Chapter 11 - The Auditory Brain

Question 1

Where is the first site of auditory processing?

Answer 1

The cochlear nucleus

Question 2

What is the first site where auditory information comes from both sides of the brain?

Answer 2

The superior olivary complex

Question 3

Describe the contralateral processing pathway.

Answer 3

1. cochlea
2. cochlear nucleus
3. trapezoid body
4. superior olivary complex
5. inferior colliculus
6. medial geniculate body
7. auditory cortex

Question 4

Describe the ipsilateral processing pathway

Answer 4

1. cochlea
2. cochlear nucleus
3. superior olivary complex
4. inferior colliculus
5. medial geniculate body
6. auditory cortex

Question 5

Describe the cochlear nucleus

Answer 5

A structure in the brain stem.
- Receives signals from Type
1 auditory nerve fibers from inner hair cells in the
ipsilateral ear (same ear)

Question 6

Describe the superior olivary complex

Answer 6

A structure in the brain stem
- Receives signals from both
cochlear nuclei

Question 7

Describe the contralateral inferior colliculus

Answer 7

A structure in the midbrain. A pit stop on the ascending
auditory pathway
‣ Some neurons project to MGB while others project to
the superior colliculus (Multisensory layers)

Question 8

Describe the medial geniculate body

Answer 8

A structure in the thalamus. Second stop in the auditory

pathway after the inferior colliculus

Question 9

Describe the auditory cortex

Answer 9

Part of the cortex, tucked into the lateral sulcus on top of

the temporal lobe

Question 10

What is motile response?

Answer 10

‣ Fine tune frequency tuning by accentuating (or limiting)
the movement of the tectorial membrane
- making it more flexible enhances the signal (top down is key for focusing sound)

Question 11

What is acoustic reflex?

Answer 11

Tensioning of the tensor tympani and the stapedius to
limit the mobility of the ossicles (Malleus and the
Stapes)
- reduces the amount of amplification in the middle ear
- useful for the sudden onset of loud sounds (but it takes about 200ms to tighten the muscles to withdraw)

Question 12

What are the three regions of the auditory cortex?

Answer 12

1. auditory core region
   a) primary auditory cortex (A1)
   b) rostral core
   c) rostrotemporal core
2. belt
   - adjacent to A1, neurons in the belt area respond to more complex characteristics of sound (responds to individual frequencies only (either a simple or a complex broken down tone - responds mostly to complex tones)
3. parabel
   - adjacent to the belt area, where neurons respond to more complex characteristics of sound (such as direction of sound and melody sound changes) in addition to input from other senses
   - important in multisensory integration (combining speech and sound - important in speech sensation because we are sensitive to the movement of the mouth and helps us work out what the sound is)

Question 13

What is a tonotopic map?

Answer 13

• low characteristic frequencies are towards the anterior part of the core
• high characteristic frequencies are towards the posterior part of the core
  
  • lower frequencies taking up a wider area because they are more broadly tuned

Question 14

What is tonotopic mapping?

Answer 14

Frequencies are mapped out in specific regions (like retinotopic mapping in the eye)

Question 15

How do neurons in A1 vary in their tuning frequencies?

Answer 15

• some are very narrowly tuned, only responding at or near their characteristic frequency (this may indicate their involvement in integrating the multiple frequencies found in complex sounds)
• others are more broadly tuned

Question 16

Do all sounds activate A1?

Answer 16

Yes

Question 17

What happens are sound signals travel further in the auditory cortex?

Answer 17

In the belt there is more selectivity (needs specific complex waveforms)
in the parabelt there is even more specificity (complex cross modal information, variations in speech pattern which convey emotions [even in music] – not just complex but conveys emotions in sound and multiple modalities as well)

Question 18

Where is the WHAT pathway located, and what does it include?

Answer 18

in the superior temporal cortex; it includes things like voice identification, musical melodies and sound signatures
- convergence with object recognition pathway

Question 19

Where is the WHERE pathway, and what does it include?

Answer 19

in the parietal lobe; location of environmental events creating sounds
- converge with the visual where/how pathway

Question 20

What is the problem with how we locate sound with our ears?

Answer 20

We can’t locate the sound just based on which ear receives the signal, because both get it

Question 21

How can we start to solve the problem of locating sound?

Answer 21

We need to start comparing signals from both ears (binaurial audition)

Question 22

What are the three locational cue planes in sound?

Answer 22

1. Distance
2. elevation
3. azimuth

Question 23

What is distance?

Answer 23

How far a sound source is from the centre of the head in any direction

Question 24

What is elevation?

Answer 24

The location of a sound source
in the up-down dimension on
the median plane
‣ Measured in degrees

Question 25

What is azimuth?

Answer 25

The location of a sound source in the side-to-side
dimension on the horizontal plane
‣ Measured in degrees (Angles increase clockwise
towards the right - 0 in front, 180 behind)

Question 26

What is the interaural level difference (ILD)?

Answer 26

• the difference in level (intensity of sound) between a sound arriving at one ear versus the other
• intensity of a sound is greatest at the ear that is closer to the sound source
  
  • smallest at azimuth of 0 (or 180 degrees)
  • largest at azimuth of 90 (or 270 degrees)

Question 27

What is an acoustic shadow?

Answer 27

Area on the other side of the head from a sound
source in which the loudness of a sound is reduced
because the sound waves are partially blocked
- the head itself creates an acoustic shadow and distorts the actual sound waves

Question 28

What is the ILD contingent upon?

Answer 28

the frequency of the sound

- ILDs for high frequency sounds are much more than for low frequency sounds

Question 29

How do the lateral superior olives help us detect where sound is?

Answer 29

It is the relay station in the brain stem where input from both ears contribute to the detection of ILDs
‣ Contains Binaural neurons
- excitatory connections to the LSO stem from the ipsilateral ear
- inhibitory connections to the LSO stem from the contralateral ear

Question 30

What is the interaural time difference (ITD)?

Answer 30

The difference in time between sounds arriving at
one ear versus the other
- this also helps us localize sounds

Question 31

What is diotic stimulation?

Answer 31

Sound arrives at both ears

simultaneously

Question 32

What is dichotic stimulation?

Answer 32

Sound arrives at one ear
faster and with a larger
intensity than the other
(binaural difference)

Question 33

How do the medial superior olives help in locating sounds?

Answer 33

Relay station in the brain stem
where input from both ears
contribute to the detection of
ITDs
‣ Contains Binaural neurons

Question 34

When do interaural time difference abilities form?

Answer 34

In the first few months of life

- by 3-4 weeks, infants can localize sound in space

Question 35

What is the cone of confusion?

Answer 35

A hypothetical cone shaped surface in auditory space
‣ When two equally distant sound sources are located on
a cone of confusion, their locations are confusable
because they have highly similar ILDs and ITDs
- moving your head will eliminate the cone of confusion problem

Question 36

How does our pinna help us locate sound?

Answer 36

It helps us perceive elevation

- the pinna modifies the amplitude of a wave and our auditory system can then infer elevation from that

Question 37

What is the inverse square law?

Answer 37

The intensity of a sound decreases
in proportion to the square of the
distance of the sound

Question 38

What is the doppler effect?

Answer 38

The apparent change in the
frequency of a wave caused by
relative motion caused by the source
of the wave and the observer
- as sound moves away from us, the pressure decreases
- as sound moves, the waves are compressed in front of the object, and are more distant behind the source of a sound (just think of the pitch in an ambulance as it is far away and gets closer, then passes you)