Lectures 15 & 16 (Annette Allen) Flashcards
Oculomotor movements & Superior Colliculus
Why do our eyes move differently from the rest of our body?
(3 reasons)
Week 8 - Occulormotor movements
- move target onto fovea
- keep target on fovea
- if we dont move eyes, our vision will fade
What is the fovea
Week 8 - Occulormotor movements
area @ back of eye, highest conc of photoreceptors, provides highest resolution vision
Roy pritchard attached a projector to the surf of his eye projecting a total stable image on his retina - what did he find and what does this show
Week 8 - Occulormotor movements
Once the images stabilised on the retina, after a few seconds of viewing, they progressively dissapeared bit by bit
- shows we need to be able to move our eyes (even if only a little bit) to see
What are the 5 types of eye movement
Week 8 - Occulormotor movements
- saccades
- smooth pursuit
- vergence
- vestibulooccular movements
- optokinetic movements
briefly describe the fundamentals of saccades as well as the movements involved
Week 8 - Occulormotor movements
shift the fovea rapidly to a peripheral target
Variable in distance (short or long), rapid movements, both eyes move together
briefly describe the fundamentals of smooth pursuit as well as the movements involved
Week 8 - Occulormotor movements
move the eyes to ensure that they keep the image of a moving target on the fovea
Smooth, continuous movement Both eyes move together. Tracking eye movement
briefly describe the fundamentals of vergence as well as the movements involved
Week 8 - Occulormotor movements
- move the eyes in opposite directions so that the image is potioned on both foveae
- if something comes towards us, tend to converge our eyes, if they move aware tend to diverge
Smooth movement , Eyes move in opposite directions
briefly describe the fundamentals of Vestibulo-ocular movements as well as the movements involved
Week 8 - Occulormotor movements
- driven by the vestibular system and keep images still on the retina during brief head movements.
- I.e eyes stay fixed looking at something even if you move your head
Smooth/Rapid movement Eyes move in the same direction to maintain fixation
briefly describe the fundamentals of optokinetic movements as well as the movements involved
Week 8 - Occulormotor movements
- hold images still during sustained movements and are driven by visual stimulation
- i.e eyes make compensatory movements to keep up with image motion like when youre on a train and looking out the window
Rapid movement, Eyes move repeatedly (pursuit then saccade like) to maintain stable view of moving object
What is the fixation system?
Week 8 - Occulormotor movements
An associated system involved in keeping eyes steady when looking at something for a period of time
We always make minor eye movements (minor saccades) to prevent vision from fading, but we still want to keep the eyes fairly fixated on something
(wont go into major detail, not a focal point)
What are micro saccades and why do we have them
Week 8 - Occulormotor movements
Minor eye movements performed to prevent vision fading
How do saccades help us investigate the world
Week 8 - Occulormotor movements
They connect our active fixations (we fixate on something then perform a saccade once we switch to a new fixation target)
How can paintings be used to see saccades
Week 8 - Occulormotor movements
When presenting a P with a painting, if you track their eye movements, you can see where people spend most of their time looking (typically at faces) and can see lines going between the spots of fixation - these lines represent saccadic eye movements
What speed do saccades occur at and why?
Week 8 - Occulormotor movements
theyre extremely fast- reaching speeds of up to 900 degrees per second - this high speed is likely due to a survival need
What kind of waveform do saccades display
Week 8 - Occulormotor movements
All saccades display a stereotypical waveform w/ a smooth increase & decrease in eye velocity
What determines the velocity of a saccade
Week 8 - Occulormotor movements
the distance moved by the eyes
How does the velocity of a saccade change over the time course of switching from the old target to the new target
Week 8 - Occulormotor movements
Velocity starts at 0 before saccade, ramps up when you start to move your eye from old target to new target., when halfway there, velocity rapidly slows down (bell shaped curve almost)
What can and cant we change about saccades
Week 8 - Occulormotor movements
we can voluntarily change the direction & amplitude of our saccades, but not their velocity - velocity always shows a pattern of rapid ramp up and ramp down
Aside from visual stimuli, what other things are we capable of making saccades for
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Auditory stimuli, Tactile stimuli,memorised locations and verbal commands
(this is also true for smooth pursuit)
How do smooth pursuits keep the eyes on a moving target
Week 8 - Occulormotor movements
by calculating how fast the target is moving
What is necessary for the smooth pursuit system to operate
Week 8 - Occulormotor movements
A moving target, otherwise you can’t pursue it
for this exact reason you cannot pursue an imaginery target (it would likely be a series of saccades rather than a true pursuit)
How fast are smooth pursuit movements
Week 8 - Occulormotor movements
Much slower than saccades- have a max velocity of 100 degrees per second
In an example where a subject is asked to make a saccade to a new target that jumps away from the centre of gaze, then slowly moves back to the centre, what would you expect to see on a graph tracking eye position
An initial small movement in the wrong direction,followed by a saccade, and then smooth pursuit in the right direction, back to the centre position.
