Reflex And Voluntary Saccades Flashcards
True or False- You can have both voluntary and reflexive saccades
True
What do we mean when we say a reflexive saccade is visually cued?
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
What are the two scenerios in which we express reflexive saccades?
Express to novel stimuli - this basically means in response to a new stimuli
When scanning an object of interest
True or False- Voluntary saccades don’t need to have a visual stimulus to innervate them
True - they can be memory guided i.e. when you’re anticipating something to appear
What are the brainstem generators for horizontal, and vertical saccades?
PPRF for horizontal saccades, riMLF for vertical saccades
Name the five higher control centres for fast saccades
Which higher control centres are heavily involved in reflexive saccades?
Midbrain Superior Colliculus (SC)
Midline Cerebellar Cortex & Nuclei
Describe the involvement of the Superior Colliculus (SC) to reflexive express saccades in reponse to novel stimuli
•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]
Summarise the Structure and Function of the Superior Colliculus SC
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
How does the superior Colliculus issue commands to brainstem saccade generators?
Via Crossed projections to the PPRF for the horizontal saccades
•& bilateral projections to the riMLF for the vertical saccades
How do we know that the Superior Colliculus is involved in Reflexive saccades in response to novel stimuli?
Unilateral lesion of the SC: abolishes reflexive express saccades into the opposite hemi-field
Are scanning saccades thought to be reflexive or voluntary?
Reflexive
Why do we make scanning saccades?
- 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)]
Where is the Parietal Eye Field located?
Inferior Parietal Lobule, in and below the Inferior Parietal Sulcus
What does the parietal eye field do?
Initiates Scanning & Visual Search
How does the parietal eye field initiate scanning and visual search- describe the pathway.
- 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
How do we know the parietal eye field is repsonsible for initiating scanning and visual search?
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).
Is reading an example of smooth pursuit or scanning saccades?
Scanning saccades - you aren’t slow tracking words hence not smooth pursuit
Where is the frontal eye field located?
Posterior bit of the Middle Frontal Gyrus
What does the frontal eye field do?
Initiates all Self-generated (i.e. voluntary) Saccades
How does the frontal eye field generate all volunatry saccades- describe the pathway.
- 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)
What does each frontal eye field contain?
Each FEF (frontal eye field) contains a Map of Saccade directions, mainly towards the opposite side
How do we know the Frontal Eyefield is responsible for all self-generated saccades?
Unilateral FEF lesions: abolish voluntary saccades away from affected side ( this is called ‘paralysis of contra-versive gaze’)
What condition does a lesion of the left frontal eye field lead to?
Right gaze palsy
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 .
We can supress reflexive express saccades in response to novel stimuli
How can we supress express/reflexive saccades?
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
True or False- the supression of an express/reflexive saccade is an ‘Executive function’
True - executive functions are anything that require thinking and aren’t instinctual
Describe the role of the Cerebellar Cortex & Deep Nuclei in saccade movements?
How do they do this?
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
What effect occurs upon saccades when we have lesions of the Cerebellar Cortex & Deep Nuclei (specifically the Vermis Cortex or Fastigial Nucleus)?
- ‘Intrusions’ = saccades occur at the wrong time
- ‘Dysmetria’ = saccades occur with the wrong size (amplitude), typically, under-shooting the intended target
