Neuroscience 13a - Eye Movements
Anil Chopra
- Describe the extrinsic eye muscles and their cranial nerve innervation.
- Describe the relevant central nuclei and connections
- Define the types of eye movements, saccades, smooth pursuit and their neural control.
- Explain the roles of the brainstem, cortex and cerebellum in saccades
- Describe the mechanisms underlying the vestibulo-ocular reflex and gaze stability.
- Describe the common types of pathological nystagmus and their underlying pathophysiology.
Eye movements are made up by movements in 4 directions or planes:
Horizontal eye movements
Vertical eye movements
Torsional eye movements – eyes roll in the head
Versional eye movements – or vergence which act to transfer the eye between near and far objects. Caused by contraction (for near) and relaxation (far) of lateral rectus muscles
Muscles and Movements of the Eyes:
The extrinsic eye muscles – arise from a fibrous ring at the back of the orbit:
- Superior Rectus – III (oculomotor)
- Lateral Rectus – VI (abducens)
- Inferior Rectus – III
- Medial Rectus - III
The intrinsic muscles of the eye – attached to the scelera of the eyeball:
- Superior Oblique – IV (trochlear) – turns the eye down and in (adduction)
- Inferior oblique – III – turns the eye up and out (abduction)
There are 2 main types of eye movement:
- Saccades – Fast Phases
These allow us to look from one object to another.
Can be made in any direction
Can be voluntary or reflex
Extremely fast, up to 600º per second!
90% accurate and if it is inaccurate then smaller saccades correct it to obtain precise fixation
Latency of saccades to a visual target is around 120m/s
They form the “fast” part of nystagmus in the vestibulo-ocular reflex.
Controlled principally by the brainstem (especially the paramedian pontine reticular formation) and rostral interstitial nucleus of the medial longitudinal fasciculus (MLF)
Role of the brainstem, cortex and cerebellum in saccades
- The posterior parietal cortex contains a large number of neurones responsive to complex visual stimuli. Important in generations of saccades to objects of visual significance – damage can lead to deficiencies in saccades and are signified by the loss of the optokinetic reflex
- The primary visual cortex and its associated extrastriate areas are also involved in saccades – through projection of V1 to superior colliculus
- Basal ganglia play a major role – control and initiation of movement
- The superior colliculus in the midbrain is important for accurate execution of saccades
- The rostral interstitial nucles of the medial longitudinal fasciculus (riMLF) is important in the control of vertical saccades
The paramedian pontine reticular formation (PPRF) is important in control of
Conditions
In cortical disease, saccades may be erratic in timing and amplitude
In basal ganglia disease (Parkinson’s disease), saccades may be smaller.
In cerebellar disease saccades are inaccurate and can often over or under shoot (Hyper/hypometria). Rapid saccades may occur involuntarily.
In brainstem disease, saccades are slow and small in amplitude.
- If the lesion is in the oculomotor nucleus, then eye movements are stopped altogether – ocular palsy or paresis.
- If only the saccades are impaired but other movements are intact then it is known as “supra nuclear palsy”
- Slow Phase Eye Movements
Allow us to stay fixed on an object when either the object or ourselves is moving. Theseinclude, notably, pursuit and vestibular ocular reflex eye movements.
Pursuit Eye Movements
Used to follow moving targets of uniform or smooth motion.
Slower than saccades around 60-80º per second.
Can follow targets that oscillate backwards and forwards at 1Hz.
Better in the horizontal than in the vertical plane.
Have a moderate predictive component
Most animals do not have predictive pursuit motion
Controlled by anterior cortical regions and superior colliculus.
Conditions
Problems with pursuit are a sign of brainstem disease
- Difficulty is the wide range of brainstem conditions.
They are also affected by ingested toxins (alcohol, antidepressants)
Vestibulo-Ocular Reflex Eye Movements
Compensate for head movements in order to keep eyes fixed on a target. It has 2 drives:
Semicircular canals – stimulated by turning head.
