In-Depth Notes on Perception, Pitch and Hearing Loss

Overview of Pitch and Hearing Loss

Frequency Analysis in the Cochlea

  • The cochlea undergoes frequency analysis to dissect different sound frequencies.

  • Basilar Membrane:

    • Oscillation occurs based on sound frequency; different locations on the membrane vibrate to specific frequencies.
    • High frequencies stimulate the base of the cochlea, whereas low frequencies stimulate the apex.

  • Traveling Wave:

    • Sound waves create traveling waves along the basilar membrane.
    • The Fourier decomposition helps in understanding how various frequencies contribute to complex tones.

Structure of the Ear

  • Outer Ear: Pinna and auditory canal
  • Middle Ear: Tympanic membrane, stapes, incus, and malleus (ossicles)
  • Inner Ear: Cochlea and auditory nerve.

Functionality of Outer Hair Cells

  • Outer hair cells amplify sound through the expansion and contraction of their membranes.
  • This amplification increases the specificity of frequency tuning and enhances auditory sensitivity.
  • Prestin: The motor molecule that causes hair cell movement, adjusting length in response to sound.

Hearing Measurements and Hearing Loss

  • Measurement of Hearing Threshold:

    • Influenced by frequency; logarithmic scale (dB SPL).
    • Threshold of hearing varies; sensitivity peaks around 4,000 Hz.

  • Types of Hearing Loss:

    • Presbycusis: Gradual loss of hearing due to hair cell damage, predominant in high frequencies, and more severe in males.
    • Noise-Induced Hearing Loss: Caused by exposure to loud sounds leading to hair cell degeneration.
    • Hidden Hearing Loss: Difficulty with speech in noisy settings despite normal tone detection; may indicate auditory nerve damage.

Theories of Pitch Perception

  • Place Theory:

    • Pitch is determined by the specific location of cell activation along the basilar membrane.
    • Low frequencies = apex activation; high frequencies = base activation.

  • Temporal Code Theory:

    • Pitch is encoded by the firing rates of auditory nerve cells; slower rates signify low frequencies and faster rates signify high frequencies.
    • Also known as Frequency Theory.

  • Phase Locking:

    • Fibers may not fire on every cycle, but combined signals create a rhythmic pattern that represents sound frequency.

Summary of Key Concepts

  • Auditory Transduction: Conversion of sound waves into neural signals by hair cells.
  • Auditory Nerve Response: Demonstrates specific frequency tuning curves indicating minimum sound intensity needed for neural response.
  • Shepard Pitch Illusion: An auditory illusion demonstrating perception of continuously increasing pitch, exploring how sound frequency is related to our interpretation of pitch.