Lecture 21

Question 1

location

Answer 1

sounds that come to the same place are assumed to belong together

Question 2

onset time

Answer 2

sounds that appear at the same time tend to belong to the same object

Question 3

similarity of timbre and pitch

Answer 3

sounds with similar timbre and pitch belong to the same object

help us break up an auditory scene: breaking up into separate streams

bach invention: two different melodies at different octaves and the notes in the left hand/right hand were more similar in pitch

Question 4

proximity in time

Answer 4

sounds that appear in rapid succession belong to the same object

bach invention: trilling two notes those two became its own auditory stream

Question 5

auditory continuity

Answer 5

if a sound of the same pitch or timbre changes in a constant fashion (even if interrupted) we think it belongs to the same object

Question 6

effect of past experience

Answer 6

like gestalt familiarity

strong top down cue, what you expect based on what you’ve heard before

if you’ve experienced a pattern of sounds before then you will attribute it to the object it came from in the past

familiar with piece or composer you know what to expect: what transition is coming up or how the two diff streams interact

Question 7

experiment by dowling and harwood

melody schema

Answer 7

Melody “Three Blind Mice” is played with notes alternating between octaves

• Listeners find it difficult to identify the song
• But after they hear the normal melody, they can then ‘hear’ (recognize) it in the modified version using melody schema

once you’ve activated this “melody schema” then you’re interpretation of the same information changes

Question 8

Auditory space

Answer 8

surrounds an observer and exists wherever there

are sound sources.

Question 9

Researchers study how sounds are localized in space along three dimensions:

Answer 9

– Azimuth - position left to right

– Elevation - position up and down

– Distance from observer

Question 10

azimuth

Answer 10

horizontal left to right

Question 11

elevation

Answer 11

position up and down

Question 12

distance

Answer 12

distance from observer

Question 13

figure-ground

Answer 13

trilling becomes the ground

main melody becomes the figure

Question 14

problem with auditory localization

Answer 14

the signal is being mixed right at the receiving organ

no separation

Question 15

Location cues are not contained in the

Answer 15

…. receptor cells (as on the retina).

Question 16

location for sounds MUST BE

Answer 16

CALCULATED

Question 17

On average, people can localize sounds which are:

Answer 17

– Directly in front of them most accurately.

– To the sides and behind their heads least accurately.

Question 18

auditory localization cues

Answer 18

• binaural

- monaural

Question 19

Binaural cues

Answer 19

location cues based on the comparison
of the signals received by the left and right ears

azimuth

two main cues:
• Interaural time difference (ITD), useful for low frequencies
• Interaural level difference (ILD), useful for high frequencies

Question 20

Interaural time difference (ITD)

Answer 20

the amount of time it takes that sound stimulus to get to one ear compared to the other ear

Question 21

Interaural level difference (ILD)

Answer 21

the amplitude of that signal hitting one ear vs. the amplitude hitting the other ear

Question 22

monaural

Answer 22

using 1 ear

elevation

Spectral cues and the head-related transfer function (HRTF)

Question 23

spectral cues

Answer 23

using frequency information in the signal

the information for location comes from the spectrum of frequencies

Question 24

Interaural time difference (ITD)

Answer 24

maximally effective when something is off to the side

• difference between the times at which sounds reach the two ears.

• When distance to each ear is the same, there are no differences in time (point A).

• When the source is to the side of the observer, the
times will differ (point B).

• This cue is better for LOW FREQUENCY tones (< 800 Hz).
− May use temporal coding (phase locking - diff neurons responding to the peaks of a frequency coming in, as you get higher and higher frequencies the phase coding becomes more difficult, so we want to use simpler sounds that are less complicated ) in cochlea.

Question 25

Interaural level difference (ILD):

Answer 25

it’s the diff in the sound pressure (amplitude) of a stimulus hitting one ear vs. another

as a sound stimulus is coming in you get a reduction in the intensity as the sound stimulus goes through your head

difference in sound pressure level reaching the two ears

• Reduction in intensity occurs for high frequency sounds for the far ear.

– The head casts an acoustic shadow, blocking out (reflecting) and absorbing some of the high-frequency pressure wave.

