Chapter 12: Auditory localization and organization

Question 1

What’s the difference between an auditory scene and auditory localization?

Answer 1

• Scene = all the sound entering the ears during a current interval of time. Includes locations and qualities of individual sounds
• Localization = perception of the location of a sound source. Much harder cause all the sound waves blend together and enter the ear at once. Dependent upon location cues

Question 2

What are the three dimensions of sound location?

Answer 2

• Azimuth (left-right)
• Elevation (up-down)
• Distance

Question 3

What’s interaural time difference (ITD)?

Answer 3

• A binaural cue
• The time difference between a sound reaching one ear versus the other
• The difference is larger for some locations (eg. directly from your right, 90 degrees right azimuth)
• Works better for low-frequency sounds since high frequency sounds are really close together

Question 4

What’s interaural level difference (ILD)?

Answer 4

• A binaural cue
• The difference in sound pressure level of sound reaching the two ears
• The head acts as a partial barrier for sound waves, creating an acoustic shadow so it makes the sound less intense when it reaches the far ear
• Differences are greater as sound is located more toward the side of your head
• More effective for high-frequency sounds

Question 5

Why is ILD better for high-frequency sounds?

Answer 5

• Longer wavelengths (lower frequencies) are able to diffract around the head with little loss of energy
• Shorter wavelengths (higher frequencies) don’t diffract as much, so more energy is lost, so sounds seem quieter at ‘far’ ear.

Question 6

What does the cone of confusion mean?

Answer 6

• The ITD and the ILD are still both somewhat ambiguous when it comes to providing input about the location in the azimuth
• Some locations can still be confused cause they provide similar ITDs and ILDs

Question 7

What’s the solution for the cone of confusion?

Answer 7

• Head movement
• Turn head towards the source of sound

Question 8

What’s a spectral cue?

Answer 8

• A monaural cue
• The bumps and ridges found in the pinna help reflect sound waves before entering the auditory canal
• Info about elevation is contained in the distribution of frequencies reaching the ear
• When the ridges in the pinnae are covered, we have poor elevation localization

Question 9

What did they Hoffman et al. study determine?

Answer 9

• They used a mould to change the ridges of the pinnae in the ear
• Tested localization over several days
• Brain was able to adapt, but was confused once the mould was removed

Question 10

What’s the auditory pathway in the brain?

Answer 10

1) Auditory nerve (inner hair cells)
2) Cochlear nucleus (in brain stem)
3) Superior Olivary Nucleus (brain stem)
4) Inferior colliculus (midbrain)
5) Medial Geniculate Nucleus (thalamus)
6) Primary Auditory cortex

Question 11

What’s the role of the Superior Olivary Nucleus in auditory localization?

Answer 11

• It’s the point at which signals from either ear first meet
• Where most of the processing occurs
• Also some further processing occurs in the Inferior Colliculus

Question 12

What’s the Jeffress model?

Answer 12

• A model for auditory localization
• A set of neurons gets signals from each ear
• These neurons must be stimulated by signals from both ears to fire
• Excitation is combined to reach the threshold. These are called coincidence detectors
• Which specific neuron fires will inform us about the specific azimuth location of the sound source
• Signals from one ear will be sent sooner than the other ear, and where the two signals meet will indicate the time difference.
• Each coincidence detector has a different location in the azimuth orientation
• Analogous to feature detectors in vision

Question 13

How does the brain detect low-frequency continuous sounds?

Answer 13

• The brain can calculate the phase difference between sounds. It just has to be able to detect the phase in the first place, which is determined through its pattern of firing

Question 14

What are ITD neurons?

Answer 14

• Found in mammals, but they have a super broad tuning curve
• So broas they extend way outside the range of possible ITDs (go outside of coincidence detector range)
• Because of this, the brain uses the pattern of firing across multiple neurons
• Can then use neurons tuned to more leftwards sounds or more rightward sounds
• Similar to trichromatic colour theory

Question 15

Place code vs. Distributed code?

Answer 15

• Place code: Having finely tuned ITD neurons. A neuron in a specific place fires, indicating the sound source location. Birds have these.
• Distributed code: The pattern of multiple neurons firing rates indicates sound source location

Question 16

What does auditory scene analysis often involve?

Answer 16

• Involves both extraction and grouping processes
• Similar to vision, it involves many of the same Gestalt principles

Question 17

What are some of auditory scene analysis cues?

Answer 17

• Location - when 2 sounds are separated in space, location can help us separate them perceptually. When a sound source moves, it usually follows a continuous path
• Synchrony - Unrelated sounds very rarely start at the same time. Single sounds tend to change in synchrony (harmonics)
• Pitch and timbre - The same source will produce sounds of the same timbre or pitch
• Experience - helps with top-down processing

Question 18

What’s auditory stream segregation?

Answer 18

• Alternating 2 tones of similar frequencies are perceived as a single auditory stream
• Also called implied polyphony or compound melodic line

Question 19

What’s a scale illusion?

Answer 19

-Occurs when 2 separate streams of notes are played to the right and left ears, so the notes jump around, but it’s perceived as a smooth continuation of notes and the listener perceives 2 smooth melodies

Question 20

What are auditory occlusion and completion?

Answer 20

• When a sound is blocked by another sound, we tend to perceptually complete it
• If the tone is interrupted by silence, it’s perceived as a broken tone
• Also works for changing/gliding tones

Question 21

What’s phonemic restoration?

Answer 21

• Occurs when we fill in speech sounds that are masked by noise

Question 22

What’s the ventriloquism effect?

Answer 22

• The tendency to localize sound based on visual cues, if visual and audio cues provide conflicting info

Question 23

What did the Seculer et al. (1997) study discover?

Answer 23

• Hearing can impact vision as well
• When people watched a video of two balls passing by each other diagonally, 63% of people perceived the balls as bouncing off of each otherwhen a click noise was added when the balls passed each other.

Question 24

What’s the McGurck effect?

Answer 24

• Depending on the visual cues provided in an image will strongly impact the auditory scene