L21 - Eye Movement types & mechanics (LA) Flashcards Preview

Visual Neuro > L21 - Eye Movement types & mechanics (LA) > Flashcards

Flashcards in L21 - Eye Movement types & mechanics (LA) Deck (29):
1

What are conjugate eye movements?

It refers to motor coordination of the eyes that allows for bilateral fixation on a single object. Several centers in the brainstem are involved.

1) Smooth pursuit - voluntary control, keeps image of moving object on fovea (only used by foveate predators and primates but not afoveate animals like rabbits). Works with saccadic system to get on target if the eyes fall behind.

2) Vestibulo-ocular reflex (ancient reflex aka 3 neuron arc) - stabilize images on the retinas (in yoked vision) during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field

3) Optokinetic nystagmus (OKN) - reflexive response to large visual field motion that allows us to adequately follow moving objects when we keep our head steady while eyes move.

2

What is disjunctive eye movement?

Vergence - the simultaneous movement of the pupils of the eyes towards or away from one another during focusing.

It allows both eyes to maintain fixation on an object when it is close to the viewer

It works with accommodation (increases lens power at near) and the pupil (constricts to aid in focusing at near)

3

How many muscles move the eyes?

6 muscles per eye - 4 approximately orthogonal rectus muscles and 2 diagonally running obliques

4

Do eyes face straight ahead when we are looking straight ahead?

No - When the muscles are nicely relaxed in the orbits, the eyes are pointing in different directions. To look straight ahead, the eyes are turned within each orbit

5

Eye movements are ___-referenced? Adduction, abduction, intorsion, extorsion, elevation, depression

Nose-referenced

Adduction -> towards the nose
Abduction -> away from nose
Intorsion -> top of eye rotates towards nose
Extorsion -> top of eye rotates away from the nose
Elevation -> up
Depression -> down

6

2 Horizontal rectus muscles

Lateral
Action - abductor
Innervation - 6th nerve

Medial
Action - adductor
Innervation - Inferior division of 3rd nerve

7

2 vertical rectus muscles

Superior
Action - elevation
Innervation - superior division of 3rd nerve

Inferior
Action - Depression
Innervation - Inferior division of 3rd nerve

8

2 oblique muscles

Superior
Action - Intorsion
Innervation - 4th nerve

Inferior (only muscle that does not originate at the back of the orbit)
Action - Extorsion
Innervation - Inferior division of 3rd nerve

9

Saccades are fast BUT the muscles resist them - what is the major hindrance?

Viscosity of the EOMs e.g. the faster you try to move the honey, the more force you need as resistance is high. If you move slowly, there is less resistance

To get the muscles to move quickly, a BURST of innervation is needed. Antagonist forces must also be turned off. After the eyes get where they're going, tonic innervation is needed to keep them in their new location

10

Where is the centre of saccadic action?

SC - excit inputs from retina and various cortical areas. Inhibit inputs from frontal cortex via striatum and sub nigra

11

What cortical areas control: reflexive saccade, reflexive saccade inhibition, intentional saccade?

Reflexive saccade - PPC
Reflexive saccade INHIBITION - DLPFC
Intentional saccade - FEF

12

T/F: Afoveate animals can also do smooth pursuit

F- they cannot since they don't have a fovea so there's no point because it wouldn't be clear

13

The property of the stimulus which elicits a smooth movement is real or perceived (illusory) target motion - the illusion of motion is enough for your pursuit system

The property of the stimulus which elicits a smooth movement is real or perceived (illusory) target motion - the illusion of motion is enough for your pursuit system

14

Why do pursuit and saccades overlap so much?

They both keep an object of interest imaged on the fovea and work together to achieve this. The ability of saccades to step in to track a moving object when pursuit falls behind may be why its rare for someone to complain that they can't pursue, whereas an inability to make saccades will bring someone into the emergency department ASAP.

15

What motions does the vestibular system sense?

Translational motion - along axes (3 possible)
Rotational motion - around axes (3 possible)

16

What type of acceleration do: semicircular canals sense? Utricle and saccule?

Angular
Linear (including gravity)

17

Cupula - gelatinous mass which has hair cell cilia embedded within it. The canals are filled with endolymph whose passage is almost blocked by the cupula. Turning your head pushes the endolymph onto the cupula - leading to depol/hyperpol.

Each canal comes in a pair. What excites one, inhibits the other. Normally, both are firing so that rotation modulates their firing rate in opposite ways

Cupula - gelatinous mass which has hair cell cilia embedded within it. The canals are filled with endolymph whose passage is almost blocked by the cupula. Turning your head pushes the endolymph onto the cupula - leading to depol/hyperpol.

Each canal comes in a pair. What excites one, inhibits the other. Normally, both are firing so that rotation modulates their firing rate in opposite ways

18

What happens when you rotate at a constant velocity (hard to do in nature) - in regards to the endolymph?

It usually lags behind and pushes on cupula BUT in this case it is able to catch up with the canal and the cupula straightens up and the response sent along the vestibular nerve goes back to its baseline level

19

What nerve innervates the ear?

8 CN

20

Gain = ? When you fixate something stationary in the environment as your head moves?

1

21

Gain = ? When you fixate something moving with you as your head moves (e.g. looking at your watch as you walk)

0

22

Is gain higher or lower in the dark?

Lower ~0.6

23

What is stimuli for vergence?

Disparity and blur

24

The phi phenomenon

The optical illusion of perceiving a series of still images, when viewed in rapid succession, as continuous motion.

25

Beta movement

Whereby a series of static images on a screen creates the illusion of a smoothly flowing scene

26

!List 4 areas crucial for smooth pursuit

FEF, LIP (Lateral Intraparietal Area), MT, PPN (precerebellar pontine nuclei)

27

!Direct VOR - 3 neurone-arc

With 3 synapses, not much delay between stimulus (head turn) and response (opposing eye movement)

Vestibular nerve -> vestibular nucleus -> Abducens nucleus -> ocular motor nuclei

28

!x fills in when VOR can't stabilize gaze. x responds nicely to constant velocity rotation where else VOR is more effective at lower/higher frequencies.

OKN fills in when the VOR can't serve to stabilise gaze

OKN responds nicely to constant velocity rotation

VOR is more effective at higher frequencies

29

!T/F - Vergence has independent access to the lateral recti - without going through that nucleus

F - Medial rectus not lateral