Eye Movments Flashcards
What are some types of eye movements?
Saccades Vergence Smooth pursuit Vestibulo-ocular reflex (VOR) Optokintetic response/nystagmus (OKR/OKN)
What are some properties of saccades?
Both eyes move together (conjugate)
Fastest eye movements
Suppress vision during a saccade so that the world doesn’t move
needs no visual input to happen
What is vergence?
A slower form of eye movement than a saccade.
Disjunctive (cross-eyed, or apart)
Hard to do without visual input
What is smooth pursuit?
Conjugate and slow tracking of a visual object … therefore it requires visual input although there have been some cases of people training themselves to do smooth pursuit without visual input.
What is VOR?
Vestibulo-ocular reflex
Requires only 3 neurons and 10 ms as it is a reflex
This reflex maintains the image on the fovea while we move
What is OKR/OKN?
Optokinetic response/nystagmus
It’s similar to smooth pursuit, but a large part of the world is moving instead of a small object. It’s reflexive and is what makes IMAX work.
How many muscles does the eye have?
Six muscles in 3 pairs of agonists and antagonists per eye
What controls horizontal movements?
Lateral/medial recti (plural of rectus)
What controls vertical movements?
Superior/inferior recti (plural of rectus)
What controls torsion and some vertical movement?
Superior oblique/inferior oblique
What is torsion?
cyclorotation of the orbit (from the bone)
What does the muscle at the top of the eye do?
Moves the eye down
What are three areas of innervation for the eye muscles?
Oculomotor Neuron
Abducens Neuron
Trochlear Neuron
What does innervation mean?
Supply nerves to
What is adduction?
Movement towards the body midline
What is extorsion?
Moving the eye to look outwards at an up or down angle. One eye’s extorsion is the other eyes intorsion.
What is intorsion?
Moving the eye to look inwards at an up or down angle. One eye’s extorsion is the other eyes intorsion.
What is elevation?
Looking up?
What is Depression?
Looking down?
Which muscles innervate to Oculomotor Neurons?
Medial Rectus, Superior Rectus, Inferior Rectus, Inferior Oblique
Which muscle innervates to Abducens Neurons?
Lateral Rectus
Which muscle innervates to Trochlear Neurons?
Superior oblique
What motion is the medial rectus involved in?
Adduction
What is abduction?
Movement away from the body’s midline
What motion is the lateral rectus involved in?
Abduction
What motion is the Suprior rectus involved in?
Elevation
What motion is the inferior rectus involved in?
depression
What motion is the inferior oblique involved in?
Extorsion
What motion is the superior oblique involved in?
Intorsion
What is the effect of a lesion to the abducens nerve?
Stops one eye moving to the right but not to the left.
What is the eye movement control hierarchy?
Cerebral centers (can skip higher order) -> Higher Order Premotor Centers -> Lower order premotor centers -> Ocular motor nuclei -> Ocular Motor nerves -> Extraocular muscles
What is true about the first three parts of the eye movement control hierarchy?
Eye movements. Different areas control for different movements.
What do all eye movements have in common?
They all share the oculomotor subsystems
What parts of the cerebral centers are involved in eye movement?
Frontal eye fields
Parieto-occipital eye fields
What parts of the higher order premotor centers are involved in eye movements?
Superior colliculus
Cerebellar Flocculus
What parts of the lower order premotor centers are involved in eye movements?
Paramedian pontine reticular formation (PPRF)
Mesencephalic reticular formation (MFR)
Vestibular nuclei
What are the numbers associated with the ocularmotor nuclei?
Oculomotor (III)
Trochlear (IV)
Abducens (VI)
How many saccades happen per second?
3
How long is fixation?
200 ms
How often are eyes steady?
60% of the time
How many degrees in a saccade per second?
800 degrees, so 2.5 circles per second
What subcortical areas contribute to saccades?
Brainstem: Pontine reticular formation (Pon RF) and mesencephalic reticular formation (MRF)
What part of a saccade is controlled by the Pon RF?
horizontal is controlled by the Pontine reticular formation
What part of a saccade is controlled by the MRF?
Vertical/torsional is controlled by the mesencephalic reticular formation
What does tonic rate refer to?
Tonic sensory input adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus
What rate do abducens motoneurons discharge at?
Tonic rate during fixation and then a burst during ipsiversive eye movement and a decrease or cessation of activity during contraversive eye movement. Tonic rate will continue to keep eye at the location after its moved.
The eye position during fixation is directly proportional to the discharge rate of the abducens neurons.
What is true about the firing rate of the paramedian pontine reticular formation?
PPRF is in the brainstem and has excitatory burst neurons for horizontal saccades. It has a strong correlation with amplitude of eye movement and number of spikes as is typical in the brain stem.
What is amplitude?
Size or extent of eye movement
Is there horizontal specialization of neurons?
Yes some excitatory burst neurons just care about horizontal movement.
What is the neural integrator?
Nucleus prepositus hypoglossi integratethe pulse of EBNs into a tonic response
What are Omnipause neurons?
