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Chapter 12

PSYB51 Ch.12.doc

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Matthias Niemeier

Ch. 12- Spatial Orientation and the Vestibular System • vestibular organs/system= set of five specialized organs- three semicircular canals and two otolith organs- located in each inner ear that sense head motion and head orientation with respect to gravity. Make a predominant contribution to our sense of tilt and our sense of self- motion ◦ sixth sense= contributes to clear vision when we move and helps us maintain balance when we stand ◦ appeared early in evolutionary history and have remained relatively unchanged= fundamental ◦ largely automatic system- vestibular perception is often relegated to the attentional background, and may responses evoked by the system are felexive • spatial orientation= a sense consisting of three interacting sensory modalities, the senses of linear motion, angular motion and tilt (our sense of tilt and self motion). • Dizzines (nonspecifically indicates any form or perveiced spatial disorientation with or without instability), spatial disorientation (any impairment of spaital orientation. More specifically, any impairment of our sense of linear motion, angular motion or tilt), imabalnce (lack of balance; unsteadiness; falling over), blurred vision, illusory self-motion= promotes awareness of our vestibular sensations • vestibular system= reflexively rotates eyeballs in the sockets to compensate for head rotation= helps stabilize images on the retina • compensate for head movement more readily than hand movement because of vestibulo-ocular reflex • often don't perceive your head rotating because vestibular system performs its job automatically • Modalities and Qualities of Spatial Orientation • sense of spatial orientation is based on three sensory modalities: ◦ angular motion= rotational motion like the rotation of a spinning top/swinging saloon doors that rotate back and forth ◦ linear motion= translational motion like the predominant movement of a train car/ bobblehead doll ◦ tilt= attaining a sloping position like that of the leaning tower of Pisa ◦ modalities like senses NOT qualities because energy is transduced (coverted from one form to another) ▪ color and brightness are different interpretations of the same light= qualities ▪ seeing and hearing involve different types of stimulation energy= light and pressure waves= modalities ▪ perceiving angular motion, linear motion and tilt= requires three different kinds of stimulation energy to be sense- angular acceleration, linear acceleration, gravity= modalities • Sensing Angular Motion, Linear Motion and Tilt • sensed b y two types of vestibular sense organs: semicircular canals and otolith organs ◦ semicircular canals= any of three toroidal tubes in the vestibular system that sense ANGULAR MOTION ◦ otolith organs= either of two mechanical structures (utricle and saccule) in the vestibular system that sense both linear acceleration (change in linear velocity) and gravity ▪ predominant contribution to sense of linear motion (sense of translation) and sense of head tilt • 2 sense organs (semicircular canals and otolith organs) but 3 modalities? ◦ Otolith organs= transduce both gravity and linear acceleration= 2 distinct modalities ▪ stimulation energies for gravity and linear acceleration are different so otolith organs attempt to separate the signals of the two ▪ so overall= 3 interacting sensory modalities= a sense of angular motion, a sense of linear motion, a sense of tilt= parallel the three different stimulation energies- angular acceleration, linear acceleration, gravity • Basic qualities of spatial orientation:AMPLITUDEAND DIRECTION ◦ each of the three spatial orientation modalities includes two quantities: amplitude and direction ◦ amplitude= speed/ strength with which a movement is carried out ◦ direction= 3 directions for angular/linear motion, and 2 directions for tilt (because “yaw” causes head rotations to align with gravity so there is no change in tilt) • Mammalian Vestibular System • vestibular organs= found in the inner ear right next to the cochlea ◦ each inner ear= has one vestibular labyrinth/system= and each system has 5 sense organs= three semicircular canals that sense rotational/angular movement, and 2 otolith organs that sense gravity and linear acceleration ◦ NEITHER otolith organs/semicircular canals respond to CONSTANT velocity= they respond to changes in velocity called acceleration ▪ constant motion, whether angular or linear does not result in vestibular signals that directly indicate motion ▪ otolith organs transduce both linear acceleration and gravity into a single neural signal sent to the brain • Hair Cells= Mechanical transducers ◦ hair cells= any cell that has stereocilia for transducing mechanical movement in the inner ear into neural activity sent to the brain; some also receive input from the brain ◦ act as mechanoreceptors= a sensory receptor that responds to mechanical stimulation (pressure, vibration, movement)= stereocilia deflection causes a change in the hair cell voltage which alters neurotransmitter release, which in turn, evokes action potentials in those vestibular nerve fibers that have 1 or more synapses on the hair cell= afferent neurons carry these action potentials to the brain ◦ absence of stimulation= hair cells have a negative charge/voltage and release neurotransmitter at a constant rake= constant rate of action potentials ▪ changes in the hair cell voltage =(receptor potential= a change in voltage across the membrane of a sensory receptor ce
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