve high visual acuity
- High visual acuity is restricted to Fovea (small circular region 1.2mm diameter) in the central retina —> Densely packed with photoreceptors - Direct Fovea to new objects of interest (Foveation) from a target already being foveated
Control of pupillary size
Constriction:
- Circular muscle
- Parasympathetic stimulation
Dilation:
- Radial muscle
- Sympathetic stimulation
***5 basic types of eye movements
Gaze-shifting movements (***Voluntary / Attention)
- Saccades
- Smooth pursuit
- Vergence
Gaze-stabilising movements (***Reflexive)
- Vestibulo-ocular reflex
- Optokinetic reflex
6 cardinal directions of movement to test muscle
Lateral rectus: Lateral
Medial rectus: Medial
Superior rectus: Lateral upper (only superior rectus can elevate eye when abducted)
Inferior rectus: Lateral lower (only inferior rectus can depress eye when abducted)
Superior oblique: Medial lower (only superior oblique can depress eye when adducted)
Inferior oblique: Medial upper (only inferior oblique can elevate eye when adducted)
Innervation of extraocular muscles
LMN:
CN3 Oculomotor nucleus (rostral midbrain): **Ipsilateral Others
CN4 Trochlear nucleus (dorsal midbrain): **Contralateral Superior oblique (特別)
CN6 Abducens nucleus (pons/medullary junction): ***Ipsilateral Lateral rectus
- Saccadic eye movement
- Conjugate movement (same direction)
- ***Rapid and Ballistic movements of both eyes between 2 phases of fixation in the same direction (conjugate eye movement)
- Driven mainly by Position signals (voluntary)
- 200-250ms
- Very small micro-saccades are involuntary —> keep image moving on retina —> prevents fading
- Range in amplitudes from small movement (reading) to large movements (gazing around a room)
- Smooth pursuit movement
- Conjugate movement (same direction)
- ***Slow, smooth eye movements —> track moving objects once foveation is achieved
- ***Quasi-voluntary (observer can choose whether or not to track, but cannot voluntarily generate smooth pursuit in absence of a moving target)
- 2 basic phases:
1. Pursuit initiation (0.1s, driven by retinal slip / velocity error —> slip of image across retina)
2. Pursuit maintenance
- Vergence movements
- Disconjugate movement (different direction of eye movement)
- Used to converge eyes onto targets at different distances
- Reflexive + Voluntary
- Driven by ***Binocular disparity of the target to be fixed
- Align fovea of each eye with targets located at different distance from observer
- Convergence / Divergence of lines of sight to see an object nearer / further away
- Vestibulo-ocular reflex (VOR)
- To stabilise eyes on a target in response to ***rapid head movements (快) —> Physiologic / Vestibular nystagmus
- Eye movement in direction ***opposite to head movement —> preserving image on centre of visual field
- Vestibular system detects **brief, transient changes in head position —> **rapid corrective eye movements
2 forms:
- Rotational VOR - driven by signals from Semicircular canals (sense head rotation)
- Translational VOR - driven by signals from Otolith organs (sense linear head acceleration)
Pathway:
Head movement —> Vestibular nerve (CN8) —> Vestibular ganglion —> Vestibular nucleus —> Oculomotor / Trochlear / Abducens nucleus —> CN3, 4, 6 —> Extraocular muscles
Example: Canal-related nystagmus
Canals on both sides operate as complementary pairs (One side ↑ firing, Other side ↓ firing)
—> Head turn **Right
—> endolymph turn **Left
—> ↑ excitability of **Right hair cells / ↓ excitability of Left hair cells
—> Synchronous ↑/↓ firing of CN8
—> Vestibular ganglion
—> Medial Vestibular nucleus (ipsilateral) (medulla)
—> PPRF (Paramedian Pontine Reticular Formation) (contralateral)
—> Ipsilateral CN3 nucleus (cross返轉頭via Medial Longitudinal Fasciculus) + Contralateral CN6 nucleus
—> Right MR + Left LR activated / Left MR + Right LR inhibited
—> Both eye balls turn **Left
(Vestibular Nucleus —(先cross去左)—> Contralateral Abducens nucleus (Pons) —(再經Medial Longitudinal Fasciculus cross翻)—> Ipsilateral Oculomotor nucleus (Midbrain))
Clinical tests for VOR
- Bedside VOR suppression test
- Visual fixation on outstretched hand
- Rotate chair from side-to-side
- Normal: eyes remain fixed on outstretched hands
- Abnormal: eyes move in opposite direction to rotation with catch-up saccades to re-fix gaze on outstretched hands - Caloric reflex test
- Irrigate ear with warm / cold water
- Warm water —> activate ipsilateral hair cells —> eye balls move to opposite side
- Cold water —> inhibit ipsilateral hair cells —> eye balls move to same side
—> Test if Vestibular nucleus / Medulla intact
- Optokinetic eye reflex
- Stabilise eyes during head movement
- Driven by **Retina slip (like pursuit) but **reflexive + involuntary