The reason for this is because the latency of our pursuit system is before the latency of our saccades (nicely highlighting that we have two seperate systems that control the same muscle groups in tandem)
In an example where a subject is asked to make a saccade to a new target that jumps away from the centre of gaze, then slowly moves back to the centre, what would you expect to see on a graph tracking eye velocity
An initial ramp up in velocity, followed by a ramp down in velocity, followed by a consistent low velocity.
The ramp up and down represent the saccadic eye movement, with the apex representing the midpoint of the movement, the consistently low velocity represents the smooth pursuit movement
In terms of how the eyes move, what differentiates vergence from saccadic and smooth pursuit movements
Week 8 - Occulormotor movements
Vergence movements are disconjugate (the two eyes move in opposite directions) whereas saccadic and Smooth pursuit movements are conjugate (eyes move in the same direction)
What happens when we look at an object that is closer to us
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Our eyes rotate inwards (they converge)
What happens when we look at an object that is further from us
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Our eyes rotate outwards (they diverge)
Why do our eyes diverge and converge depending on the distance of the fixation object
Week 8 - Occulormotor movements
to ensure that the object of fixation is on the fovea of each retina
There are 6 extraocular muscles attatched to each eye that control eye movement, what are they
Week 8 - Occulormotor movements
four rectus muscles (superior,inferior, medial & lateral) & two obliques (superior and inferior)
Define the 2 eye movements that happen on the torsional axis
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Intorsion rotates the top of the cornea toward the nose.
Extorsion rotates the top of the cornea away from the nose.
Define the 2 eye movements that happen on the vertical axis
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Elevation rotates the eye vertically up.
Depression rotates the eye down.
Define the 2 eye movements that happen on the horizontal axis
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Abduction rotates the eye away from the nose
Adduction rotates the eye towards the nose.
What are the three axis of rotation that the orientation of the eye can be defined by
Week 8 - Occulormotor movements
horizontal,vertical & torsional
What muscle adducts the eye (towards the nose)
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medial rectus
What muscle abducts the eye (away from the nose)
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lateral rectus
Vertical and torsional rotations are carried out by a combination of 4/6 muscles connected to the eye, this depends on the horizontal position of the eye.
When the eye is adducted, what muscles are responsible for intorsion, extorsion, depression and elevation
Week 8 - Occulormotor movements
superior rectus- intorsion
inferior rectus- extorsion
superior oblique- depression
inferior oblique - elevation
Vertical and torsional rotations are carried out by a combination of 4/6 muscles connected to the eye, this depends on the horizontal position of the eye.
When the eye is abducted , what muscles are responsible for intorsion, extorsion, depression and elevation
Week 8 - Occulormotor movements
superior rectus- elevation
inferior rectus- depression
superior oblique- intorsion
inferior oblique - extorsion
There are three cranial nerves that control the extraocular muscles, what are they and where in the brain are they located
Week 8 - Occulormotor movements
- the abducens nerve (CN VI) - in pons
- the oculomotor nerve (CN III) - In mesencephalic reticular formation
- the trochlear nerve (CN IV) - In medial longitudinal fasiculus
Where are the nuclei to the cranial nerves that control the extraocular muscles
Week 8 - Occulormotor movements
in the brain stem
What does damage to the cranial nerves or extraocular muscles result it and why
Week 8 - Occulormotor movements
Double vision (diplopia) - because damage to the nerves/muscles causes the images of a fixated object to no longer fall on the same locations of both retinae.
