Oculomotor movements Flashcards
eye movements and circuits that control them
what is ocular motor control
eye movements
What is the primary purpose of eye movements?
To move the target of interest to a specialized part of the retina, the fovea
what is the fovea / what does it do
has a high density of cone photoreceptors, providing the highest acuity vision.
back of retina
What are the two main reasons we move our eyes?
- To move them voluntarily to a target of interest
- To track moving objects
What happens to images if we do not move our eyes?
Images fade because the photoreceptors in the retina respond to change
This can be demonstrated by fixating on a central point and observing fading images.
Who conducted an experiment in the 1960s involving a projector attached to the eye?
Roy Pritchard
shows how we need to be able to move our eyes in order to see/ without is grey field of light
what are the 5 types of eye movement
- saccades
- smooth persuit
- vergence
- vestibulo-ocular
- optokinetic
what are saccades
shift the fovea raidly to peripheral target
What is a saccadic eye movement?
Rapid eye movements that move the eyes from one location to another - eyes move together
What is a smooth pursuit movement?
movement of the eyes to ensure that they keep the image of a moving target on the fovea (in the visual field)
- smooth and continous, both eyes move together
typically for moving object
What are vergence eye movements?
Eye movements that occur in opposite directions, such as convergence and divergence, so image is positioned on both foveae
- smooth
What are vestibular ocular movements?
Eye movements driven by the vestibular system that maintain fixation/ still image on the retina while the head moves
- smooth/rapid, eyes move in same direction
What are optokinetic movements?
Eye movements that stabilize images during sustained movements, such as looking out a moving train window. driven by visual stimulation
- rapid
- eyes move repeatedly (persuit ten saccade like) to maintain stable view
What is the fixation system?
A system that governs the ability to hold the eyes still during focused/ intent gaze, involving tiny eye movements called micro saccades
what are saccadic movements for
to investigate the world around us via a series of fixations that are connected by saccades
What is the maximum speed of saccadic movements?
Up to 900 degrees per second
extremley fast
True or False: Saccadic eye movements can be made voluntarily.
True
Fill in the blank: Smooth pursuit movements require a _______ to be effective.
visual target
What happens when a moving object exceeds the speed of smooth pursuit movements?
Saccadic eye movements are used to catch up with the object
What is a key characteristic of saccadic movements?
They have a stereotypical waveform with a smooth increase and decrease in velocity
what determines the velocity of a saccade ?
the distance moved by the eyes
How do saccadic eye movements relate to survival?
They allow for quick movements away from approaching objects or to chase prey
The ability to track objects efficiently is evolutionarily important.
what can we voluntarily change about saccadic eye movements?
direction and amplitude but NOT velocity
maximum velocity of smooth persuit eye movements
100 degrees per second
if object is moving faster you use saccadic eye movements to catch up
smooth pursuit targets
- needs to be moving if imagery
- but can pursuit auditory ones
What is the significance of saccades during fixation?
They prevent images from fading on the retina
Fill in the blank: The fovea is the _______ part of the retina for high acuity vision.
central
What happens during a saccade when a target moves away from the center of gaze?
A jump to the new location followed by smooth pursuit
- as pursuit system has a shorter latency
- shows different systems controlling eye movements
This involves a quick movement to the target and then a gradual tracking of its motion.
what are conjucate eye movements
when both eyes move in the same direction
smooth pursuit / saccades
what are disconjugate eye movements
eyes move in different directions
vergence movements
when do eyes converge
when we look at an object that is closer to us and our eyes rotate inwards (towards nose)
when do eyes diverge
when we look at an object that is further away from us and ur eyes rotate outwards (away from nose)
what are the 6 muscles attached to each eye ?
extraocular muscles
- 4 rectus muscles: superior, inferior, medial, lateral
- 2 oblique: superior and inferior
what are the three axis of rotation for the eyes ?