- Semicircular canal cause response to a number of head movements including:
- Up and down – pitch head movements
- Side to side – yaw head movements
- Tilling to right or left shoulder – roll head movements
- They can respond to head rotations of around 180º per second.
- Can respond to oscillations of around 8Hz.
- Generally used in faster head movements – slower head movements are maintained by pursuit eye movements.
- There are 3 canals in each labyrinth (2 labyrinths):
- Vertical anterior
- Vertical posterior
- Horizontal
- Head rotation stimulates the vestibular apparatus on the side to which the head is turned (e.g. right turn stimulates right side) this drives the eyes in the opposite direction. The eye and head movements are opposing so they cancel each other out leaving the direction in which the eyes are looking stationary in space.
Otolith organs–stimulated by linear accelerations or tilt.
- Inertial force sensors that detect movement of the head e.g. jumping up and down, side to side, running forwards and backward.
- Control the linear vestibulo ocular reflexes which are not as strong as the angular ones caused by stimulation from the semi-circular canals.
- Comprise of:
- Horizontal plate – urticarious(side to side movements)
- Vertical plate – succlus(up and down movements)
- These contain hair cells within a gelatinous matrix which are stimulated when the head moves by the inertia of the fluid within them.
- Hair cells in the lateral part of the utricle generate the horizontal compensatory eye movements (with the utricle generating movement in the opposite direction)
- Generally only active when looking at nearby targets.
Conditions
Disorders of the vestibular ocular reflex reduce vision acuity during movements of the head.
Disorders can cause oscillopsia i.e. when the eyes cannot maintain focus on a still object when the head is moved.
Because there are constant tonuses (signals causing push of eyes in the opposite direction) from both the vestibular apparatus when still, the eyes do not move. If there is dysfunction with one of the vestibular apparatus, then the tonus will proceed unopposed resulting in a vestibular nystagmus.
Loss of the vestibulo-ocular reflex most often occurs in diseases of the labyrinth or of the VIII nerve. Can also be caused by brainstem vascular, neoplastic or demyelinating disease.
Optokinetic Eye Movement
This is the movement of the eyes in response to movement of large areas.
It is present in all mammals not just humans.
It causes an optokinetic nystagmus – this is when a pattern of slow phase drifts and fast saccades are produced in response to the large excursion.
It is also accompanied by the illusion of self motion – vection.
Vergence movement
Itcombines slow and moderately fast disconjugate eye movements to look between near and far and to maintain accurate binocular alignment during all eye movement. To look at an object closer by, the eyes rotate towards each other (convergence), while for an object farther away they rotate away from each other (divergence).
Conditions
Normal subjects have poor vergence in one eye due to an overdominance in one eye.
Patients with strabismus (eyes not properly aligned) cannot converge.
Problems with vergence imply lesion in the mesencephalon.
Gaze Holding
The ability to hold eyes steady even in eccentric positions.
Organised by brainstem- cerebellar circuitry.
Conditions
If gaze holding fails, the eyes drift back to the centre so the elastic forces in the stretched antagonist pull them.
It can be interrupted by saccades in certain conditions which results in a gaze paretic nystagmus. Often signs of cerebellar (moderate) or brainstem disease (severe).
Co-Ordination of Eye and Head Movements
In a typical movement of the body the following will occur:
Eyes saccade in the direction of visual orientation.
At the same time the head moves however, the eyes reach the new orientation before the head does.
The eyes execute a vestibular ocular reflex movement which is compensatory to the movement of the head.
This allows the fastest shift of gaze with the least disturbance.
In pursuit of a moving object with head movement, the following occurs:
Target is initially focused on with pursuit eye movement.
When the head is turned, the vestibular ocular reflex needs to be over-ridden if pursuit of the object is desired. (If it wasn’t, then vision would stay at the fixed point!). This is unachievable if the head is moving at over 70º per second.
Conditions
One should be able to follow objects moving in the field of vision at around 50º per second. If not then it can be indicative of non-specific cerebral disease or intoxication.