Eye Movements Flashcards
Name 5 types of EYE Movements
Vestibulo-ocular reflex (VOR) Optokinetic reflex (OKN) Saccades Smooth pursuit Vergence
Name Differences in 5 types of EYE movements
VOR and OKN are:
- old
- no fovea necessary
- stabilize eye when head moves
Saccades, Smooth pursuit and Vergence are:
- new
- fovea necessary
- keep fovea on visual target
Name 3 complimentary pairs of eye muscles
- lateral rectus & medial rectus —-> abduction & adduction
- superior rectus & inferior rectus —-> elevation/intorsion & depression/extorsion
- superior oblique & inferior oblique —-> depression/intorsion & elevation/extorsion
Measuring EYE Movements:
What is an EOG?
An Electro-oculogram is a charge at the back of the retina that moves with the eye.
Measuring EYE Movements:
What is a Scleral search coil?
- magnetic fields induce currents in a coil on the eye
- either surgically implanted on the eye (animals) or embedded within a contact lens (human)
- current is proportional to the angle of the coil
Measuring EYE Movements:
What is video-based eye tracker?
It’s infrared, iris match
Name all 6 EYE vs SOMA MUSCLE properties, part 1.
- All 6 eye muscles participate in all 5 eye movement types
- Smaller fibers with high innervation ratio
- # nerve terminals : # muscle fibers = 1:10
- 8ms twitch time (2-3 x faster than somatomotor fibers)
- Fast, strong and very precise
Name all 6 EYE vs SOMA MUSCLE properties, part 2.
- Eye muscles never get tired.
- No recurrent inhibition, but a lot of connective tissue
- Each muscle has 2 layers:
- Global layer – inserts on eyeball
- Orbital layer – inserts on connective tissue
- More proprioceptors, especially muscle spindles (detect stretch)
- WHY? Oculomotor muscles never deal with changes in load
Name 3 attributes of the Vestibulo-Ocular Reflex (VOR)
- Functions to stabilize the eye during head motion
- Fast , stereotyped response (7 - 15 ms latency)
- Active almost all the time and allows clear vision while moving
Talk about the Mechanism of the Vestibulo-Ocular Reflex (VOR)
- Semicircular canals of the vestibular system detect head rotation
- The oculomotor system rotates the eyes by the same amount, but in the opposite direction
What are the 2 types of Vestibular-Ocular Reflex (VOR)?
- Rotational VOR (rVOR)
when the head rotates with a certain speed and direction, the eyes rotate with the same speed but in the opposite direction - Translational VOR (tVOR)
When the head translates, the eyes rotate in the opposite direction by an amount proportional to the distance of the object and is a function of gaze angle
BASIC BRAINSTEM CIRCUIT for the rVOR:
The rVOR consists of a 3-neuron arc, name them
- semicircular canal (SCC) afferent neurons
- vestibular nucleus (VN) interneurons
- oculomotor abducens neurons
BASIC BRAINSTEM CIRCUIT for the rVOR:
Describe with a rightward head rotation.
- right SCC afferents excited
- ipsilateral (right) VN cells excited
- VN interneurons project across the midline and excite the contralateral (left) abducens nucleus
- Abducens motoneurons excite left lateral rectus
- Abducens interneurons project back across the midline to excite right oculomotor neurons, which excite the right medial rectus
- Opposite inhibitory path from left canal
Describe 7 Properties of Nystagmus
- the VOR generates the slow phases of nystagmus
- when eye approaches the edge of the oculomotor range, a saccade (fast phase) is generated in the opposite direction
- normal nystagmus is seen during large head rotations
- during constant velocity rotation…..habituation of the VOR
- time constant of habituation = 5 seconds
- brainstem “velocity storage” extends this to 25 seconds
- Post-rotatory nystagmus occurs in opposite direciton
Describe Saccades
- rapid (up to 900o/s), ballistic eye movements to change direction of fixation
- no visual feedback
- ~3 saccades per second
- microsaccades during fixation
- like fast phase of nystagmus
- used for redirecting gaze, visual search, reaching, REM sleep
Saccades:
Describe Saccade Generation
- both the SC and the Frontal Eye Fields (FEF) can initiate saccades
- both the SC and FEF use population coding to generate saccades
- takes into account the activity of all active neurons and weighs each according to the strength of its firing rate
Moving to the study of Optokinetic reflex (OKN):
Describe the OKN
- Uses visual rather than vestibular input, therefore sluggish, but will maintain its response indefinitely.
- Complements the VOR.
Gain = signal output / signal input
Gain VOR = eye movement / head movement
Describe the 2 pathways of the Optokinetic reflex (OKN)
- sub-cortical via brainstem accessory optic system
- cortical via dorsal cortical pathway
BOTH pathways converge on the vestibular nucleus
- explains why OKN produces sensation of motion
- e.g., waiting at traffic light, waterfall illusion, IMAX theatre
Saccades:
Describe Saccade Main Sequence.
Velocity of the saccade depends on distance of the target
- one CAN voluntarily change the amplitude and direction of a saccade but one CANNOT change its velocity
Similarly, the Duration of the saccade depends on the distance of the target
- one CANNOT change its duration
Saccades:
Describe the Superior Colliculus (SC)
- the SC contains a topographical map of eye movement vectors
- activity in a specific SC region produces an eye movement directed to a distinct spatial location (relative to the fovea)
- SC cells fire just prior to an eye movement, eliciting a “go to” command
Smooth pursuit:
Describe Smooth Pursuit
- smooth pursuit moves the eyes to keep moving targets on the fovea
- requires a moving stimulus (with some exceptions)
- maximum velocity = 100o/s (degrees per second)
- smooth pursuit is a closed loop system that uses visual motion feedback
Vergence:
Describe Vergence
- convergence brings the eyes together for near targets
- divergence takes the eyes apart for far targets
- associated with accommodation response of the lens
- involves primary visual cortex (VI) for detecting visual target, area MT for measuring angle disparity, and brainstem neurons to move the eyes proportional to target distance
- very slow ~25o/s
- goal: to maintain or obtain singular binocular vision
Final Common Pathway:
Describe the specifics
- eye velocity signals drive eye to a specific location in space
- to keep the eye at this location, position signal is necessary
- Q: How can one get position from velocity?
- A: Integration
- The brainstem final common pathway integrates velocity commands and then adds the integral to the command.
- velocity = pulse
position = step - pulse-step is seen in the spiking rate of oculomotor neurons
- separate final common pathways for horizontal and vertical/torsional components
Which cranial nerves are used in eye movements?
Superior Oblique 4 (trochlear nerve)
Lateral Rectus 6 (abducens nerve)
all the rest of the muscles use 3 (oculomotor nerve)