Neurons in the Paramedian pontine reticular formation whose tonic firing stops during a saccade.
They monosynpatically inhibit EBNs
How are vertical & torsional saccades generated?
The pulse is generated by the riMLF (rostral interstitial neuron of the MLF) in the mesencephalic reticular formation near III & IV neuron.
The tone (step) is created by the adjacent INC . (interstitial neuron of Cajal)
They converge on the MN of the 4 vertical muscles in the III & IV neuron.
What kind of map does the superior colliculus have?
topographic. Just like in the occipital cortex, the fovea is over-represented.
Direction here is mapped linearly
Also, ipsilateral for space map.
The map is present in each of its 7 layers individually, so a stimuli in one location in space will activate cells in all layers as it maps the location of the stimulus in each layer and the location of excitation will match the map of the other layers.
Is the superior colliculus involved in saccades?
Yes. It’s a major hub.
How many layers does the Superior Colliculus have?
Seven: Superficial grey Superficial white Optic Layer Intermediate grey Intermediate white deep grey deep white
What are the superficial layers of the superior colliculus involved in?
Superficial grey and white areas involved in purely visual sensory processing.
What re the intermediate and deep layers for the superior colliculus involved in?
Mostly visual processing, but also convergence of other modalities and most cells have a visual motor response. There’s more motor the deeper you get into the colliculus. They can elicit motor without a visual response and motor output is not limited to saccades.
SC
Superior Colliculus
What does it mean that the SC is a laminar structure?
It’s arranged into layers.
FEF
Frontal Eye Fields
Compare FEF to SC in saccades
Burst of firing is higher in the superior colliculus than in the frontal eye fields.
What happens in the Intermediate layers of the Superior Colliculus during a saccade?
The neurons have a movement field (like a receptive field) and discharge for movements within that amplitude and direction. This also seems to use population vectors or maybe not since # of spikes doesn’t indicate direction … movement vector?
I’m thinking of this like as dot where activity is the “hot spot” and the surrounding activity declines so that it all adds up to the hot spot
How do topographic maps in the intermediate and deep layers relate to maps of the superficial layers in the SC?
The movement fields in the deeper layers coincide with visual response fields in the superficial layers.
What kind of scheme do neurons in the SC use?
Spatial or place coding scheme
of spikes cannot indicate saccade amplitude and direction, only the location of the neuron on the SC map can determine movement vector. It’s still weighted.
Define vector
A quantity, such as the velocity of an object or the force acting on an object, that has both magnitude and direction.
How does the map related to the size of the movement?
The size of the movement relates to what part of the map you’re in. I imagine that since the fovea has a larger presence that movement within that area requires less of a saccade that double that distance within the map to some other area of the eye.
What is a gaussian distribution?
A normal distribution
Efficacy
The ability to produce a desired or intended result.
What is a bouton?
An enlarged part of a nerve fiber or cell, especially an axon, where it forms a synapse with another nerve. Synaptic button
How did they determine weights in the colliculus?
They injected anterograde tracers into the animals and counted the boutons in the PPRF. There was a linear relationship with greater weight/more boutons at the back of the colliculus.
What is anterograde tracing?
A research method which is used to trace axonal projections from their source (the cell body or soma) to their point of termination (the synapse).
What are 4 key points of the saccadic system?
1) Direct (velocity) and indirect (neural integrator) pathways
2) Spatial to temporal transformation.
3) Vector decoding mechanisms in “spatial structures”
4) Feedback control maintained by corollary discharge, not sensory feedback
PPRF
paramedian pontine reticular formation
Explain the brainstem control of saccades
1) SC neurons deliver desired eye movements commands contralaterally.
2) Omnipause neurons (OPNs) that preserve fixation cease their tonic activity. They inhibit EBNs. This is in the PPRF
3) EBNs discharge a high-frequency burst (pulse) to drive the eyes at high velocity.
4) Ipsilateral to the PPRF, Nucleus prepositus hypoglossi (npt) integrates the pulse of EBNs into a tonic response.
This area is fed by the PPRF in to the nph in the indirect pathway. The tonic signal is from the medial vestbular complex.
5) Extraocular motoneurons (abducens are one example) sum the outputs of the EBNs and neural integrator. The high frequency burst moves onto the eyes to a eccentric location and the tonic activity maintains the new location.
6) OPNs resume activity to end saccade
EBN
Excitatory burst neuron
What are OPNs?
Omnipause neurons
What do OPNs do?
Pause during a saccade.
What happens if OPNs are stimulated during a saccade?
The saccade stops midflight when a drug is injected on OPNs. After stimulation is over, the saccade resumes going to its intended direction. This shows that an internal representation of movement is still active and memory guided.
Therefore there must be a feedback system that says that the end point for the saccade has not been reached.
What happens if EBNs are inactivated via an injection of a drug?
The eyes move more slowly but still get to the target. This is another memory guided saccade task.
What is the feedback control system for saccades?
A saccade is made and feedback drives continuation of the saccade until error goes down to zero. The nph reports back the error to a system between the sc and OPN/EBN.
