Ch. 12

• Sound Localization: the ability to localize the location of a sound source in a sound field
• Precedence Effect


  When a sound is followed by another sound separated by a sufficiently short time delay (below the listener's echo threshold), listeners perceive a single auditory event
• Auditory Stream Analysis: the ability to separate each of the sound sources and separate them in space

  • we use different cues to differentiate
• Perceptual Grouping: putting parts together into a whole

  • we use the original source of the sound to localize where the sound is coming from, provided that the reverberating sounds are not too far seperate from it
• Comparing location information for vision and hearing involves understanding how these two sensory systems process spatial information differently, yet effectively, to help us navigate our environment
• Both senses are crucial for spatial awareness, but they rely on distinct mechanisms and properties to locate objects and events in space
• Auditory Space: surrounds an observer and exists wherever there is sound
• Tones with the same frequency activate the cochlea (hair cells) in the same way regardless of where they are coming from
• Localization Cues


  • Azimuth coordinates: position left to right
  • Elevation coordinates: position up and down
  • Distance coordinates: position from observer
• People can localize sounds most accurately directly in front of them and least accurately to the sides and behind their heads
• Location cues are not contained in the receptor cells like on the retina in vision, thus, location for sounds must be calculated
• Binaural Cues


  Location cues based on the comparison of the signals received by the left and right ears
• Interaural Time Difference (ITD): difference between the times that sounds reach the two ears
• Interaural Level Difference (ILD): difference in sound pressure level reaching the two ears

  • better for high frequency sounds (low frequencies don't go right through)
  • reduction in intensity occurs for high frequency sounds from the far ear
• Cone of Confusion


  Describes a region of space where the location of a sound source can be ambiguous
• ILD and ITD are not effective for judgments on elevation, since in many locations they may be zero
• Mold experiment: How localization changes when a mold is placed in the ear

  1. Listeners were measured for performance locating sounds differing in elevation

  2. They were then fitted with a mold that changed the shape of their pinnae

  3. Right after the molds were inserted, performance was poor for elevation but was unaffected for azimuth

  4. After 19 days, performance for elevation was close to original performance

  5. Once the molds were removed, performance stayed high
• Jeffress Neural Coincidence Model

  • Neurons are wired so they each receive signals from the two ears
  • Coincidence detectors; inferior colliculus
  • ITD detectors; place code
  • ITD tuning curves
• Broad ITD Tuning Curves in Mammals

  • Coding for localization based on broadly tuned neurons in the right hemisphere that respond when sound is coming from the left, and in the left hemisphere that respond when sound is coming from the right
  • The location of a sound is indicated by relative responses of these two types of broadly tuned neurons
• What (ventral stream)


  Starts in the anterior portion of the core and belt and extends to the prefrontal cortex, used to identify sounds
• Where (dorsal stream)


  Starts in the posterior core and belt and extends to the parietal and prefrontal cortices, used to locate sounds
• Direct Sound


  Sound that reaches the listener's ears straight from the source
• Indirect Sound


  Sound that is reflected off of environmental surfaces and then to the listener
• Experiment by Litovsky et al.


  1. Listeners sat between two speakers: a lead speaker and a lag speaker

  2. When sound comes from the lead speaker followed by the lag speaker with a long delay, listeners hear two sounds

  3. When the delay is decreased to 5:20 msec, listeners hear the sound as only coming from the lead speaker: the precedence effect
• Reverberation Time: the time it takes for a sound to be reduced in intensity by 1/1000th of its original
• Intimacy time: difference between direct sound & first reflected sound
• Bass ratio: ratio of low to middle frequencies reflected from surfaces

  • sound is better when it has a higher base ratio
• Spaciousness Factor: fraction of all the sound received by listener that is indirect
• Auditory Scene: the array of all sound sources in the environment
• Auditory Scene Analysis: process by which sound sources in the auditory scene are separated into individual perceptions
• Simultaneous Grouping: process in auditory perception that enables us to organize sounds that occur at the same time into coherent groups

  • helping us differentiate between multiple sound sources
• Sequential Grouping: process in auditory perception that allows us to organize sounds over time into coherent sequences

  • helping us to distinguish and make sense of different sound sources in our environment
• Experiment by Bregman and Campbell


  1. Stimuli were in alternating high and low tones

  2. When stimuli played slowly, the perception is hearing high and low tones alternating

  3. When the stimuli are played quickly, the listener hears two streams, one high and one low
• Experiment by Deutsch: the scale illusion or melodic channeling


  1. Stimuli were two sequences alternating between the right and left ears

  2. Listeners perceive two smooth sequences by grouping the sounds by similarity in pitch
• Experiment by Warren et al.

  1. Tones were presented interrupted by gaps of silence or by noise

  2. In the silence condition, listeners perceived that the sound stopped during the gaps

  3. In the noise condition, the perception was that the sound continued behind the noise
• Experiment by Dowling: Effect of past experience

  1. Melody "Three Blind Mice" is played with notes alternating between octaves

  2. Listeners find it difficult to identify the song

  3. However, after they hear the normal melody, they can then hear it in the modified version using melody schema
• Visual capture or the ventriloquist effect


  An observer perceives the sound as coming from the visual location rather than the source for the sound

  
• The interaction between vision and hearing is multisensory in nature
• Thaler et al. (2011): used expert blind echolocators to create clicking sounds and observed these signals activated the brain
• Elevation is difficult to tell when the sound is coming from directly in front of you or directly from the side

  • because it's not amenable to ITD & ILD