Reflex and voluntary saccades Flashcards
list the 4 components that make up the ocular motor control hierarchy, and state what each components are responsible for
- supra-nuclear: higher centres
- initiates eye movements for different purposes
- sends motor commands to the relevant… - supra-nuclear: brainstem generator
- is the lower level of the supra nuclear
- function is to activate burst cells (in PPRF and riMLF) which fire particular patterns of activity
- sent via their axons in the… - the inter-nuclear pathway
- = medial longitudinal fasciculus (MLF) to… - ocular motor neurons
- activating them and causing similar firing patterns
- required for slow or fast contraction of their EOMs
what is the purpose of the supra-nuclear higher centres
- initiates eye movements for different purposes
- sends motor commands to the relevant supra-nuclear brainstem generators (lower part of supra-nuclear)
what is the purpose of the supra-nuclear brainstem generators
- function is to activate burst cells (in PPRF and riMLF) which fire particular patterns of activity
- sent via their axons in the inter-nuclear pathway
what is the purpose of the inter-nuclear pathway
it contains the medial longitudinal fasciculus (MLF) which sends axons to the ocular motor neurons
what is the purpose of the ocular motor neurons
- activating the axons and causing similar firing patterns
- required for slow or fast contraction of their EOMs
what type of gaze is a saccade
fast, conjugate, re-directions of bi-foveal gaze of up to 1000 deg/s
i.e. redirect our foveas to a new object of interest in our FOV
name the two types of saccades
- reflexive: visually cued
- voluntary: internally generated
describe the two types of reflexive saccades
- express to novel stimuli:
- a short latency
- quick shift gaze to find out what stimulus is - when scanning an object of interest
describe the voluntary type of saccade
it is cognitively or memory guided from learned motor routines e.g. reading
it is internally generated by absent will and don’t need something in your FOV at that moment in order to initiate that particular eye movement
name the two brainstem saccade generators and their action
- PPRF: horizontal saccades
- riMLF: vertical saccades
list the 5 higher control centres for mediating fast saccadic eye movements
- frontal eye field
- supplementary eye field
- parietal eye field
- midbrain superior colliculus
- midline cerebellar cortex and nuclei
which eye fields are responsible for voluntary saccades
frontal eye field and supplementary eye field
what type of saccade is the parietal eye field responsible for
reflexive saccades for scanning visual search type
what type of saccade is the midbrain superior colliculus responsible for
reflex express saccades to novel stimuli
what is the midline cerebellar cortex and nuclei responsible for
making sure the saccade happens at the right time and with the right amplitude
what are the two layers of the superior colliculus and what do they do
- superficial layers:
- receive direct retinal inputs from the two retinas and contain maps of the opposite hemifield
- centre to peripheral VF, anterior to posterior SC
- lower to upper VF, lateral to medial SC
- superficial layers connect to deeper layers
- deep layers:
- contain pre motor neurons and a map of the saccade directions, in register with the visual map in the layers above
- centre to peripheral gaze, anterior to posterior SC
- down to up gaze, lateral to medial SC
- the 2nd deeper down layer connects directly to the brainstem generator saccades
activity to which part of the superior colliculus causes saccades around central fixation
anterior activity
saccades around which area does activity in the posterior part of the superior colliculus cause
saccades out into the periphery
which direction does activity in the lateral part of the superior colliculus cause the eyes to move
downwards
which direction does the activity in the medial part of the superior colliculus cause the eyes to move
upwards
what do deep layer neurons of the superior colliculus in the saccade map issue commands to
the brainstem saccade generators
in which two ways can the deep layer neurons of the superior colliculus in the saccade map issue commands to the brainstem saccade generators via
- crossed projections of axons to the PPRF = horizontal saccade generator
and connect to
- bilateral projections of axons to the riMLF = the vertical saccade generator
what does a unilateral lesion of the superior colliculus (which is isolated to that structure) cause
abolishes reflexive express saccades into the opposite hemi-field
list the steps of how a reflex express saccade to the left is made
- RGCs are activated in the left and right retinas
- the RGCs send axons down the optic pathway
- to the superficial layers of the superior colliculus
- which make connections with the deeper layers of the superior colliculus neurons
- these neurons send axons across the midline to the excitatory burst cells in the PPRF on the left side of the brainstem
- those excitatory burst cells activate two neuron populations in the abducens nucleus: one which leads to directly to the LR of the L eye and one via the internuclear pathway which activates the neuron in the oculomotor complex and makes the MR of the R eye contract
- so both eyes look towards the left
= reflex express saccade
what are the purposes of making several scanning/visual search saccades followed by brief fixation every second (250,000 every day)
- to examine different regions of interest on objects by bi-fixating them with out high acuity foveas
- so acquiring more detailed knowledge about them
where in the brain is the parietal eye field located
inferior parietal lobule (in posterior part of parietal cortex)
what does the parietal eye field initiate
scanning and visual search
from where does the PEF receive visual inputs from
dorsal stream extra-striate areas e.g. v5/MT
where do the PEF neurons send descending projections to and what does this allow for
PEF sends descending projections via the posterior limb of the internal capsule to deep layer neurons in the saccade map of the ipsilateral superior colliculus
allows the PEF to issue commands to these superior colliculus neurons and to hotwire their direct access to the brainstem saccade generators