What does damage to the abducens nerve (CN VI) cause
Week 8 - Occulormotor movements
damage to the lateral rectus muscle causing a loss of abduction beyond the midline, causing diplopia
Easy diagnosis - lost ability to move eye beyond midline
What does damage to the oculomtor nerve (CN III) cause
Week 8 - Occulormotor movements
- results in a loss of eye movements medially or upward from the mid position. It also leads to a drooping eye lid, mydriasis, and a downward and lateral gaze
- Innervates 4 extra ocular muscles ipsilaterally so damage to this nerve can be quite pronounced
Lesions rare but lead to unilateral failure of almost all eye move
What does damage to the trochlear nerve (CN IV) cause)
Week 8 - Occulormotor movements
- leads to a skew deviation (where eyes are at different vertical positions in the orbit).
- also leads to deficits in intorsion/extorsion, elevation/depression, and a torsional deficit.
What happens to the firing rate of extraoculor motor neurons during a saccade
(2 things)
Week 8 - Occulormotor movements
Firing rate increases in a pulse of activity as the eye velocity goes from 0 to 900 degrees/ second
The extraoculor motor neurons change their baseline firing rate to reflect the new position of the eye at the end of the eye movement - this is known as a step change in intensity
What are horizontal saccades generated by
Week 8 - Occulormotor movements
Motor neurons in the pontine reticular formation
Which brain area are vertical saccades generated by?
Week 8 - Occulormotor movements
Saccades in the mesencephalic reticular formation (in midbraid)
What are responsible for the step (2) and pulse (4) component of motor signal respectively?
Week 8 - Occulormotor movements
step - neurons in the medial vestibular nucleus and the nucleus prepositus hypoglossi
Pulse - Omnipause cells, medium lead burst neurons, long lead burst neurons and inhibitory burst neurons
How do burst cells control the motor component of motor signal
Week 8 - Occulormotor movements
they fire at a burst of high frequency spikes just before & during ipsilateral saccades
In terms of horizontal movements
What do omnipause cells do in terms of generating pulse
Week 8 - Occulormotor movements
fire continuously except around the time of a saccade. They are GABA-ergic and inhibit the medium-lead burst neurons
In terms of horizontal movements
What do medium-lead burst neurons do in terms of generating pulse
Week 8 - Occulormotor movements
make direct connections to oculomotor neurons
In terms of horziontal movements
What do long-lead burst neurons do in terms of generating pulse
Week 8 - Occulormotor movements
drive the medium lead burst cells & recieve inputs from higher centres
In terms of horizontal movements
What do inhibitory burst neurons do in terms of generating pulse
Week 8 - Occulormotor movements
they are driven by medium-lead burst cells and suppress contralateral abducens neurons
What interaction is required from omnipause cells and long-lead burst cells for saccadic movement (Pulse) to occur? Why?
Week 8 - Occulormotor movements
the omnipause cells to pause their firing and the excitation of long-lead burst cells.
This ensures the stability of the system and that unwanted saccades occur infrequently
In terms of horizontal movements
neurons in what region control the step response
Week 8 - Occulormotor movements
medial vestibular nucleus and the nucleus prepositus hypoglossi
What do lesions in the medial vestibular nucleus and the nucleus prepositus hypoglossi result in
Week 8 - Occulormotor movements
Lesions in these regions cause the eyes to drift back to a central viewing position but do not affect saccades per se
Which structure controls the output of saccades?
Week 8 - Occulormotor movements
superior colliculus
What are sensory-motor systems necessary for?
Survival.
What does colliculus mean?
Little hill
What is the name for the Superior Colliculus in lower vertebrates?
Optic Tectum.
Where is the SC found in mammals?
The dorsal surface of the midbrain, found below the cerebral cortex in mammals.
Describe the relationship of size and location of the SC in lower to higher vertebrates.
