PSY 345

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PSY 345

• The Vestibular Sense

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The Vestibular Sense

• What we generally call balance is the VESTIBULAR
  sense.
• The Vestibular Organs are structures in the inner
  ear on each side of the head.
• Changes in the position of the vestibular organs
  cause receptors to be stimulated.
• These receptors tell the brain the direction of
  tilt, amount of acceleration and position of the
  head with respect to gravity.

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The structure of the vestibular system

• Each vestibular organ is made up of
• Two otolith organs
• Utricle
• Saccule
• Three semi-circular canals
• Oriented in the three planes (X, Y and Z)

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The structure of the vestibular system
semicircular canals.

• The canals detect angular head acceleration
• Not constant motion
• Not linear acceleration
• The canals contain a jellylike substance called
  endolymph and are lined with hair cells.
• Rotational acceleration causes the jellylike
  substance to move the hair cells, causing them to
  hyperpolarize or depolarize depending upon the
  direction of acceleration.
• Cup example

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Hair Cells
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The structure of the vestibular system otolith
organs.

• The Utricle and Saccule detect head orientation
  with respect to gravity and linear acceleration.
• Utricle Horizontal
• Saccule Vertical
• Each contains hair cells embedded in jelly that
  contains small crystals
• Otoconia
• Pressure of otoconia against hair cells signals
  position of head relative to gravity.

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The Otoliths
Otoconia
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otoconia
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The Vestibular Pathway

• Output from the vestibular organs is carried to
  the vestibular nucleus by Cranial Nerve VIII.
• There it is integrated with information from the
  visual and somatosensory systems.

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The Vestibular Pathway
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The vestibulo-occular reflex

• The VOR allows us to keep our eyes focused in one
  direction while our head is moving about.
• When the head turns to the left
• The eyes are told to turn to the right

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Visually induced illusions of movement

• Self-vection the perception of movement while
  stationary.
• Eyes see movement
• Vestibular system reports nothing
• Brain assumes that we are moving instead of the
  visual field being in motion.

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Visually induced illusions of movement

• Postural Sway Movement of the visual
  surroundings produces a compensatory response
  from the observer.
• Demonstration of the importance of visual input
  in bodily orientation
• Addition of tactile stimulation

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Motion Sickness

• Sensory Conflict Theory
• Information from the visual system and the
  vestibular system is continuously coordinated to
  maintain equilibrium
• If information from each system is not congruent
  then we can experience motion sickness.
• Unusual passive movement
• Vomiting is believed to be an adaptive response
  to certain alkaloid poisons