6 - hearing, taste and movement

• Hearing alerts us to useful information
• Sound waves


  Periodic compressions of air, water, or other media
• Amplitude


  Intensity of a sound wave
• Frequency


  Number of compressions per second, measured in hertz (Hz)
• Pitch


  Perceived aspect of sound related to frequency
• Timbre


  Tone quality or tone complexity
• Prosody


  Conveying emotional information by tone of voice
• Outer ear


  • Pinna helps locate sound source
  • Auditory canal
• Middle ear


  • Tympanic membrane vibrates
  • Three tiny bones transmit vibrations to oval window
• Inner ear


  • Cochlea contains hair cells that respond to fluid vibrations
  • Auditory nerve transmits signals
• Place theory


  Basilar membrane has areas tuned to specific frequencies
• Frequency theory


  Basilar membrane vibrates in synchrony with sound, causing auditory nerve to fire at same frequency
• Volley principle


  Auditory nerve produces volleys of impulses for sounds up to 4000 Hz
• Basilar membrane


  • Stiff at base, floppy at apex
  • Hair cells along membrane act as tuned resonators
• 4% of people have amusia, impaired detection of frequency changes
• Absolute pitch


  Ability to identify a note by hearing it
• Pitch information reaches some parts of the brain and not others
• Amusia


  • Many relatives of a person with amusia have the same condition, so it probably has a genetic basis
  • People with amusia have a thicker than average auditory cortex in the right hemisphere but fewer than average connections from it to the frontal cortex
• Brain's response to hearing two slightly different tones in people with amusia

  Initial response is about the same as in other people, indicating the ears are properly registering the information, but they fail to process the information further
• P300 wave


  A strong wave of brain activity about 3/10 second later that is produced by a changed or surprising stimulus, but a small change in sound fails to produce that wave in people with amusia
• Absolute pitch


  The ability to hear a note and identify it
• Absolute pitch


  • Genetic predisposition contributes, but early musical training is also important
  • Not everyone with musical training develops absolute pitch, but almost everyone with absolute pitch had early musical training
  • More common among people who speak tonal languages where the meaning of a sound depends on its pitch, so people learn from infancy to pay close attention to slight changes of pitch
• Through which mechanism do we perceive low-frequency sounds (up to about 100 Hz)?
• How do we perceive middle-frequency sounds (100 to 4000 Hz)?
• How do we perceive high-frequency sounds (above 4000 Hz)?
• What evidence suggests that amusia depends on special experiences?
• Auditory cortex


  • Organization strongly parallels that of the visual cortex, with a "what" pathway sensitive to patterns of sound and a "where" pathway sensitive to sound location
  • Just as the visual cortex is active during visual imagery, area A1 responds to imagined sounds as well as real ones
  • Development depends on experience, as rearing an animal in constant noise impairs auditory development
• Damage to the primary auditory cortex does not produce deafness, but just impairs processing of information
• Tonotopic map


  The auditory cortex provides a map of sounds with an area responsive to lower tones up to areas responsive to higher and higher tones
• Cells in auditory cortex

  • Some are tuned sharply to a single tone, others respond to neighboring tones
  • Most respond best to a complex sound with a dominant tone and several harmonics
  • Surrounding the primary auditory cortex are areas that respond best to identifiable, meaningful sounds
• Conductive deafness results from diseases, infections, or tumorous bone growth preventing the middle ear from transmitting sound waves properly to the cochlea
• Nerve deafness results from damage to the cochlea, hair cells, or auditory nerve, and can be inherited, result from disease, or exposure to loud noises
• Tinnitus


  Frequent or constant ringing in the ears, sometimes due to a phenomenon similar to phantom limb where damage to part of the cochlea leads to axons representing other parts of the body invading the auditory cortex
• Hearing problems in older people despite hearing aids

  • Brain areas responsible for language comprehension become less active, possibly due to prolonged degradation of auditory input
  • Decrease in inhibitory neurotransmitters in auditory portions of the brain makes it harder to suppress irrelevant sounds and attend to the important one
  • Auditory cortex has delayed, spread-out responses to each sound, so the response to one sound partly overlaps the response to another
• Nerve deafness would be more common among members of rock bands due to exposure to loud noises damaging the synapses and neurons of the auditory system
• Reasons older people have trouble hearing speech despite hearing aids

  • Brain areas responsible for language comprehension become less active
  • Decrease in inhibitory neurotransmitters makes it harder to suppress irrelevant sounds and attend to the important one
  • Auditory cortex has delayed, spread-out responses to each sound
• Sound localization


  Determining the direction and distance of a sound by comparing the responses of the two ears, using differences in time of arrival and intensity between the ears
• Labeled-line code


  Each receptor responds to a limited range of stimuli, and the meaning depends entirely on which neurons are active
• Across-fiber pattern code


  Each receptor responds to a wider range of stimuli, and the meaning depends on the pattern across receptors
• Bakery communication example


  • High and medium bells = need flour
  • Medium and low bells = need sugar
  • High and low bells = need eggs
  • All three bells = need vanilla extract