- horizontal (pink)
- vertical (blue)
- torsional (green) (twisty)
abduction
horizontal axis
rotates the eye away from the ose
adduction
horizontal axis
rotates the eye towards the nose
elevation
vertical axis
rotates the eye vertically up
depression
vertical axis
rotates the eye down
intorsion
torsional axis
rotates the top of the cornea towards the nose
extorsion
torsional axis
rotates the top of the cornea away from the nose
what muscle is adduction controled by
medial rectus
what muscle is abduction controlled by ?
lateral rectus
what are the verticle and torsional rotations controled by and what does it depend on?
remaining four muscles, depends on horizontal position of the eyes (adducted or abducted)
what are the 3 cranial nerves that control the extraocular muscles ?
- abducens nerve (VI)
- oculomotor nerve (III)
- trochlear nerve (IV)
nuclei of nerves in brain stem
what is diplopia ?
double vision
how would someone get diplopia ?
patients with cranial nerve or extraocular muscle damage as coordination of movement fails and images are no longer on same location of both retina
what happens if there is damage to abducens nerve ?
damage to lateral rectus causing loss of abduction beyond midline…. diplopia
what happens if there is damage to the oculomotor nerve ?
loss of eve movements medially or upward from mid position
… dropping eyelids, mydriasis, downward and lateral gaze
what is mydriasis
pupil dilation
what happens if ther eis damage to the trochlear nerve ?
leads to skew deviation - eyes at different vertical positions - and problems with horizontal/verticle and torisonal movement deficits
instead look around with their head
most likely cause of damage to nerves involved in eye movement
tumor
what does the activity of extraocular motor neurons signal ?
eye position and velocity of the eye
what happens to the firing rate of extraocular motor neurons in the eye during a saccade ?
- increases in a pulse of activity as eye velocity increases
pulse of activity as they move the eye
after a saccade, what do extraocular motor neurons do ?
change their baseline firing to reflect new position of the eye at the end of movement - this is called a step change in intensity
step-pulse response of ocularmotor neurons in a graph and what they signal for a saccade
- size of saccade: height if step
- speed of saccade: height of pulse
where are horizontal saccades generated ?
by motor neurons in the pontine reticular formation
where are vertical saccades generated ?
by motor neurons in the mesencephalic reticular formation (in midbrain)
what type of neurons is the pulse driven by ?
burst cells
what are brust cells
fire at a burst of high frequency spikes just before or during saccadic movements
neurons involved in generating pulse for horizontal saccadic eye movements
- medium-lead burst neurons
- long-lead burst neurons
- inhibitory burst neurons
- omnipause cells
what are medium-lead burst neurons
make direct connections to oculomotor neurons
what are long-lead burst neurons
drive the medium-lead burst cells and recieve input from higher centers
what are inhibitory burst neurons
are driven by medium-lead burst cells and suppress contralateral abducens and neurons (inhibits movement in unwanted direction)
what are omnipause cells
fire continuously to inhibit medium-lead burst neurons EXCEPT around the time of a saccade.
- GABA-ergic
how are unwanted saccades controlled to be infrequent ?
stability of the pulse system
saccadic eye movement (pulse) require both omnipause cells to pause their firing and the excitation of long-lead burst cells
were are the neurons involved in controlling the step response for horizontal movements ?
- medial vestibular nucleus
- nucleus prepositus hypoglossi
allow eyes to move to and maintain a particular location
what happens if you have lesions in the MVN or NPH?
cause eyes to drift back to a central viewing positionafter saccade (but do not affect saccades)
what cells provide motor signals for saccades ?
pontine and mesencephalic burst cells
what are the pontine and mesencephalic burst cells output controlled by ?
superior colliculus
what is the Superior colliculus ?
layered, multi-sensory structure
what does upper layer of the SC do ?
recieves visual signals from retina
what do lower layers of the SC do ?
process multiple signals from various other parts of the brain
in the smooth persuit system, where are neurons that signal eye velocity ?
- medial vestibular nucleus
- nucleus prepositus
- hypoglossi
- pons
in the smooth pursuit system where is input recieved from ?
- cerebellum
- visual areas
- FEF (frontal eye fields)
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