The vestibular organ and the vestibulo-ocular reflex

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