Week 7 (Overview of Hearing, Anatomy, Auditory Processing in Cochlea)

Overview of Hearing

  • Hearing is the ability to perceive soundwaves

  • The ear has structures that can sense, transmit, and convert vibrations to signals the brain can interpret

  • There are three main properties of sound waves that ears can detect'

    • Amplitude

      • Reflects intensity

      • Greater amplitude sound louder

      • measure is decibels

    • Frequency

      • Reflects pitch

      • measured in hertz

      • Can hear in between 15hz - 20,000hz

      • Children can hear higher frequencies than adults

    • Timbre

      • Reflects the tone quality or complexity

      • How we distinguish different sounds

Anatomy of the Ear

  • Outer ear

    • Pinna

      • Receives sound vibrations and helps locate source of the sound

    • Auditory Canal

  • Middle ear

    • Tympanic membrane

      • Vibrates at the same frequency as received sound waves and transmits this to ossicles

    • Ossicles

      • 3 tiny bones that transmits vibrations to oval window

        • Hammer

        • Anvil

        • Stirrup

  • Inner Ear

    • Oval window

      • Thin membrane of the inner ear that vibrates fluid in the cochlea

    • Cochlea

      • Where vibrations are converted to nerve impulses

      • Organ of Corti produces nerve impulses in response to vibrations

        • Tectorial Membrane

          • Top membrane of the organ of corti

        • Basilar Membrane

          • Bottom membrane of organ of corti

        • They move hair cells up and down in response to auditory vibration

        • Hair cells

          • Auditory receptors between the basilar and tectorial membranes

      • Contains 3 fluid-filled chambers

        • Scala Vestibuli

        • Scala Media

        • Scala Tympani

Auditory Processing Process in the Ear

  • Sound enters the Pinna of the ear and travels down the auditory canal

  • This causes the tympanic membrane to vibrate

  • Vibrations transferred to the 3 ossicles

  • Vibrations in the stapes trigger vibrations in the oval window

  • The oval window vibrates the cochlea

Functions of Processing in the Cochlea

Pitch Processing

  • Two main theories concerning pitch processing

    • Place theory

      • We hear different pitches because specific places on the basilar membrane of stimulated

      • Each membrane are tuned to a specific frequency

      • Some parts of the basilar membrane are too stiff to resonate

    • Frequency theory

      • We hear different pitches because the speed of sound waves matches/determines the speed of action potentials traveling to the brain

      • Vibrate in synchrony with sound waves

      • However the maximal firing rate of a neuron is shorter than the highest frequencies we hear

  • Current theory for pitch processing

    • Frequency theory for low frequency sounds

      • Up to 100Hz, the basilar membrane vibrates with soundwaves, and the auditory nerve fires one action potential per wave

        • Frequency of impulse determines pitch

        • Softer sounds activate fewer neurons

    • Place theory for medium-high frequency sounds