Eye Movements

• Eye movements are paired even when they move in different directions
• Levels of neural control
• Reflex, subcortical level
• Cortical control:

• Frontal cortex: voluntary activity (frontal eye field are connected to the extraocular nuclei via the paramedian pontine reticular formation)

• Occipital cortex and superior colliculus: coordinating centres
• Interneuronal connections:
• Paramedian pontine reticular formation
• Vestibulo-ocular and cervico-ocular reflexes

• Occur around three axes passing through the centre of rotation at right angles to each other (Fick’s axes)

• 2 horizontal axes of Fick (X and Y) and a vertical Z axis
• Secondary gaze positions are achieved by rotation around the X or Z axes

• Tertiary gaze (ie. oblique) positions are achieved by rotation around both X and Z axes simultaneously

• Torsional movements occur around the Y axis

• Centre of rotation is 13.5/14mm behind corneal apex and 1.6mm nasal side to the geometric centre

• Near-fixation is approximately 0.33m in front of the eyes and requires convergence and accommodation

• Primary position of gaze: position taken up by both eyes fixating on a distant object directly ahead

• Secondary positions: any other positions such as near-fixation, cardinal positions, midline vertical positions etc.

Binocular movements

• Isolated agonist model: primary action of muscles occurs when the muscle contracts in the primary position

• Versional (conjugate) movements: the eyes move in the same direction and the axes remain parallel. 

• Controlled by pairs of muscles
• Main types of conjugate eye movements
• Saccades (supranuclear control): align the fovea with the target

• Smooth pursuit (supranuclear control): maintain fixation once the saccadic system has found the target

• Non-optical reflex movements: maintain eye positions without conscious input as body/head position changes

• Vergence (disjunctive) movements: simultaneous movement in opposite directions (eg. convergence)

• Slower than versional movements
• Stimulated by retinal disparity

• In the fusion-free position of physiological rest, the eyes are slightly divergent

• Amplitude of convergence: the difference between the converging power for near and far points of convergence

• Sherrington’s law of reciprocal innervation: increased contraction of the prime mover EOM is associated with diminished contractile activity of the antagonist muscle

Yoked muscles

• Contract to move both eyes in the same direction
• Right lateral rectus and left medial rectus both move the eyes to the right
• Right superior oblique and left inferior rectus both move the eyes down and to the left
• Right inferior rectus and left superior oblique both move the eyes down and to the right

Smooth pursuit movements

• Smooth, continuous tracking movements
• Keep an object on the fovea

• Stimulated by “retinal slip” ie. the movement of the target across the retina

• Velocity of 30-50 degrees/second (beyond this it breaks down)
• Latency of 150ms
• Complex ipsilateral supranuclear control: PPRF, superior colliculi, cerebellum…

Saccadic movements

• Short, sharp movements
• Place an object in the peripheral field onto the fovea (rapid relocation of fixation)
• Visual acuity is reduced during saccades
• Associated with selective suppression of motion detection
• Increased visual threshold during the saccade

• Fastest eye movements: velocity of 800-1000 degrees/second
• Latency of 100ms

• Controlled by the contralateral frontal cortex (supranuclear): the premotor frontal eye fields (FEF)

• Impulses for horizontal movement pass to the contralateral paramedian pontine reticular formation (PPRF): horizontal gaze centre

• Neurones then pass
• Directly to the ipsilateral sixth nerve nucleus

• Indirectly via interneurones which cross to the contralateral MLF and to the medial rectus subnucleus of the contralateral third nerve

• Vertical pathways are less well mapped: bilateral mediation via the midbrain vertical gaze centre (rostral interstitial nucleus of the MLF)

Fixation movements

• Move the retinal image slightly to prevent image fade due to bleaching of the photoreceptor pigments (Troxler’s phenomenon)

• Drifts occur monocularly
• Microsaccades are binocular and occur with sustained foveal fixation
• Aim for the best perceived image with fresh cones

Visual reflexes

• Mediated by the vestibular system 
• Sends efferents to the horizontal gaze centre
• Fixation reflex
• Ability to fixate in bright light
• Developed within a few days of birth
• Demonstrated by tests for optokinetic nystagmus
• Oculovestibular reflexes
• Eye movements responding to head/trunk positional changes 

• Utricle and saccule detect head tilting/gravitational effects: input to the inferior and medial vestibular nuclei

• Semicircular canals detect rotatory movements: input to superior and medial vestibular nuclei

• Floculonodular lobe of the cerebellum controls vestibular inputs to the oculomotor system

• Require intact brainstem function
• Rotational movements of the head demonstrate this reflex (doll’s head)

• Oculocervical reflex
• Proprioceptors in the neck provide information about neck position
• Lateral movements of the head about the vertical axis produces this reflex

Optokinetic nystagmus

• Stationary observer and moving scene or vice versa
• Elicited by a striped drum revolving at 30-100 degrees/second

• Slow phase (smooth pursuit component) when eye follows target and fast flick (saccadic movement) when they relocate to new target position

• Requires intact cortex and patient’s attention
• Defective response implies cerebral cortex lesion

Downbeat nystagmus

• Usually indicative of cervicomedullary structural disease
• Causes:
• Arnold-chiari malformation
• Stroke
• MS
• Platybasia (Paget’s disease and Gorlin Goltz syndrome)