what does a unilateral lesion to the PEF cause
contralateral hemi-spatial ‘neglect’ and/or impoverished scanning in the opposite hemi-field (sticky fixation)
it abolishes the contralateral hemi field
list the steps that occur within the brain, of the descending projections from the PEF to the deep layers of the superior colliculus which activates the pre motor neurons
- the PEF contains a pre motor neuron cell body in the grey matter of the inferior parietal lobule
- they send motor commands down the white matter into the posterior limb of the internal capsule
- then to the cerebral peduncle (underneath the midbrain): the axons in the white matter will exit and travel to deep layers of the superior colliculus
- this activates pre motor neurons in that midbrain structure
what does a lesion to the right PEF cause
reduced scanning and neglect in the left hemi-field
so scan paths will only be in the right hand side of the image and rarely onto the left side which is ignored = sticky fixation
why do we do voluntary saccades
these are performed as self (or internally) initiated acts of will
- because you decide to look at something different
- because you are instructed to “look at this”
what are voluntary saccades influenced by
cognitive factors: because you want to inspect something specific
and are executed as part of learned motor routines e.g. scanning across words on a page while reading i.e. so not making smooth pursuit eye movements but instead bouncing across a page and picking and computing what the words mean
where in the brain is the frontal eye field located
middle frontal gyrus
what does the FEF initiate
all self generated saccades
list the steps of how the FEF initiates self generated saccades
- via direct projections to the brainstem saccade generators
- descending axons run in the anterior limb of the internal capsule and then to the cerebral peduncle
- they cross over to the PPRF and make connections with the EBCs (in the upper pons) on opposite side of the brain (horizontal saccades) and go bilaterally to the riMLF and make connections with the EBCs (vertical saccades)
what does each FEF contain a map of
saccade directions, mainly towards the opposite side/hemi-field
what does a unilateral lesion to the FEF cause
abolishes voluntary saccades away from the affected side (into the opposite side of the VF) causing paralysis of the contra-versive gaze)
e.g. damage to the right FEF = cannot make voluntary saccades towards the left
where abouts in the brain does the FEF axons cross to the opposite PPRF
in the upper pons
as the FEF axons cross over and make connections with the PPRF EBCs on the opposite side of the brain for horizontal saccades, what does this cause as a result
activity in the left FEF causes saccades towards the right
what do FEF have direct connections with
supplementary eye fields
what does a left FEF lesions cause
a supra nuclear right gaze palsy (can’t make voluntary saccades to the right)
what will still be intact with a lesion to the left FEF
- left gaze (as right FEF still intact and signalling to the left PPRF to make leftward saccades)
- right VOR: dolls eye reflex
which aspect of scanning do FEF and SEF control
cognitive aspects of scanning i.e. when you want to inspect something specific
what can FEF and SEF inhibit
express saccades
what are FEF and SEF connected to
- each other
- and to the ipsilateral PEF via dense inter-cortical projections
what do the direct visual search and scan paths produced by the FEF and SEF depend on
the specific information that needs to be extracted and as part of a complex, learned routine
what do the FEF and SEF have direct descending projections to
to the deep layers of the superior colliculus
how do the FEF and SEF inhabit express saccades
when the FEF and SEF make direct descending projections to the deep layers of the superior colliculus, instead of activating the pre motor neurons that generate express saccades, they inhibit the deep payer neurons = preventing the occurrence of unwanted express saccades during more purposeful voluntary visual search
what is the inhibition of express saccades, caused by the FEF and SEF called
executive functions
they relate to anticipatory or predictive brain activities e.g. when reading and during cognitive control of saccades
what is meant by the cognitive control of saccades
different scan paths generate to retrieve different information
e.g. if a person was given no instructions when shows a picture, they will make scan paths to all the various/main components of the picture
if the person was asked if the visitor brings good or bad news in the picture, the person will make scans on the body of the person in the picture, to see clues of body expression and face
if the person was asked to assess the wealth of the household when shown the picture, they will make scan paths on the furniture etc
what are the cerebellar cortex and deep nuclei involved in
saccade ‘gain control’ = to make sure eye movements occur with the right amplitude, size and at the right time
they function as a common ‘supervisor’
list the steps of the connections that are made in order for the cerebellar cortex and deep nuclei involved to produce saccade ‘gain control’
- the midline cerebellar cortex, mainly the vermis receives motor signals from all 3 cortical eye fields (via the pontine nucleus)
- purkinje cells in these cortical regions (in the vermis), project to the deep cerebellar nuclei, mainly to the fastigial nucleus found at the midline
- which sends instructions directly to the brainstem saccade generators, the PPRF and riMLF
what two things to lesions to the vermis cortex or fastigal nucleus (deep down) in the cerebellar cortex result in
- intrusions = saccades occur at the wrong time
- dysmetria = saccades occur with the wrong size (amplitude), typically undershooting the intended target
how do the FEF and SEF for voluntary saccades make connections with the brainstem nulclei
they make direct connections with the brainstem generator nuclei
how does the PEF make connections with the brainstem nuclei
it makes indirect connections with the brainstem generator nuclei via the superior colliculus
