S & P Chapter 11 Notes

• Sound


  Pressure changes that occur in a medium (e.g., air) that allows us to hear
• Sound


  • Results from movement or vibration occurring from an event in our environment
  • Condensation: Increased air pressure caused by mechanisms pushing air molecules together
  • Rarefaction: Decreased air pressure caused by mechanisms that create space for air molecules to spread out
• Sound wave


  Patterns of pressure changes
• Frequency


  The number of pressure change repetitions that occur per second, measured in Hertz (Hz) and humans can perceive ranges from ~20 to 20,000 Hz
• Amplitude


  The difference in pressure between the peaks and troughs of the wave, measured in decibels (dB) and referred to as sound intensity
• Example 1: Conversation


  • Pressure is approx. 60 micropascals, frequency is unknown
• Example 2: Subway train


  • Pressure is unknown, frequency is unknown
• Complex tones


  More common in our environment and consist of a mix of multiple tones combined together
• Harmonics


  • Each individual tone in the mix is a multiple of the fundamental frequency of the complex tone
  • 1st Harmonic: Usually the lowest frequency tone (same as fundamental frequency; multiple = 1)
  • 2nd Harmonic has frequency double that of the fundamental, 3rd Harmonic has triple, and so on...
• Loudness


  Our subjective experience of the intensity of a pure or complex tone (measured in dB)
• Pitch


  Our perception of sound as having a high, medium, or low tone
• Tone height


  How our perception of pitch changes as the frequency of the tone increases or decreases
• Tone chroma


  Perceptions of similarity due to tones being octave multiples of each other
• Timbre


  Differences in sound quality irrespective of their pitch or loudness
• Auditory transduction


  • Begins with sound waves flowing through the outer ear and auditory canal
  • Sound waves reverberate the tympanic membrane (eardrum) and these vibrations are sent to the structures in the middle ear
  • The ossicles (smallest bones in the body) help concentrate the vibrations on the oval window onto a localized area through lever action
  • The oval window vibrates, transmitting these vibrations into the fluid-filled cochlea of the inner ear
• Organ of Corti

  • Consists of hair cells, basilar membrane, and tectorial membrane
  • Inner hair cells: One row of ~3,500 cells responsible for transducing vibrations into neural signals
  • Outer hair cells: Three rows of ~12,000 cells that amplify the vibrations of the basilar membrane
• Auditory transduction


  1. Vibrations of the oval window are transmitted through the fluid-filled cochlea, causing the organ of Corti to move up/down

  2. This movement causes the hair on the inner and outer hair cells to bend

  3. 1 direction of bending → tip links open, leads to influx of ions (K+) in membrane of hair cell, release of neurotransmitters at synapse with auditory nerve (causing fiber to fire)

  4. Other direction of bending → tip links close, leads to efflux of ions (K+)
• Basilar membrane

  • Moves back and forth as a traveling wave to different frequencies, with high frequencies evoking movement closer to the base and low frequencies evoking movement closer to the apex
• Place theory

  Each location on the basilar membrane vibrates maximally for particular frequencies
• Tonotopic map

  Orderly representation of frequencies for auditory processing that are organized based on their relative location within structures of the auditory system
• Cochlear amplifier

  Differential movement of the outer hair cells (expansion and contraction) to amplify the vibrations of the basilar membrane
• Auditory pathway to the brain

  1. Auditory nerve fibers leave the cochlea and synapse to subcortical structures: Cochlear nucleus → Superior olivary nucleus → Inferior colliculus → Medial geniculate nucleus

  2. From the MGN, the signals continue through to the auditory cortex
• Auditory cortex

  • Core area contains primary and neighboring auditory fields
  • Belt area surrounds the core
  • Parabelt area is inferior to the belt
• Pitch-selective neurons

  Neurons within the auditory cortex and surrounding areas that respond maximally to sounds containing different harmonic structures but the same fundamental frequency
• Sensitivity to pitch is strongest in the anterior auditory cortex and becomes progressively weaker as you move to the posterior