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

Disturbance of supranuclear eye movement function leads to gaze palsies (further explored in Part 2 package)

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

Slower than versional movements
Stimulated by retinal disparity

Transection of the corpus callosum inhibits vergence movements.

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

Hering’s law.

During voluntary movements of the eye, equal and simultaneous innervation occurs from the CNS to the muscles of both eyes allowing gaze in a particular direction ie. the extent of movement of one eye is equal and symmetrical to the other
Applies to all voluntary conjugate movements

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

Primary, secondary and tertiary actions of the EOMs

Muscle	Primary	Secondary	Tertiary	Yoke (contralateral eye)
Medial rectus	Adduction	nil	nil	Lateral rectus
Lateral rectus	Abduction	nil	nil	Medial rectus
Superior rectus	Elevation(maximal in abduction)	Intorsion (maximal in adduction)	Adduction	Inferior oblique	Synergists for medial rectus and antagonists for lateral rectus
Inferior rectus	Depression (maximal in abduction)	Extorsion (maximal in adduction)	Adduction	Superior oblique
Superior oblique	Intorsion	Depression (in adduction)	Abduction	Inferior rectus	Synergists for lateral rectus and antagonists for medial rectus
Inferior oblique	Extorsion	Elevation (in adduction)	Abduction	Superior rectus

## 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…

Note that smooth pursuit supranuclear control is ipsilateral compared to saccadic control which is contralateral

Smooth pursuit movement is lost on the ipsilateral side in occipitoparietal lesions

Causes asymmetric optokinetic nystagmus (Cogan’s law)

## 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)

Caloric testing

Stimulation of a semicircular canal leads to nystagmus in the plane of that canal with the slow phase occurring in a direction opposite to that causing the nystagmus (the direction of the nystagmus is described relative to the fast saccadic part)

Warm water causes endolymph to rise and stimulates the semicircular canal (caloric testing)

Warm water causes saccadic movement to the ipsilateral side
Cold water causes saccadic movement to the contralateral side
Mneumonic: cold opposite, warm same (COWS)

Bilateral stimulation causes vertical nystagmus
The response to caloric testing:

Reduced by: fixation on an object, optokinetic stimulus
Increased by: scotopic conditions, high plus lenses

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

A central scotoma increases the frequency of optokinetic nystagmus

## Downbeat nystagmus

Usually indicative of cervicomedullary structural disease
Causes:

Arnold-chiari malformation
Stroke
MS
Platybasia (Paget’s disease and Gorlin Goltz syndrome)