Reflex And Voluntary Saccades

Question 1

True or False- You can have both voluntary and reflexive saccades

Answer 1

True

Question 2

What do we mean when we say a reflexive saccade is visually cued?

Answer 2

Saccade occurs as a result of looking at a new stimulus or could be due to the arrival of a new stimulus in our visual field

Question 3

What are the two scenerios in which we express reflexive saccades?

Answer 3

Express to novel stimuli - this basically means in response to a new stimuli

When scanning an object of interest

Question 4

True or False- Voluntary saccades don’t need to have a visual stimulus to innervate them

Answer 4

True - they can be memory guided i.e. when you’re anticipating something to appear

Question 5

What are the brainstem generators for horizontal, and vertical saccades?

Answer 5

PPRF for horizontal saccades, riMLF for vertical saccades

Question 6

Name the five higher control centres for fast saccades

Answer 6

Question 7

Which higher control centres are heavily involved in reflexive saccades?

Answer 7

Midbrain Superior Colliculus (SC)

Midline Cerebellar Cortex & Nuclei

Question 8

Describe the involvement of the Superior Colliculus (SC) to reflexive express saccades in reponse to novel stimuli

Answer 8

•Superficial layers of the superior colliculus receive direct retinal inputs & contain maps of the opposite visual hemi-field

[–The organization of the SC map is as follows:

–Centre-to-peripheral VF, anterior-to-posterior SC

–Lower-to-upper VF, lateral-to-medial SC]

• Superficial neurons of the SC connect to Deeper layers
• Deep layers contain premotor neurons & a map of saccade directions (which get triggered).

[Organisation of the map of saccade directions is:

–Centre-to-peripheral Gaze, anterior-to-posterior SC

–Down-to-Up Gaze, lateral-to-medial SC]

[Picture shows organisation of SC]

Question 9

Summarise the Structure and Function of the Superior Colliculus SC

Answer 9

Structurally it has two layers - a superficial one which has a map of the visual field so the SC can determine where in our field of view a new stimulus appears, and a deep layer (of neurones) where there is a map of saccade directions so it can issue commands to brainstem saccade generators

Question 10

How does the superior Colliculus issue commands to brainstem saccade generators?

Answer 10

Via Crossed projections to the PPRF for the horizontal saccades

•& bilateral projections to the riMLF for the vertical saccades

Question 11

How do we know that the Superior Colliculus is involved in Reflexive saccades in response to novel stimuli?

Answer 11

Unilateral lesion of the SC: abolishes reflexive express saccades into the opposite hemi-field

Question 12

Are scanning saccades thought to be reflexive or voluntary?

Answer 12

Reflexive

Question 13

Why do we make scanning saccades?

Answer 13

• To examine different regions of interest on objects by bi-fixating them with our high acuity foveas
• So acquiring more detailed knowledge about them

[Fun Fact:

Although unaware that we are doing so, we

make several scanning (or visual search)

saccades – followed by a brief fixation – every

second (~250,000 every day)]

Question 14

Where is the Parietal Eye Field located?

Answer 14

Inferior Parietal Lobule, in and below the Inferior Parietal Sulcus

Question 15

What does the parietal eye field do?

Answer 15

Initiates Scanning & Visual Search

Question 16

How does the parietal eye field initiate scanning and visual search- describe the pathway.

Answer 16

• It Receives visual inputs from Dorsal Stream Extra-Striate areas (e.g., V5/MT)
• It Sends descending projections (via the internal capsule, posterior limb) to Deep layer neurons in the Saccade Map of the ipsilateral SC.
• This allows the Parietal Eye Field to issue commands to these SC neurons & to ‘hotwire’ their direct access to the Brainstem Saccade Generators

Question 17

How do we know the parietal eye field is repsonsible for initiating scanning and visual search?

Answer 17

A Unilateral lesion of the PEF leads to :

contralateral, hemi-spatial ‘neglect’ &/or impoverished scanning in the opposite hemi-field (known as ‘sticky fixation’)

(Basically you can’t scan on opposite side of the field to where the lesion is - hence sticky fixation - your fixation is stuck).

Question 18

Is reading an example of smooth pursuit or scanning saccades?

Answer 18

Scanning saccades - you aren’t slow tracking words hence not smooth pursuit

Question 19

Where is the frontal eye field located?

Answer 19

Posterior bit of the Middle Frontal Gyrus

Question 20

What does the frontal eye field do?

Answer 20

Initiates all Self-generated (i.e. voluntary) Saccades

Question 21

How does the frontal eye field generate all volunatry saccades- describe the pathway.

Answer 21

• It does this Via Direct projections to the Brainstem Saccade Generators
• Descending axons run in the Internal Capsule (anterior limb), then through cerebral peduncle and ..
• Cross over to the PPRF (for horizontal saccades) & go bilaterally to riMLF ( for vertical saccades)

Question 22

What does each frontal eye field contain?

Answer 22

Each FEF (frontal eye field) contains a Map of Saccade directions, mainly towards the opposite side

Question 23

How do we know the Frontal Eyefield is responsible for all self-generated saccades?

Answer 23

Unilateral FEF lesions: abolish voluntary saccades away from affected side ( this is called ‘paralysis of contra-versive gaze’)

Question 24

What condition does a lesion of the left frontal eye field lead to?

Answer 24

Right gaze palsy

Question 25

In the following scenerio how are we able to maintain fixation and not get distracted (i.e. our reflexive saccade doesn’t occur):

You are reading a book and don’t automatically look at the new person who enters the room as you don’t care for who may have entered the room .

Answer 25

We can supress reflexive express saccades in response to novel stimuli

Question 26

How can we supress express/reflexive saccades?

Answer 26

The frontal and supplementary eye fields (FEF & SEF) control Cognitive aspects of Scanning & can INHIBIT Express Saccades (i.e. you basically think to yourself that you don’t want to look).

• Alongside other roles the FEF & SEF have direct descending projections to the Deep Layers of the Superior Colliculus
• These inhibit the deep layer neurons, preventing the occurrence of unwanted express saccades during more purposeful voluntary visual search

Question 27

True or False- the supression of an express/reflexive saccade is an ‘Executive function’

Answer 27

True - executive functions are anything that require thinking and aren’t instinctual

Question 28

Describe the role of the Cerebellar Cortex & Deep Nuclei in saccade movements?

How do they do this?

Answer 28

They basically Make sure saccades occur at the right time and at the right amplitude.

“They are responsible for saccade gain-control”

Vermis cortex in the cerebellar cortex receives input from all three cortical eye fields. Purkinje cells in these cortical regions project to Deep Cerebellar Nuclei - the Fastigal Nucleus.

Which sends instructions directly to the Brainstem Saccade Generators, PPRF & riMLF

Question 29

What effect occurs upon saccades when we have lesions of the Cerebellar Cortex & Deep Nuclei (specifically the Vermis Cortex or Fastigial Nucleus)?

Answer 29

• ‘Intrusions’ = saccades occur at the wrong time
• ‘Dysmetria’ = saccades occur with the wrong size (amplitude), typically, under-shooting the intended target