W5: Eye Movement & Gaze

Question 1

What’s the Troxler effect?

Answer 1

Eyes need to move to keep our vision refreshed

Question 2

Describe the extra-ocular muscles and there innervations

Answer 2

CN II (Ocularmotor Nerve):
Superior Rectus: upwards
Inferior Rectus: downwards
Medial Rectus: inwards
Inferior Oblique: RE anticlockwise vv for LE

CN IV (Trochlear): Superior Oblique: RE clockwise vv for LE
CN VI (Abducens): Lateral Rectus: outwards

Question 3

What’s the consequence of damage to eye muscles/nerves?

Answer 3

Double vision or Diplopia

Question 4

Describe gaze shifting via saccades and vergences

Answer 4

Saccades shift fovea rapidly to target, conjugate eye movement in same direction to keep image on fovea
Vergence moves target image to foveae; function of horizontal rectus muscles innervated by neurons in ocular motor nucleus

Question 5

Describe the step-pulse model of a saccade

Answer 5

Pulse: extra-ocular motor neurons; firing rate increases during saccade (up to 900deg/sec max) in activity pulse tells us speed of saccade

Step: extraocular motor neurons change baseline firing rate to new eye position tells us size of saccade

Question 6

Describe the superior colliculus structure and function with it’s input/output

Answer 6

Input from retina and dorsal pathway
Superficial layer for visual
Deep layers for motor coding occularmotor activity
Output to cerebral cortex

Question 7

What happens in the superior colliculus during fixation and saccades?

Answer 7

Fix: Neurons at front-edge/foveal zone hold eye steady
Saccade: Neurons activated in region where saccade will be directed
Before saccade, activity builds in target location and decreases in rest of superior colliculus

Question 8

Describe how Frontal Eye Fields trigger saccades

Answer 8

Trigger intentional saccades by activating superior colliculus which activates burst neurons in brainstem
Supplementary eye fields organise saccades into sequences

Question 9

Describe fixational eye movements with 3 examples

Answer 9

Eye continually moving (involuntarily) when fixating on static object
Microsaccades (could be error correction for ocular drift to refresh retina), Drift, Tremor

Question 10

Describe gaze holding

Answer 10

1. Smooth Pursuit movements keep moving image on fovea; eye velocity neurons receive input from FEF/MT/MST
2. Vestibulo-ocular reflexes keep images still on fovea during brief head movement
3. Optokinetic reflexes keep images still during sustained head movement

Question 11

Describe the vestibular system

Answer 11

3 Semi Circular fluid-filled canals for rotational movement
Utricle and Saccule with otoliths for linear/translational movement

Question 12

Explain 3 vestibula-ocular reflexes

Answer 12

Rotational VOR: head rotation (canals)
Linear VOR: linear movement (otoliths)
Ocular Counter-Rolling VOR: head tilt (otoliths)

Question 13

Explain nystagmus due to rotational VOR

Answer 13

Reflex causes eye rotation opposite to head rotation detected by vestibular system
Sustained rotation doesn’t drive eyes beyond edge of orbit so they make a rapid resetting movement
Vestibular signal drives slow phase
Brain stem circuit drives quick phase

Question 14

Why is the vestibular system flawed and how is it countered?

Answer 14

Habituates, semi circular canals don’t respond well to slow movement; optokinetic system stabilises the visual scene

Question 15

Describe optokinetic reflex and how it works with the vestibular system

Answer 15

Begin rotating and accurate vestibular input into vestibular nuclei allows clear vision. This decays whilst optokinetic input increases to compensate

Question 16

Describe circular vection

Answer 16

Movement of whole visual scene makes someone think they’re moving so optokinetic reflex results in illusions of self motion