It’s a highly conserved and ancient part of the mammalian midbrain.
The size changes relative to the forebrain:
- Lower vertebrates have a large SC than forebrain.
- Higher vertebrates have a much larger forebrain than SC.
What role does the forebrain play in processing SC detected stimuli?
The nuanced appraisal of these stimuli.
The SC is more reflexive - hence lower vertebrates react as apposed to higher who can act
What is consistent within the structure of the SC between animals?
There is a highly conserved, mulit-layered structure in the midbrain.
These alternating fibres vary due to species but they do maintain layering.
Many inputs and outputs to the different layers are common across vertebrates
What are the three main layers of the SC and what are they concerned with?
Superficial: Dorsal
- Visuosensory inputs.
Intermediate and Deep: More ventral
- More sensory and motor related functioning.
What are the 7 layers of the SC?
- Stratum Zonale
- Superficial Grey Layer
- Optic Layer
- Intermediate Grey Layer
- Intermediate White Layer
- Deep Gray Layer
- Deep White Layer
What are the inputs to the superficial SC?
Primarily visual.
It is the primary target of the retina in some mammals (>90% of all glutamatergic RGCs project here in mice).
Supplemented by extensive projections from the visual cortex.
Detail the relationship of inputs from RGCs to the layers of the SC.
RGCs travel and meet the more superficial layers of the SC to synpase target neurons.
Their targeting of the outer layer is very specific.
Each cell responds to a receptive field - the location varies due to where in the SC you record.
What organisation is there in the SC?
For visual information, retinotopic organisation
What types of information are input into the SC and where do they come from?
Visual:
- Frontal Eye Fields (FEF)
- Lateral interparietal cortex
Auditory/Somatosensory:
- Trigeminal Complex
- Barrel Cortex
- Inferior Colliculus
- Nucleus of the brachium of the inferior colliculus.
Provide a general overview of the inputs to the intermediate/deep SC.
Inputs become more diverse as you enter deeper layers as they receive cortical and subcortical inputs.
What types of control and input occur in the intermediate/deep SC?
Auditory and Somatosensory Inputs.
Recent experience (FEF, M2)
Target Value (Substantia Nigra)
Saccade Control (FEF, Cerebellum)
How does ‘map alignment’ refer to the organisation of the SC?
Why is this important?
The different layers of the neurons in the SC contain mutually aligned maps of space for each sensory modality.
- Allows multisensory facilitation.
- Straightforward transformation from sensory cue to motor commands (e.g., in orienting movements).
What is multisensory facilitation and why is it important?
It is the combination of information from more than one sensory modality.
- It is important for detecting biologically relevant events due to the amount of sensory modalities it entails.
- Less salient single modality stimuli can be ignored.
What are the overarching areas that the SC projects to?
Cerebral Cortex & Nuclei.
Thalamus
Hypothalamus
Midbrain
Pons
Medulla
Cerebellum.
Important to know that these projections show the significant role the SC plays.
(VERY complex outputs - see slide 10 of lecture 16)
What does having diverse inputs and outputs allow the SC to do? (3)
- Recieve, process and integrate sensory information.
- Register it spatially.
- Direct appropriate behaviours.
Outline the Sahbizada, Dean & Redgrave (1986) research into SC.
METHODS:
- Inserted stimulating electrodes into different areas of the SC.
- Stimulated them and recorded the behaviour types elicited.
RESULTS:
- A range of behavioural responses were stimulated by the SC (see slide 12 , L16).
- This showed that their outputs are highly diverse.
- Most of these could be organised into orienting and defensive in their role.
Which layers of the SC code for oculomotor activity?
The intermediate and deep layers.
What is special about the neurons in the layers of the SC that code for oculomotor activity?
They show a larger response to stimuli if an animal is going to make a saccade towards those stimuli.
This is different to V1 cells that simply respond due to changes in light intensity in their receptive field.
Outline the study by Goldberg and Wurtz (1972) into neurons coding oculomotor activity.
METHODS:
- Recorded from neurons in the SC of a monkey.
