The vestibular organ and the vestibulo-ocular reflex

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The vestibular organ and the vestibulo-ocular
reflex

• Bijan Pesaran
• 29 April, 2008

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Vestibular organ
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Bony and membranous labyrinth
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Cupula and otoliths move sensory receptors
Cristae
Maculae
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Dynamics of semicircular canals

• Torsion-pendulum model
• MD2T(in) MD2T(out) rD T(out)k T(out)
• System is over-damped
• For frequencies up to 20 Hz, cupula motion
  reflects velocity of head motion

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Hair cell orientation varies across the maculae
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Hair cells respond to cupula motion
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Canal afferents in vestibular nerve code velocity

• S-curve is common
• Can be excitatory and inhibitory
• Different cells have different ranges
• Population code

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The oculomotor muscles
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Rotational degrees of freedom
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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

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A neural integrator model can maintain eye
position
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Nystagmus has two phases
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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

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rVOR gain varies with frequency

• Almost perfect gt 1Hz
• Low gain for low frequencies (0.1Hz)
• Sensory mechanisms can compensate (optokinetic
  reflex)

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rVOR plasticity

• Motor plasticity is necessary to compensate for
  changes in muscle properties
• Adaptation to magnifying lens takes days
• Depends on cerebellum to learn
• Doesnt depend on cerebellum to maintain
• Cerebellum provides error signal

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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

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tVOR depends on viewing distance
Rightward
Leftward
Short latency of 10-12 ms but longer than rVOR
5-7 ms
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Motion velocity
Motion direction
Viewing distance
Eye position

• tVOR depends on viewing angle

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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