L28 Neural control of eye movements

Question 1

What is the role of eye movements?

Answer 1

• Under normal circumstances, the fixation or angle of gaze is steady enough for perception of the world to occur
• Small movements of the eye don’t interfere with vision, insstead enhance it
• Images brought to the center of acuity (fovea) without moving the head
• Brain generates neural signals that control orientation of eyes

Question 2

Why do we need to move our eyes?

Answer 2

Eye movements will stabalise gaze (keep images steady on the retina)
Shift gaze (redirect line of sight to a new object)

1) Resolve visual field while we are in motion - stabalises images which would otherwise flow over the retina:
- vestibulo-ocular reflex
- opto-kinetic nystagmus

2) Resolve independently moving objects - keep the moving object on each eye fovea:
- Smooth pursuits and saccades

3) Explore visual field - direct eyes towards potential areas of interest - saccades

Question 3

What is the function of superior colliculus?

Answer 3

• Receives input from all sensory modalities
• Involved in eye movements and visually guided tasks
• Has direct input from retina
• Involved in reflexive saccadic movements and volitional movements
• Controls fixation and initiates saccades
• Has a retinotopic organisation.

Question 4

What are the three fundamental visual sensory-motor tasks?

Answer 4

See while we move - self-motion
See objects that move - object motion
See the field focusing on different objects at a time: explore space and shift attention

Question 5

What are the 5 main eye movements?

Answer 5

Saccades: Rapid shifts in gaze to move fixation points (focusing on new area)
Pursuits: Smooth tracking movements to follow objects
Vergence: Movements that coordinate the two eyes to focus on near objects
Vestibulo-ocular reflex (VOR): movements that stabalise gaze during head movements
Optokinetic nystagmus (OKN): involuntary eye movements in response to a moving visual scene.

Question 6

Are pursuits and saccades voluntary or involuntary?

Answer 6

Combination of both - we choose where to direct our gaze, but the smoothness and stability of fixations rely on involuntary reflexes

Saccades and vergences are primarily involuntary, although they can be influenced by our attention to a certain degree

Question 7

Are VOR and OKR voluntary?

Answer 7

No, they’re entirely involuntary reflexes that maintain visual stability without any concious effort,

Question 8

What are the mechanisms to stablise images on the retina while we move our head?

Answer 8

Vestibulo-ocular reflex (VOR)
Visually mediated reflexes (optokinetic and smooth pursuit tracking) - rotation of eye in response to movement of object, i.e. car goes by

Both work in a synergistic fashion to maximise the eye compensation reponse to any head movement.

Question 9

Vestibulo-ocular reflex:

Answer 9

Vestibulo-ocular Kinesthesia is the sense of our body’s movement that helps our eyes stay stable. Stabilises gaze and allows clear vision during head movement.

Eye movement in the opposite direction to head movement

Question 10

Vestibulo-ocular reflex is triggered by:

Answer 10

Triggered in response to acceleration of fluid in semi-circular canals (in the ear and otoliths organs) which determine where we move eyes.

VOK experiment found that VOK can be stimulated by hair cells (i.e. with cold water)

Independent of visual signals - generated by receptors in inner ear

Question 11

VOK can account for head movements up to…
Pursuits up to …

Answer 11

VOK: 300˚ per second
Pursuits: 100˚ per second

Question 12

VOR pathway:

Answer 12

• Vestibular system follows the route of vestibular nerve (VIII) through Scarpa’s ganglion to vestibular nucleus in the brainstem.
• Fibres cross to the contralateral abducens nucleus, projecting directly to the lateral rectus (abducens nerve 6)
• Fibres from abducens nucleus to oculomotor nucleus, activating the ipsilateral medial rectus muscle of the eye (CN3)

vestibular nucleus through ascending tract of Dieters to the ipsilatral medial rectus nucleus; inhibitory vestibular pathways to the ipsilateral abducens nucleus

Vestibular apparatus (semi-circular canals) –vestibular nerve–> Vestibular nuclei (brainstem) > Motor nuclei of cranial nerves > EOMs

Question 13

How many synapses in the VOR pathway?

Answer 13

3 synapses; vestibular nucleus > motor neuron nucleus > oculomotor nucleus

Question 14

What is Optokinetic nystagmus?

Answer 14

A normal reflexive response of the eyes in response to large-scale movements of the visual scene –> i.e. in bus, seeing trees move by. Visually guided

OKN is an alert response where the eyes move toward a novel moving target in the periphery and follow it in toward primary gaze

Responds optimally if the stimulus is movement of all, or large portion of the retinal image.

Question 15

How OKN different from pathological nystagmus

Answer 15

Different to pathological nystagmus that can result from damage to the vestibular system (cerebellum)

Question 16

Optic flow on the retina judges tasks like:

Answer 16

• judging the speed and direction of our own movement
• maintaining balance and spatial orientation
• perceiving depth and 3D structure of the environment
• Guiding eye movements to track objects

Question 17

What is optic flow on retina?

Answer 17

Form of visually streaming which occurs as we are moving continuously in one direction
- retina contains ganglion cells that respond exclusively to motion in certain directions or orientations.

Question 18

What is the OKN pathway?

Answer 18

Optic flow on retina > LGN > visual cortex (V1 area)

Moving target > retina > LGN > cerebellum > vestibular nucleus > ocular motor region

Question 19

When should you use OKN tests?

Answer 19

A number of visual problems can generate abnormal OKN. A normal OKN pathway means the oculomotor pathway from the retina to the brainstem and back to the eye muscles is functioning properly.

Thus, if the patient is un-cooperative, you can use OKN. A normal OKN result is sufficient evidence for reasonally intact vision.

Question 20

The ____ is critical in controlling visual fixation. A subset of neurons (fixation cells) discharge tonically when a subject actively fixates a target spot and pause during execution of saccadic eye movements.

Answer 20

Superioris Colliculus

• tiny involuntary eye movements are seen even during fixation to compensate for slight drifts.