- Presented a stimulus in the middle of visual field, or in an area requiring a saccade.
RESULTS:
- They found that there was a larger response in the cells if a saccade to the stimulus was required.
- Less response to it presented stimulus alone.
What do the intermediate/deeper regions code for? What is special about their encoding?
Oculomotor response.
They response to the detection of the stimulus, not the stimulus itself.
What are distinct movement-related cells in the SC? Outline how they fire and why that is important.
They are cells that have movement fields and detect the presence of a stimulus, not the stimulus itself.
Eye movements are encoded by a population code as individual movement cells have large overlapping movement fields.
The most rostral portion of the SC, often called the fixation zone, is believed to be important for maintaining fixation.
What is the rostral part of the SC referred to as and what do lesions here cause?
The ‘fixation zone’.
Lesions here mean you are more likely to saccade to distracting stimuli even when instructed/rewarded for focusing on a set stimulus.
What does the SC target to control its outputs?
The mesencephalic regions and pontine reticular formations.
Which areas input the SC to allow it to have all its outputs? How do they interact?
FEF and LIP:
- Both drive excitation of SC.
Substantia Nigra:
- Drives inhibition of SC.
The FEF begins vision and activates the SC directly, it also indirectly activates it via the LIP and Parietal Cortex.
Also excites the caudate nucleus which inhibits the substantia nigra - so mitigates inhibtion.
Detail the Shang et al. (2019) study into SC involvement with prey capture.
METHODS:
- Injected tetanus neurotoxin into SC which permanetly silenced neurons in which it was expressed.
- Watched to see the effect on a mouse’s ability to catch a cricket.
RESULTS:
- The speed of attack was severely lower when SC silenced.
- Same as time for capture.
Describe the Hoy et al. (2019) study into different types of neurons within the SC.
METHODS:
- Used chemogenetics and expressed a GPCR which is only activated by CNO (synthetic drug) in mice.
- Therefore, only upon the injection of CNO will the downstream processes be activated.
- In this example it reduces neuronal firing upon introduction (slide 19, L16 for cascade).
- Targetted 3 types of neurons: Wide Field, Narrow FIeld and Parvulbumin expressing neurons.
RESULTS:
- They found that the introduction of CNO DID reduce firing rate (so method worked).
- In behavioural cricket test they found that when the WF or NF neurons were inhibited there was an increased time of capture.
- This was not present for PV.
Breakdown the Hoy et al. (2019) study results for WF neurons.
In a cricket capture behavioural test silenced WF cells resulted in:
Lower speed of detection:
- This could be due to the reduced number of approaches occuring
- This suggests WF neuron silencing leads to reduction in ability to detect prey.
Once detected accuracy was not inhibited (so orienting was fine)
SUMMARY:
- Disrupted prey detection and approach initiation.
Breakdown the Hoy et al. (2019) study results for NF neurons.
In a cricket capture behavioural test silenced NF cells resulted in:
Lower speed of approach:
- But there was no effect on the ability to see and detect from distance.
The accuracy/orienting of appoach was reduced heavily.
What is a looming stimulus and what is it a good way to test?
A stimulus that appears overhead.
A good way to test defensive behaviour.
Describe the Evans et al. (2018) study into what is occuring in the SC during defensive behaviours.
METHODS:
- Used calcium imaging by expressing GCaMP6: a fluorescent reporter of how much calcium is in the neuron.
- Used a high res camera to image this.
- Presented a looming stimulus and looked to see the response.
RESULTS:
- Many neurons in the dorsomedial SC were activated - which encoded the danger of stimulus and the response.
Describe the iChloC study into looming stimulus response from mice.
METHODS:
- Used inhibitory optogenetic approach.
- Expressed a light sensitive protein in the neuron which will either be excitatory or inhibitory in response to light exposure.
- iChloC means CL was expressed which was inhibitory when light exposed.
- Expressed in the dorsomedial SC and measure mice response to looming stimulus.
RESULTS:
- WIthout the light, almost all mice escaped.
- With the light, the probability of escape was very low.
- Inhibition of glutamatergic neurons in the dmSC reduces probability of escape response.
