Chapter 6: Other Sensory Senses

• Sound

  Periodic compressions of air, water, or other media
• Sound waves

  • Vary in amplitude
  • Vary in frequency
• Amplitude


  The intensity of a sound wave
• Sounds of greater amplitude

  Seem louder, but exceptions occur
• Frequency


  The number of compressions per second, measured in hertz (Hz, cycles per second)
• Pitch


  The related aspect of perception, sounds higher in frequency are higher in pitch
• Most adult humans hear sounds starting at about 15 to 20 Hz and ranging up to almost 20,000 Hz
• Children hear higher frequencies than adults, because the ability to perceive high frequencies decreases with age and exposure to loud noises
• Larger animals like elephants hear best at lower pitches, and small animals like mice hear higher pitches, including a range well above what humans hear
• Timbre


  The third aspect of sound, tone quality or tone complexity
• People communicate emotion by alterations in pitch, loudness, and timbre
• Prosody


  Conveying emotional information by tone of voice
• Anatomical parts of the ear

  • Outer ear
  • Middle ear
  • Inner ear
• Pinna


  The familiar structure of flesh and cartilage attached to each side of the head, alters reflections of sound waves to help locate sound sources
• Tympanic membrane or eardrum


  The soundwave vibrates the eardrum as it reaches the middle ear, connecting to three tiny bones that transmit the vibrations to the oval window
• Three tiny bones in the middle ear

  • Hammer (malleus)
  • Anvil (incus)
  • Stirrup (stapes)
• Cochlea


  The snail-shaped structure of the inner ear, where vibrations in the fluid set the hair cells into motion
• Auditory receptors or hair cells

  Lie between the basilar membrane and tectorial membrane, their vibration opens ion channels and stimulates the auditory nerve
• Place theory of pitch perception


  • The basilar membrane is tuned to specific frequencies, each frequency activates hair cells at a specific place along the membrane
• Frequency theory of pitch perception

  • The entire basilar membrane vibrates in synchrony with the sound, causing auditory nerve axons to fire at the same frequency
• Modification of place and frequency theories

  • For low frequencies, the basilar membrane vibrates in synchrony and neurons fire one action potential per wave
  • For high frequencies, neurons fire on some but not all waves, with action potentials phase-locked to the sound wave peaks
• Volley principle of pitch discrimination


  The auditory nerve produces volleys of impulses for sounds up to about 4000 Hz, beyond which it cannot keep pace with the sound waves
• Most human hearing takes place below 4000 Hz, the approximate limit of the volley principle
• Mechanism for perceiving high frequencies

  • A high-pitched sound sets up a traveling wave that peaks at a specific point along the basilar membrane, identifying the frequency
• Primary auditory cortex (area A1)


  • Responds to imagined sounds as well as real ones
  • Provides a tonotopic map of sounds, with some cells preferring single tones and most preferring complex sounds
• Damage to the primary auditory cortex does not produce deafness, but impairs speech and music processing
• Surrounding auditory cortex areas

  • Respond best to relevant natural sounds like animal calls, birdsong, machinery, music, and speech
• The auditory cortex is important not just for hearing, but also for thinking about concepts related to hearing
• Sound localization

  • Uses differences in time of arrival, intensity, and phase between the two ears
• Humans localize low frequencies by phase differences, and high frequencies by loudness differences
• Sudden sounds of any frequency can be localized by the times of onset
• Sound localization requires learning, as the distance between the ears changes as the head grows
• Tone deafness or amusia


  Individual differences in the ability to perceive pitch
• Localization of low frequencies

  By phase differences
• Localization of high frequencies

  By loudness differences
• Localization of sudden sounds

  By times of onset
• Localization requires learning as head size changes
• Becoming deaf in one ear

  At first, sounds seem to come directly from the side of the intact ear
• Over time with one-sided deafness

  People learn to interpret loudness cues to localize sounds
• Accuracy with one-sided deafness does not match that of people with two ears