L3 - Oculomotor Systems Flashcards
What are 3 functions of eye movement?
- To move the eye across the visual scene and to foveate important/interesting aspects.
- To converge/diverge the eyes at different distances
- To stabilise the image of our visual scene despite motion of the eye or the scene.
Rods are found in the ____ and work well in ____ _____ conditions.
Periphery, Low light
Cones are found in the _____ and work well for ______ environments.
Centre of vision, colourful.
Which muscles move the eye left and right?
Medial and lateral rectus
Which muscle moves the eye downwards?
Inferior rectus
Which muscle moves the eye upwards?
Superior rectus
What are extra-ocular rotations and which muscles are responsible for their initiation?
Movements of the eye that do not involve vertical or horizontal rotation. Controlled by superior and inferior obliques
What are ductions?
Movements of the eye towards or away from the midline.
What is an adducting movement?
Movement of the eye inwards, i.e towards the nose
What is an abducting movement?
Movement of the eye outwards, i.e towards the ear.
What are versions?
Movements of both eyes in the same direction.
What are vergences?
Movements of the eyes in opposite directions
Most gaze shifts involve what?
Versions and vergences
What is the function of gaze stabilising mechanisms?
Keeping the visual scene as still as possible to ensure a still image.
What is the purpose of gaze shifting mechanisms?
Used by animals with fovea’s to actively search around the visual scene.
Is gaze fixation active or passive?
Using muscles to fixate eyes - active
What is the optokinetic reflex and which type of mechanism is it?
A powerful reflex which maintains gaze position driven by whole field visual motion. Type of gaze stabilising mechanism
What is visual slip?
Motion across the retina. Means that retina is not correcting movement properly.
What is an optokinetic nystagmus?
A movement of the eye seen when an individual follows a moving object with their eyes, which then moves out of the field of vision at which point their eye moves back to the position it was in when it first saw the object.
What does OKN stand for?
Optokinetic Nystagmus
Why is the Optokinetic reflex slow and incomplete?
It is driven by an error signal. Therefore, processing is required to detect the error before a correcting mechanism can occur.
What is a Nystagmus?
Alternation of slow drift of the eyes (slow phase) and rapid saccades (fast phase).
What is the vestibulo ocular reflex?
Automatic reflex that maintains gaze position despite head movement. It is driven by movement of the head, which is detected by the semi circular canals in the inner ear. Counter-rotation is initiated in the eye according to the movement of the head.
What are ocular motor neurons?
Motor neurons that send signals to the eye.
Where does the vestibular nucleus send signals to?
Ocular motor neurons
What is head velocity detected by?
Semi-circular canals in the vestibular system.
In a vestibulary ocular reflex, how is the reflex strength and speed controlled so that the movement of the eyes does not overshoot?
There is an error signal from the visual system.
What does PPH stand for?
Prepositus Hyperglossi
What is PPH?
A nucleus which integrates the input it gets from the vestibular nucleus, stores it as a memory and then feeds it back out as a tonic signal. This allows the eye position to be held after the head has stopped rotating.
What is a tonic signal?
A physiological response that is slow and often works in opposition to a fast response.
What percentage of head movement is counter-acted by the Vestibulo Ocular Reflex?
90-95%
What type of pathway is PPH’s effect on eye position?
Indirect
What does VOR stand for?
Vestibulo Ocular Reflex
What is the key feature of PPH that allows it to keep the eye in position after the head has stopped moving?
It is self-excitatory
What is smooth pursuit?
Slow simultaneous movements of both eyes in the same direction, that requires suppression of the optokinetic reflex. Allows tracking of moving objects across a stationary background.
What are the limitations of smooth pursuit?
- useful when object being tracked is slow moving
- is based on a slow visual feedback mechanism.
During a saccade, what occurs to visual input and why?
Visual input is suppressed, because vision would be so blurred that only a very limited amount of information could be picked up.
Saccades are controlled by what?
Feed-forward mechanism which does not rely on visual information - the initial aspect of the visual scene that was interesting is the target.
If the target is missed, a feedback system then kicks in to decide on and initiate a ‘catch-up’ saccade.
What is ballistic control?
Feedforward movements that don’t have feedback during the movement itself.
The input layer of the superior collicuous has what?
A retinotopic map of the world
The output layer of the superior colliculus does what?
Motor command needed to re-centre an object onto the fovea.
What do omnipause neurons do?
Inhibits movement and holds the eye in place if the signal coming into the superior colliculus refers to an object/aspect of the visual scene that is already on the fovea.
What does OMN stand for?
Ocular motor neurons
What does PPRF stand for?
Paramedian Pontine Reticular Formation
What does the PPRF do?
Releases a burst of action potentials to allow saccadic movements to foveate the aspect of the visual scene that is in the periphery.
What does FEF stand for?
Frontal eye fields
What do FEFs do?
Control voluntary saccades and saccades to remembered targets.
What does SEF stand for?
Supplementary eye fields.
What do SEFs do?
Controls sequential eye movements - e.g. when reading.
Where are saccadic movements controlled?
Systems in the midbrain, in the superior colliculus.
What does SC stand for?
Superior colliculus
