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Chapter 12

PSYB51H3 Chapter 12: PSYB51 Chapter 12

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Matthias Niemeier

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PSYB51: Perception and Cognition Clara Rebello PSYB51 Chapter 12: The Vestibular System and Our Sense of Equilibrium  When you spin around until you feel dizzy o Your dizziness arose from contributions of the vestibular organs to your sense of equilibrium o Vestibular organs: The set of five sense organs (three semicircular canals and two otolith organs) located in each inner ear that sense head motion and head orientation with respect to gravity o Equilibrium: Our vestibular sense comprised of spatial orientation perception (encompassing our perception of linear motion, angular motion, and tilt), combined with reflexive vestibular responses like posture, vestibule-autonomic reflexes, and vestibule-ocular reflexes o The senses of tilt and self-motion comprise our sense of spatial orientation: A sense consisting of 3 interacting modalities  Perception of linear motion  Angular motion  Tilt  Vestibular system: The vestibular organs and the neural pathways directly associated with these sense organs o Contributes to clear vision when we move and helps us maintain balance when we stand  The fundamental nature of this system is emphasized by the fact that the vestibular organs appeared very early in evolutionary history and have remained relatively unchanged o Vestibular perception is often relegated to the attentional background, and many responses evoked by the vestibular system are reflexive o Only when we experience problems like dizziness, vertigo, spatial disorientation, imbalance, blurred vision, illusory self-motion are we likely to become more aware of our equilibrium sense  Vertigo: A sensation of rotation or spinning  Term often used more generally to mean any form of dizziness o Aristotle didn’t include equilibrium or vestibular sense when he catalogued our sensory systems  Why?  It wasn’t until the 19 century that scientists understood that the vestibular system is a specialized set of sense organs PSYB51: Perception and Cognition Clara Rebello  Until then, the vestibular system had been considered an entrance to the cochlea  Not entirely satisfactory explanation  Aristotle had catalogued other senses without detailed anatomical or physiological knowledge  Another explanation  Inconspicuous nature of the vestibular sense (vestibulo- ocular reflex VOR: A shot-latency reflex that helps stabilize vision by counterrotating the eyes when the vestibular system sense head movement)  You don’t typically perceive your head motion, because the vestibular system usually performs its job automatically (little conscious awareness) Vestibular Contributions to Equilibrium  The vestibular system makes crucial contributions to balance but doesn’t provide the sensory foundation for balance o Kinesthesia does o Balance: The neural processes of postural control by which weight is evenly distributed, enabling us to remain upright and stable  It also helps stabilize our eyes during head motion o Makes crucial contributions to clarity of sight but doesn’t provide the sensory foundation  Our eyes and visual system do  Also helps maintain blood flow to the brain via contributions to cardiac regulation but isn’t foundational there o Somatosensation is  Our equilibrium sense combines information flowing from our brain to our muscles with information flowing inward to the brain from various sensory systems (especially the kinesthetic, visual, and vestibular systems) o Kinesthetic: Perception of the position and movement of our limbs in space o Active sensing balances information derived from efferent commands flowing outward from the brain to the periphery with information from various afferent signals flowing from sensors inward to the brain  Active sensing: Sensing that includes self-generated probing of the environment  Humans (vision, touch, equilibrium), animals (includes the use of echoes by whales and bats, the use of electrical signals by some fish, and the use of whiskers/antennae) PSYB51: Perception and Cognition Clara Rebello  Efferent commands: Information flowing outward from the central nervous system to the periphery  Example: motor commands that regulate muscle contraction  Afferent signals: Information flowing inward to the central nervous system from sensors in the periphery  Passive sensing would rely exclusively on such sensory inflow, providing a traditional view of sensation  Our equilibrium is composed of many fundamental reflexes and perceptual modalities, but the vestibular system doesn’t exclusively provide the sensory foundation for any perceptual modality Modalities and Qualities of Spatial Orientation  Our perception of spatial orientation includes 3 sensory modalities o Sense of angular motion: Perceptual modality that senses rotation o Sense of linear motion: Perceptual modality that senses translation o Sense of tilt: Perceptual modality that senses head inclination with respect to gravity  Why are they called modalities? o Key lies in the energy transduced: To convert from one form of energy to another o Colour and brightness are different interpretations of the same energy (qualities) o Seeing and