Oculomotor System I Flashcards
Why do we make eye movements?
To optimize visual function!
What are two types of movements of the eye
- movements that stabilize the eye when the head moves
- movements that align the fovea with a visual target (these are fliogenetically newer elements cuz not all animals have a fovea)
What kinds of movements are those that stabilize the eye when the head moves?
- saccade
- optokinetic
- vestibulo-ocular
- visual fixation
vestibulo-ocular and optokinetic
What kinds of movements are those that align the fovea with a visual target?
- saccade
- optokinetic
- vestibulo-ocular
- visual fixation
saccades and visual fixation
What is the vestibulo-ocular reflex?
Sensing head movements, and for every head movement theres a small eye movment going equal and opposite to stabilize that.
- ideal to compensate for high frequency head motions (eg those when youre walking around, jumping, etc (basically just normal day to day activities)
This reflex adapts, which means that if youre outgoing a constant rotational velocity, within 20 secs, the brain is told that you are no longer spinning. (20 seconds of constant velocity to adapt, then this reflex is no longer functioning, WHICH IS AN ISSUE, as theres then huge blurred vision on the retina)
What is an issue with the ability of the vestibulo-ocular reflex to adapt?
This reflex adapts, which means that if youre outgoing a constant rotational velocity, within 20 secs, the brain is told that you are no longer spinning. (20 seconds of constant velocity to adapt, then this reflex is no longer functioning, WHICH IS AN ISSUE, as theres then huge blurred vision on the retina)
Describe how the eye position in orbit, head position in space, and eye position in space change in the vestibulo-ocular reflex.
Eye position in orbit is equal and opposite to teh head position in space (eyes in orbit move equal and opposite to the head movement)
Eye position in space never moved because he keeps looking straight at us
What is the input of each of these, and what kind of movements are they?
vergence
vestibulo-ocular
saccade
visual fixation
optokinetic
smooth pursuit
Movements that stabilize the eye when the head moves:
- vestibulo-ocular (sensory input from the vestibuloocular canal)
- optokinetic (sensory input is visual. when the whole visual field moves on your retina. phylogenetically variable)
Movements that align the fovea with a visual target:
- saccade (when you quickly move from one point to another. visual input is lost because its so fast)
- visual fixation (when the eye is not moving. can be an extreme motor action as well. youre locked down doing something with your visual system)
- smooth pursuit ( like optokinetic eye movements (drifting gradients) but 1 huge difference: Needs a visual target but the target needs to be small (now its not the visual field moving, its some object or animal in the visual field moving and you want to follow it with your fovea. Eg watching a bird flying). Making a smooth eye movement to track the movement with your fovea. This requires attention (you have to want to do this)
- vergence (Disconjugent eye movements, the eyes move in opposite directions.)
What kind of stimulus would the stimulus of a train beside you starting to move be?
Optokinetic stimulus. You almost think youre moving…the perception of the visual field moving is what makes you think this
What are saccades?
type of movement that aligns the fovea with the visual target.
they allow us to scan the visual field and intermittently focus our attention on the parts of the scene that convey the most significant information
T or F: eyes move smoothly across the page when reading
F: a series of saccades and fixations occur when reading
What is the difference between optokinetic and smooth pursuit movements?
Optokinetic:
- a movement that stabilizes the eye when the head moves
- Unvoluntary movement compared to smooth pursuit. Also involves the changing of the whole visual field
Smooth pursuit
- a movement that aligns the fovea with a visual target
- like optokinetic eye movements (drifting gradients) but 1 huge difference:
- Needs a visual target but the target needs to be small (now its not the visual field moving, its some object or animal in the visual field moving and you want to follow it with your fovea. Eg watching a bird flying). Making a smooth eye movement to track the movement with your fovea. This requires attention (you have to want to do this)
Describe the eye movement following a flying bird changing directions?
Eyes don’t start to move until about 200 ms, MT figures out the velocity (speed and direction) of the moving stimulus and the eye starts moving at the same velocity. But theres an error due to the 200 ms delay.
The saccadic eye movement helps to correct this error and put the fovea on target, then smooth pursuit continues
(eg the bird youre watching fly suddenly changing directions)
Which movements are conjugate eye movements, and which are disconjugent?
vergence
vestibulo-ocular
saccade
visual fixation
optokinetic
smooth pursuit
vergence = disconjugate, eyes move in opposite directions
everything else is conjugate, the two eyes move together
what is vergence necessary for?
necessary to look at targets at different lengths
convergence: to fixate on near objects (driven by crossed disparity)
divergence: to fixate on far objects (driven by uncrossed disparity)
Retinal disparity: the difference in location of image between the two eyes. The disparity signal is derived in extrastriate cortex (_______ stream). This signal is critical for ________________.
Retinal disparity: the difference in location of image between the two eyes. The disparity signal is derived in extrastriate cortex (dorsal stream). This signal is critical for depth perception.
What are the different extraocular muscles?
6 extraocular muscles organized into 3 orthogonal pairs
- lateral and medial rectus rotate the eye horizontally
- inferior and superior rectus pull the eye vertically
- superior and inferior oblique spin the eye in a tortional direction
review table 13.1 in oculomotor system I
Which cranial nerves carry axons of motoneurons to the extraocular muscles? Where do each of these nerves originate?
III (oculomotor nerve), IV (trochlear nerve), VI (abducens nerve)
The oculumotor and trochlear nerves originate from the MIDBRAIN
The abducens originates from the PONS
What is the function of cranial nerves III, IV, and VI in eye movement?
Cranial nerves III (oculomotor), IV (trochlear), and VI (abducens) carry motor neuron axons to the extraocular muscles, controlling eye movements.
Their motoneuron cell bodies are located in their respective brainstem cranial nerve nuclei, allowing precise coordination of gaze.
What do motor neurons (MNs) encode?
eye velocity (E with dot) and eye position (E)
MN firing frequency is linearly related to eye position
MN firing is directly proportional to eye velocity
T or F: There are motor neurons specialized for each kind of eye movement.
F: There are no motor neurons unique for saccades/smooth motor pursuits/etc. They’re NOT specialized for the different kinds of eye movements
Which type of neuron encodes which kind of signal:
Motor neuron, tonic neuron, burst neuron
Motor neuron: both
Tonic neuron: step change
Burst neuron: velocity signal
What is the neural integrator, and where is it located?
Integrates eye velocity (E dot) command to produce eye position command (E)
- located in reticular formation and vestibular complex of brainstem
- part of final common path for all eye movements
