Control of Eye Movements

Question 1

What are the four types of eye movements?

Answer 1

1. Smooth pursuit: tracking → to keep an object on the fovea)
2. Saccades: rapid, ballistic “jumps” → to bring an object onto the fovea
   •There is suppression of visual information during a saccade
   •Why you don’t see a blur of the visual world during a saccade
   •Average of 3 saccades per minute during the day!
3. Reflexes – neither is monosynaptic
   •VOR = Vestibular ocular reflex (vestibular system)
   •OKN = optokinetic nystagmus (visual object movement)
4. Vergence: moving the fovea to an object closer (convergence) or farther away (divergence)

Question 2

What is the difference between conjugate and vergence eye movements?

Answer 2

Conjugate eye movements: both eyes move in the same direction. Initiated by a variety of sensory inputs, visual and vestibular inputs most important

Vergence eye movements: eyes movie in opposite directions. Example – both eyes turn nasally (“cross-eyed”) during the near reflex

Question 3

What is a saccade and how is it generated?

Answer 3

Saccade: a rapid movement that brings the eyes to a predetermined target or position at a rate of up to 700˚/second (contrast to smooth pursuit, which tops out at 50˚/second).

Saccades can be used to restore the eye toward the center of orbit during some tracking tasks, whether it was visually or vestibularly evoked. Saccades can also be used in a more voluntary way to rapidly foveate an interesting peripheral stimulus, moving the eye away from the center of orbit.

The frontal eye field in the frontal lobes of the cortex can activate saccades by two pathways:

1) Direct to the reticular formation
2) Via the superior colliculus and then tot he reticular formation

Question 4

What are smooth pursuit eye movements, how fast can they be, and why are they limited to relatively slow speeds?

Answer 4

A smooth pursuit eye movement is elicited by any large moving stimulus, and it is an attempt to foveate that stimulus for more extended visual examination. Once the object is on/near the fovea, slower movements (pursuit) are used to track the moving object.

Smooth pursuit eye movements can only maintain foveation at a rate of 50 degrees per second or less. This is because coordinating smooth pursuit requires a lot of analysis in the visual cortex. You analyze the position, direction of movement and speed in the visual cortex, and this takes time.

Question 5

Describe the control of the VOR for a person sitting on a chair that is rotated to the right (clockwise rotation if you look down on them from above).

Answer 5

1) As the head rotates to the right, fluid in the horizontal semi-circular canal lags behind, resulting in deflection of the cupula in both horizontal canals. In the right horizontal canal the deflection results in depolarization of the hair cells, in the left horizontal canal the hair cells are hyperpolarized.
2) Excitation on the right is transmitted to the right vestibular nuclei. Cells in the vestibular nuclei project by way of the medial longitudinal fasciculus (MLF) to excite left lateral rectus motor neurons in the abducens nucleus.
3) Any stimuli that excite abducens motor neurons ALSO excite internuclear interneurons, which have axons that cross over and ascend in the medial longitudinal fasciculus to excite right medial rectus motor neurons. This ensures coactivation of the left lateral rectus and right medial rectus.
4) Conjugate eye movement to counter head rotation results.

Question 6

What is internuclear ophthalmoplegia? Describe what you observe for the patient, how you can decide if the medial rectus motoneurons and/or nerve are intact, and what the most likely structure that is affected is.

Answer 6

Internuclear opthalmoplegia describes the disconnection of the coordination of medial and lateral recti during horizontal gaze movements due to MLF damage.

The medial rectus muscle and motor neurons may be normal even though the muscle does not function properly during horizontal gaze. You can tease this out by doing a vergence test. If a patient is unable to participate in horizontal gaze saccades or pursuit, yet can perform convergence eye movements, then the patient likely has internuclear opthalmoplegia (as opposed to, for example, CN III palsy).

Question 7

What is nystagmus? Give examples where nystagmus occurs.

Answer 7

Nystagmus: if the head turns to the right, the eyes counter rotate left. If the head continues to turn, the eyes rotate slowly left until the limit of eye rotation is reached, then snap quickly back to a new fixation point. The sawtooth movement of the eyes – slow ramp opposite to head rotation, fast saccade to center of eye position – is called nystagmus.

• The direction of nystagmus is determined by the direction of the rapid saccade → “right-beating nystagmus”
• Normal feature of some eye movements (prolonged rotation of the head)
• Inappropriate nystagmus can be an indication of various pathologies