Eye Movements and Sensory Integration

Question 1

What is foveation?

Answer 1

When eye movements direct the fovea to new objects of interest in the visual field

Question 2

What are saccades?

Answer 2

A rapid conjugate eye movement that shifts the centre of gaze from one part of the visual field to another

Question 3

What are the 5 basic types of eye movements?

Answer 3

2 functional categories
1. Shift the direction of gaze
- Saccades
- Smooth pursuit movements
- Vergence movements
2. Stabilise gaze
- Vestibulo-ocular reflex
- Optokinetic nystagmus movements

Question 4

Describe smooth pursuit movements

Answer 4

Slower tracking movements of the eyes designed to keep a moving stimulus on the fovea once foveation is achieved

Question 5

Describe vergence movements

Answer 5

Align the fovea of each eye with targets located at different distances from the observer
Commonly employed when abruptly shifting the direction of gaze
Vergence movements are disconjugate - they involve either a convergence or divergence of the lines of sight of each eye to foveate an object that is nearer or farther away

Question 6

Describe the vestibulo-ocular reflex and optokinetic nystagmus eye reflex

Answer 6

Operate together to move the eyes and stabilise gaze relative to the external world, thus compensating for head movements
Vestibulo-occular reflex = fixating on a point whilst you shake your head
Optokinetic reflex = eye movement to focus on a moving train - the eye follows the surround movement so that the image stays stable on the retina

Question 7

What is the basic circuitry of the VOR?

Answer 7

1. Primary vestibular afferents (axons from semicircular canals) synapse on neurons in the medial vestibular nucleus
2. Medial vestibular nucleus projects to abducens nucleus
3. Abducens nucleus causes the lateral rectus muscle to contract
4. Abducens nucleus projects to the oculomotor nucleus which causes the medial rectus muscle to contract

Question 8

What is the classic 3-neuron reflex arc in the VOR?

Answer 8

1. Sensory neurons signal head movement
2. These synapse with interneurons in the vestibular nuclei
3. These synapse onto oculomotor neurons that drive the eye muscles

Question 9

What is the basic circuitry of the OKR?

Answer 9

Similar to VOR except the input comes from the retina and pretectum (rather than semicircular canals) and it signals movement of the whole retinal image
Output of the reflex affects the retinal slip
Retinal slip is the input to the reflex
So therefore it is feedback control

Question 10

Why do we have 2 stabilising eye reflexes?

Answer 10

OKR on its own is inadequate - does not work for rapidly changing movements
OKR works well for low-frequency movements, but poorly for high-frequency movements because of feedback delays in the retina
VOR good for high frequency head movements with a very fast response time but poor for low frequency constant movements because of semi circular canal mechanics

Question 11

How does the VOR know how big an eye movement command to send to the eye muscles?

Answer 11

Need constant maintenance - learning
Need to learn a simple motor skill
This learning uses the cerebellum located on the side-path to the main reflex
Uses retinal slip as a teaching signal to keep the reflex accurate
Uses the cerebellum for constant calibration using retinal slip as an error signal

Question 12

What are the main eye muscles that control eye movement?

Answer 12

Lateral (abduction) and medial (adduction) rectus muscle (horizontal movements)

Superior and inferior rectus muscle
Superior and inferior oblique muscles
(Coordination of both muscles for vertical movements)

Question 13

What 2 tasks are involved in moving the eyes to fixate on a new target?

Answer 13

1. The amplitude of the movement
   = amplitude of saccadic movement is encoded by the duration of neuronal activity in the lower motor neurons of the oculomotor nuclei
2. The direction of the movement
   = controlled by local circuit neurons in two gaze centres in the reticular formation