hearing  Perceiving angular motion, linear motion, and tilt requires that 3 different stimuli (angular acceleration, linear acceleration, gravity) be transduced  Semicircular canal: Any of three toroidal tubes in the vestibular system that sense angular motion o Senses angular acceleration: The rate of change of angular velocity o This signal makes a predominant contribution to our sense of angular motion  Otolith organs: Either of two mechanical structures (utricle and saccule) in the vestibular system that sense both linear acceleration and gravity o They transduce both linear acceleration (The rate of change of linear velocity) and gravity o Provide a predominant contribution to sense of head tilt and to sense of linear motion, also referred to as sense of translation  The semicircular canals and otolith organs establish at least two sensory modalities o Perception of tilt results from the brain’s estimate of orientation with respect to gravity, and perception of linear motion results from the brain’s estimate of linear acceleration o Tilt perception is fundamentally different from translation perception PSYB51: Perception and Cognition Clara Rebello  Each spatial orientation modality includes two qualities o Amplitude: The size(increase/decrease) of a head movement  For linear motion, we can perceive translation having high or low velocity  We can perceive rotational velocity with high or low amplitude  Tilt amplitudes can be small or large o Direction: The line one moves along or faces, with reference to the point or region one is moving toward or facing  We need three axes to describe the 3-D space we move in  X-axis  Always points forward  Y-axis  Always points out the left ear  Z-axis  Always points out the top of the head  The head can rotate in 3 independent ways, which can be represented  With a roll angular velocity (Rotation around x-axis)  With a pitch angular velocity (Rotation around y-axis)  With a yaw angular velocity (Rotation around x-axis)  Three directions to our sense of linear motion  Stepping forward or backward along the x-axis  Sliding from right to left along the y-axis  Translating up or down along the z-axis  When head rotations align with gravity, there’s no change in head tilt  There are 3 directions for angular velocity and translation, but only two for tilt (pitch and roll) The Mammalian Vestibular System  The vestibular organs can be found in the inner ear next to the cochlea o Each inner ear has one vestibular labyrinth, and each vestibular labyrinth includes five sense organs (three semicircular canals and two otolith organs)  Neither the otolith organs nor the semicircular canals respond to constant velocity o They respond to changes in velocity (acceleration) o The sensitivity of the vestibular system to acceleration demonstrates that this system is principally sensitive to changes in motion PSYB51: Perception and Cognition Clara Rebello o Constant motion doesn’t result in vestibular signals that directly indicate motion  Hair cells act as mechanoreceptors in each of the five vestibular organs o Mechanoreceptors: A sensory receptor that responds to mechanical stimulation (pressure, vibration, or movement) o Head motion causes hair cell stereocilia to deflect  This deflection causes a change in the hair cell voltage, which alters neurotransmitter release, which evokes action potentials in those vestibular- nerve fibers that have one or more synapses on the hair cell o The rate of action potentials transmitted by afferent neurons increases or decreases following the hair cell receptor potential: A change in voltage across the membrane of a hair cell in response to stimulation  Receptor potential is proportional to the bending of the hair cell bundles and control the rate at which hair cells release neurotransmitter to the afferent neurons  Hair cell bends toward tallest stereocilia  Hair cell voltage becomes less negative  Depolarization: Hair cell becomes less polarized than the negative resting potential  Hair cell depolarization increases the release of neurotransmitter, causing increase in the action potential rate (excitation)  Hair cell bends away from tallest stereocilia  Cell potential becomes more negative (hyperpolarizes), causing decrease in the release of neurotransmitter and decrease in the action potential rate (inhibition) o The fact that the hair cells respond oppositely for deflections in opposite directions is also crucial for the coding of vestibular stimuli  Otolith organs  Acceleration in one direction increases the receptor potentials of some hair cells, while acceleration in the opposite direction decreases those receptor potentials  Three semicircular canals  Horizontal (or lateral), anterior (or superior), and posterior o Osseous canal (like tubes) is carved out of the mastoid bone (filled with fluid called perilymph) o The cross section for each canal swells substantially near where the canals join the vestibule  Ampulla: (swelling) An expansion of each semicircular canal duct that includes that canal’s cupula, crista, and hair cells, where transduction occurs PSYB51: Perception and Cognition Clara Rebello  Within the endolymph space of each ampulla, the angular-motion detectors are assembled into a sensory epithelium call
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