Task 8- the auditory scene

Question 1

How is having 2 ears useful for localizing sound in space?

Answer 1

1. Because pressure waves dont reach both ears at the same time (ITD)
2. Because sound intensity is greater at the ear closer to the sound source (ILD)

Question 2

Interaural Time Difference (ITD)

Answer 2

The difference in time between the sounds arriving at one ear and the other, works best at low frequencies

Question 3

Azimuth

Answer 3

The angle of a sound source on a horizontal plane relative to our head in the center
Is read clockwise, straight ahead is 0, behind the head is 180, the left part is then descending with a minus (-160, etc)

Question 4

When is the ITD the smallest, when the largest?

Answer 4

The smallest ITD occurs at 0 and 180 degree azimuth, the largest at 90 and -90 degree azimuth

Question 5

Physiology of ITD

Answer 5

• Information from both cochleas reaches the cochlear nucleus and travels then to both LSO ( Lateral superior olive) and MSO ( medial superior olive) of each side
• according to the Jeffress modell, there are ITD detectors, that fire when information received indicates a specific point (place coded, only birds)
• mammals have broadly tuned neurons with a distribution code -> response is a combination of many neurons

Question 6

The what pathway

Answer 6

It extends from the anterior core and belt to the prefrontal cortex, responsible for identifying sound (these neurons respond very good to complex sounds)

Question 7

The where pathway

Answer 7

It extends from the posterior core and belt to the prefrontal cortex, responsible for localizing sound in space (these neurons have a much better spatial tuning, than A1)

Question 8

Interaural level difference (ILD)

Answer 8

• Sounds are more intense at the ear closer to the sound source, because the head works as an acoustic shadow
• correlates with azimuth in a similar matter as ITD (biggest at 90/-90)
• only works for frequencies above 1000hz

Question 9

Physiology of ILD

Answer 9

• neurons to identify ILD are found in the LSO (lateral superior olive)
• they receive excitatory input from the ipsilateral cochlea and inhibitory input from the contralateral cochlea
• these inputs compete, the neurons fire stronger at the cochlea closer to the sound source
• thus, the side closer to the sound source has stronger excitation, and enhanced inhibition contralateral

Question 10

Cones of confusion

Answer 10

A region in space, which extends around our head, and has the same ILD and ITD for every location in this region, making it hard to localize the sound source
-> moving our head solves this problem

Question 11

Directional transfer function (DTF)

Answer 11

• Describes how pinna, ear canal, head and torso change the intensity of sounds with different frequencies that arrive at each ear from different locations in space
• Changes over time due to growth of head, and ear, experience is important in using DTF to localize sounds

Question 12

The pinna as a cue for sound localization

Answer 12

• The pinna is unique for individuals and provides additional information about the sounds localization
• It enhances some frequencies different than others, and some locations will be reflected differently than others from it, thus it helps localizing sounds and identifying them

Question 13

Auditory distance perception

Answer 13

• Relative intensity as a cue, because sound becomes less intense with distance (inverse-square law)
• spectral composition as a cue, as air dampens high frequency pressure waves more than low frequency (only noticeable for distances over 1000 meter)
• the relative amounts of direct vs indirect/ reverbant sound, because the closer a sound is the more direct the energy is, the farther away, the more reverbant energy

Question 14

Auditory scene analysis/ source segregation

Answer 14

distinguishing different sounds in an auditory scene

Question 15

Spatial seperation between sounds (cue for auditory scene analysis)

Answer 15

sounds that come from the same location in space can be treated as if the arose from the same sound source

Question 16

spectral/ temporal segmentation (cue for auditory scene analysis)

Answer 16

sounds with a similar pitch are being treated as coming from the same source and put into one “auditory stream”. -> auditory stream segmentation

Question 17

Grouping by timbre (cue for auditory scene analysis)

Answer 17

sounds with the same timbre usually arise from the same sound source

Question 18

Grouping by onset (cue for auditory scene analysis)

Answer 18

sound components that begin at the same time will be tend to be heard as coming from the same sound source

Question 19

getting familiar with sounds (cue for auditory scene analysis)

Answer 19

familiar sounds (like one’s name) will be easier to pick up than unfamiliar sounds

Question 20

Continuity and restoration effect (cue for auditory scene analysis)

Answer 20

our brain is able to restore sound that is interrupted as if it was continuing

Question 21

acoustic startle reflex

Answer 21

very rapid motor response to a sudden sound, very few neurons involved, onset around 10ms after sound, anxiety can enhance motor response