Audition: Sound, the Auditory System and Pitch Perception

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Audition Sound, the Auditory System and Pitch
Perception
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Week 11 Outline

• Auditory Functions
• The sound stimulus
• Anatomy of the Ear
• Auditory Physiology
• Frequency Coding in the Auditory System
• Auditory Pathology

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Auditory Functions

• Omnidirectional Space Perception
• Alert to stimuli outside of visual field
• echolocation
• Communication
• Speech and Social Isolation
• Demonstration watch a recorded TV program with
  the sound off and take notes about what you think
  is going on, then rewatch the same program with
  the sound on and compare
• Pleasure
• Perception of Music

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The Sound Stimulus

• Mechanical vibrations of a fluid or solid medium
  transferred to the inner ear
• Typically sounds are generated by vibrations of
  air molecules that are transmitted to the inner
  ear through the outer and middle ears
• compression waves of air pressure
• Speed about 340 m/s through air, considerably
  faster through denser materials (e.g., liquids or
  solids
• Sounds cannot travel through a vacuum

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Defining Sound Physically

• Physical sounds are defined by their Fourier
  spectra of amplitude (SPL) and phase as functions
  of frequency (Hz)

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Defining Sound Physically (cont.)

• Amplitude of Sound the Sound Pressure Level
  (SPL) expressed in decibels (dB)
• The loudest sound we can perceive is 10 million
  times louder than the softest sound we can
  perceive
• To represent such a large range of sensitivity,
  SPL is generally expressed in decibels (dB)
• 1 dB 20log(p/p0), where
• p max pressure of sound
• p0 pressure at threshold for hearing
• For SPL, p0 .0002 dynes/cm2
• Frequency of Sound
• Human Sensitivity 20-20,000 Hz

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The Sensory and Perceptual Qualities of Sound

• Three Sensory Dimensions
• Loudness
• the intensity of the sound
• dependent on physical SPL and frequency
• Pitch
• the highness or lowness of the sound
• related to fundamental frequency
• Timbre (pronounced tamber)
• tonal quality of the sound
• dependent on frequency spectrum and loudness
  envelope
• Two additional Perceptual Qualities
• Locus the place from which the sound originates
• Object Recognition identifying the source of the
  sound

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Anatomy of the Outer Ear

• The Outer Ear
• Structures
• Pinna(e)
• Auditory Canal
• Functions
• gather and amplify sounds
• Enhance localization
• Protect middle and inner ear

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Anatomy of the Middle Ear

• The Middle Ear
• Structures
• Tympanic membrane
• Ossicles
• Middle ear muscles
• Eustachian tubes
• Functions
• Amplify most sounds
• Dampen loud sounds to protect inner ear

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Anatomy of the Inner Ear

• Cochlea
• Structurally, a long tube rolled into a spiral
• Divided along its length into 3 parts (Scala) by
  the Reissners and Basilar membranes
• Scala vestibuli
• Scala tympani
• Cochlear partition (scala media) contains the
  organ of corti

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Anatomy of the Inner Ear (cont.)

• Organ of Corti
• Inner hair cells
• Outer hair cells

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Auditory Physiology Auditory Pathways

• Ascending Pathways
• Cochlear Nerve
• Brain Stem
• Cochlear Nuclei
• Superior Olivary Nuclei
• Nuclei of Lateral Lemnisci
• Midbrain
• Inferior Colliculi
• Medial Geniculate Nuclei on Thalamus
• Temporal Cerebral Cortex
• Primary Auditory Receiving Area (A1)
• Descending Pathways from brain stem nuclei to
  cochlea

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Auditory Physiology Auditory Receptive Fields

• Neurons throughout auditory pathways are tuned to
  frequency
• Cells become more narrowly tuned to frequency as
  stimulus proceeds upward through ascending
  pathway to the cortex
• Auditory Cortex tonotopic maps

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Frequency Coding in the Auditory System

• The Place Code specific hair cells on the
  cochlea represent particular frequencies in a
  tonotopic map
• Helmholtzs Resonance Theory (late 1800s)
• Bekesys Traveling Wave Hypothesis (1928)
• Physiological Evidence
• Culler et al. (1943) tonotopic map of cochlea
• Russel Sellick (1977) frequency tuning of hair
  cells in cochlea

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Frequency Coding in the Auditory System

• The Timing Code (Frequency Matching Theory)
  frequency of neural impulses (action potentials)
  directly codes the frequency of the sound
• First proposed by Rutherford (1886)
• Limited by refractory period of neural firing
• Wevers (1949) Volley Principle
• Phase Locking and the Volley Principle

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Frequency Coding in the Auditory System

• Human auditory system uses both place and timing
  codes
• Place Code ? middle to high frequencies
• Timing Code ? low to middle frequencies
• Both code middle frequencies critical for?

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Auditory Pathology

• Tinnitus persistent ringing in the ears
  resulting from cochlear damage
• Affects about 1 of the general population, 10
  of adults gt 60 years old
• Hearing Loss (partial deafness)
• 16 of adults have a 25dB or greater hearing loss
• Presbycusis (old hearing) loss of high frequency
  sensitivity
• Relationship to Paranoia
• Conduction vs. Sensorineural Hearing Loss
• Tonal gaps in hearing loss