Eye movements and sensory integration Flashcards
What is the vestibulo-occular reflex (VOR)?
A stabilising eye movement - to avoid image drift
As head rotates, eyes move in opposite direction
Problem = how to balance image quality with head movement - visual information is irretrievably lost when moving eye
Solution = counter-rotate eyes to exactly offset head movement
What is the optokinetic reflex (OKR)
A second stabilising eye movement
Response to movement of surroundings - eyes follow the surround movements so image stays stable on the retina
What are saccades?
A target identification eye movement
Very fast flicks of the eye - most common (3/s when waking) -
Cannot see own in mirror as vision is suppressed but can see other’s ie when reading
Necessary to bring items into foveal vision where acuity and colour vision is best
What is smooth pursuit?
A second target identification mechanism
For keeping slow moving targets on the fovea
Hard to do voluntarily - can be seen when fixating on a pendulum
What is vergence?
A third mechanism of target identification
For perception of stimuli changing their distance from you
Can be slow or fast
What is the basic circuity of VOR?
Motion/velocity of head sensed by three sets of semicircualr canals each in different orientations (damaged by some antibiotic overdose)
Sensory axons = primary vestibular afferents with cell bodies in Scarpa’s ganglion (outside the brain)
Synapse with interneurons in the medial vestibular nucleus
Project to abducens nucleus that project motor neurons to the lateral + medial rectus muscles (horizonal plane)
VOR interrupted when drinking, reduction in dynamic visual acuity
Why is the VOR reflex an example of feedforward/ballistic control?
Output is movement of the eyes but input is velocity of the head
Output has no effect on input; output does not serve to reduce its input (useful as can keep sensing further changes)
What is the basic circuity of OKR?
Similar to VOR but input from retina and pretectum = movement of the whole retinal image (retinal slip) ie image velocity (then vestibulo-occular nuclei - eye movement)
A feedback mechanism as output does effect input - correction of the view of the world stops needing correction of the world
Why are two stabilising reflexes required?
OKR on its own is inadequate as does not work for rapidly changing/high frequency movements - as frequency increases, gain reduces; due to a delay in the feedback loop - retinal processing of slip takes 50-100ms
VOR is good for high frequency head movements ie walking/running as processing takes c.14ms; but poor for low frequency constant movements - semicircular canals stop responding to constant head movements
How does the VOR
arc know how much to correct eye movement by each time?
Genetic/inherited? unlikely:
Needs to be adaptable to deal with growth of semicircular canals; injury, age etc
Likely a continuously adaptive process - cerebellar region of the flocculus (bit below the main lobes, coming off the bottom of the vermis) is central - inactivation of this region block VOR
Can also wear spectacles that alter the size of retinal image and so alter what VOR has to do - induce this VOR adaptation