- Operate effectively in response to ***slow (below 1Hz) (慢) head movements (where VOR has low gain) —> Keep eyes on target (e.g. in train eyes automatically track objects until they reach end of their excursion)
- Alternating slow and fast movement of eyes in response to any stimuli —> **Optokinetic nystagmus (normal reflex of Visual and Oculomotor systems in response to **large-scale movements of visual scene)
- Optokinetic eye reflex + VOR act complementarily
—> VOR: rapid, transient head movement
—> Optokinetic eye reflex: slow head movement
Visual pathway and visual deficits associated with lesions
Left visual cortex (in occipital lobe) receives information from Right visual field (vice versa)
Left optic nerve lesion: Only Right eye vision left
Optic chiasm lesion: Bilateral temporal hemianopia
Left optic tract lesion: Only Left visual field (by Right optic tract) ok
Neural control of eye movements
Eye muscle contraction directly controlled by neurons in CN
- **Burst-tonic neuronal firing pattern:
- Neuron fire a burst of activity that precedes and extend throughout movement (Nerve爆發去郁隻眼)
- in between saccades, neurons have tonic firing —> monotonically related to static eye position (定住隻眼)
**Brain areas involved in selecting visual targets for **Saccades generation + ***Planning eye movements
- Superior colliculus
- Frontal eye fields (Brodmann’s area 8)
- Lateral intraparietal area
—> heavily interconnected network
Both SC, FEF —> Saccade planning + Generation
- SC: general role in Planning particular metrics (***Amplitude, Direction) of a saccade
- FEF: higher level role in ***Target selection for saccade
SC, FEF: saccade vector produced by microstimulation is the same independent of starting eye position —> contain place code for saccade vectors
Lesions to SC, FEF
Made to either one alone: Transient deficit with saccades, but largely recover
Made to both: Saccades permanently abolished
Visual receptive field and Movement field
Neurons in SC, FEF, Lateral intraparietal area have both Visual receptive field + Movement field
—> Spatially overlapping
Visual receptive field: region in which neurons are activated by ***Visual stimuli
Movement field: region in which neurons responds ***prior to execution of saccadic eye movement
Connection of Sensory and Motor structures in Saccadic + Pursuit eye movements
- Visual signals —> processed by ***Dorsal spatial vision pathway
- Sensory / Attentional signals / Target selection —> guided by ***Frontal eye field
These cortical areas interact with subcortical structures
—> **Initiation and **Coordination of eye movement by:
1. Superior colliculus
2. Vestibular nuclei
3. Oculomotor motor centres in Reticular formation
***3 types of neurons in SC, FEF, Lateral intraparietal area
3 types of neurons (with 3 types of responses) in ALL 3 areas:
- Visual neurons: respond briskly to ***onset of visual target in receptive field (visually triggered response may be maintained during delay, NO burst of activity around time of saccade) (當目標出現了, nerve爆發, 之後維持stable但唔會爆發)
- Visual/movement neurons: have brisk response to visual target + burst of activity beginning right before saccade
- Movement neurons: no visual response but clear burst of activity around saccade (nerve爆發去郁眼)
Superior colliculus
Planning particular metrics (***Amplitude, Direction) of a saccade
3 layers —> 3 responses
- Superficial layer: Visual response
- Intermediate layer: Visual/movement cells
- Deep layer: Movement response
Neural control of ***Direction of eye movements
Determined by relative activations of different eye muscles
- Horizontal movement (左右郁) - ***Paramedian Pontine Reticular Formation (PPRF) (Horizontal gaze centre)
- Vertical movement (上下郁) - ***Rostral Interstitial Nucleus
Frontal eye field
- Brodmann’s area 8
- 2 routes to influence eye movements:
1. Direct: projections to contralateral PPRF
2. Indirect: projections to ipsilateral SC —> in turn projects to contralateral PPRF
***Synaptic circuitry responsible for horizontal eye movements to Right
Activation of neurons in ***Right PPRF (Paramedian Pontine Reticular Formation) by FEF —>
- Right Abducens nucleus —> ↑ activity of LMN of CN6 —> Right LR activated
- Internuclear neuron (within Right abducens nucleus) —> Medial Longitudinal Fasciculus (cross to Left side) —> Left Oculomotor nucleus —> ↑ activity of LMN of CN3 —> Left MR activated
Overall effect: eyes move to ***Right
***Clinical relevance of PPRF (Paramedian Pontine Reticular Formation)
Example using right gaze
- Lesion between FEF to right PPRF —> gaze of both eyes to right impaired
- Lesion of MLF —> impaired adduction of left eye
- Lesion of LMN of CN3 —> impaired adduction of left eye + Ptosis (Levator palpebrae superioris innervated by CN3)
- Lesion of Right PPRF —> eyes cannot move to right, intact Left PPRF move eyes to left
