The vestibular organ and the vestibulo- ocular reflex Bijan Pesaran 29 April, 2008
Jan 09, 2016
The vestibular organ and the vestibulo-ocular reflex
Bijan Pesaran
29 April, 2008
Vestibular organ
Bony and membranous labyrinth
Cupula and otoliths move sensory receptors
Cristae Maculae
Dynamics of semicircular canals
• Torsion-pendulum model
• MD2Θ(in) = MD2Θ(out) +rD Θ(out)+k Θ(out)
• System is over-damped
• For frequencies up to 20 Hz, cupula motion reflects velocity of head motion
Hair cell orientation varies across the maculae
Hair cells respond to cupula motion
Canal afferents in vestibular nerve code velocity
• S-curve is common• Can be excitatory and
inhibitory• Different cells have
different ranges• Population code
The oculomotor muscles
Rotational degrees of freedom
Static VOR
• Stabilize eyes due to tilt of head
• In humans weak, dominated by dynamic VOR and vision
• Easily demonstrated in rabbits
• Complicated by stimulation of proprioceptors in neck (COR)– Move head with body
• In humans, torsional counter-roll due to tilt
A neural integrator model can maintain eye position
Nystagmus has two phases
Quick phase
• Not due to eye position– No effect of removing eyes– Not at consistent eye position
• Related to eye velocity
• Periodic phase intervals– Unimodal at low head speeds– Multimodal with 0.5s interval at higher speeds
rVOR gain varies with frequency
• Almost perfect > 1Hz• Low gain for low
frequencies (0.1Hz)• Sensory mechanisms
can compensate (optokinetic reflex)
rVOR plasticity
• Motor plasticity is necessary to compensate for changes in muscle properties
• Adaptation to magnifying lens takes days
• Depends on cerebellum to learn
• Doesn’t depend on cerebellum to maintain
• Cerebellum provides error signal
Translation VOR
• Compensates for linear motion as opposed to rotational motion
• Only studied in primates – rudimentary in lateral-eyed species
• Only stabilizes one point – fovea– Consider optic flow during motion
tVOR depends on viewing distance
Rightward
Leftward
Short latency of 10-12 msbut longer than rVOR 5-7 ms
• tVOR depends on viewing angle
Motionvelocity
Viewingdistance
Eye position
Motion direction
Gaze shifts require eye and head movements
• Need to move eye and head to new position with stable vision
• Keep eye velocity equal and opposite to head movement
• During saccade itself, turn off VOR
• At this time, see no compensation for changes in head position