• This effect doesn’t occur for low frequency sounds. Works best above 800 Hz.

the sound pressure waves interfered with by the head

Question 26

acoustic shadow

Answer 26

blocking out (reflecting) 
and absorbing some of the high-frequency pressure wave.

Question 27

Interaural level difference (ILD) is largest at locations….

Answer 27

…farther to the side.

– This is shown in psychophysics experiments where they measure the frequencies hitting the ears.

Question 28

Limitations of ITL & ILD

Answer 28

they are not effective for detecting differences in elevation.

• An additional cue is necessary for reliable elevation judgments (monaural cues).

Question 29

ILD and ITD provide useful cues for judging locations along most of the….

Answer 29

…azimuth plane.

Question 30

cone of confusion

Answer 30

– Consider points A and B. The level and time differences of signals reaching each ear is zero.

– There is a similar ambiguity at points C and D.
– These points are said to lie on a cone of confusion.

– Multiple (infinite!) cones of confusion are possible.

all the points are the same distance and can have the same shadowing effect: not getting any time or level differences

Question 31

the pinna

Answer 31

is a spectral filter

when sound comes in it bounces off them and the frequencies are affected

Question 32

the head-related 
transfer function (HRTF) 

what kind of cue?

Answer 32

unique to each head

how the shape of the pinna and the head uniquely affect the intensities of frequencies

– This is a spectral cue since the information for location comes from the spectrum of frequencies.

Question 33

pinna exp.

Answer 33

Measurements have been 
performed by placing small 
microphones in ears and 
comparing the intensities of 
frequencies with those at the sound source.

“what it’s like to sound like you”

Question 34

Hoffman et al. (1998) psychophysics experiment
investigating spectral cues

an example of what else?

what was unaffected? what does that tell us?

Answer 34

manipulated the pinna

– Listeners’ accuracy was measured while locating
sounds differing in elevation (baseline).

– They were then fitted with an ear mold that changed the shape of their pinna.

– If the shape of the pinna is important for elevation
judgments, subjects should show a large decrease in
performance (compared to baseline)…

– Right after the molds were inserted, performance was poor for elevation but
was unaffected for azimuth (spectral cues not important for left and right).

– After 19 days (they kept the molds in this whole time), performance for elevation was close to original performance.

– Once the molds were removed, performance remained high (didn’t go back to some baseline where they had to re-learn).

– This suggests that there might be two different sets of neurons—one for each
set of spectral cues (new and old) = a sparse code OR a more broadly distributed population code that could adjust and encode for both.

– Nice example of experience-dependent
plasticity.

Question 35

Which auditory cue would be most useful for locating a low frequency tone coming from the top of the screen?

Answer 35

it’s a low frequency but where you’re sitting in the room changes whether it’s an ITD or HRTF (based on the altitude one is sitting in the room)

Question 36

Why are we best at localizing sounds in front of us?

Answer 36

– It appears that ITD and ILD azimuth cues are most effective to the side, but we have difficulty detecting those on the cone of confusion.

– Remember that perception is an active process!! No single observation is used to localize a sound. We orient to make ITD and ILD zero in front of us (till you’re staring right at the object).

– Spectral cues (HRTF) are always used: very good when straight on cause that’s what we have the most experience with.

– We also use expectations (top-down information) to help localize.

Question 37

auditory localization cues: binaural and monaural. They work together effectively, but…

Answer 37

are mainly for orienting.

In real applications, we move our heads and take many ‘auditory samples’ with all localization cues.

Question 38

Physiological representation of auditory space

Answer 38

Two basic approaches have been proposed (using ITD as a model)

1. Narrowly tuned ITD neurons
2. Broadly tuned ITD neurons

Question 39

Narrowly tuned ITD neurons

Answer 39

neurons responsive to small changes

respond to small differences in timing

• Respond to specific time differences only. One neuron gives a location. This is a form of specificity coding.

Question 40

Broadly tuned ITD neurons

Answer 40

lots of neurons responding to large changes in timing differences

• Respond to broad range of time differences. Location
is calculated from a range of neural responses. This is
a form of distributed coding.

Question 41

Jeffress Model for narrowly tuned ITD neurons

Answer 41

how the system is sensitive to small changes

– These neurons receive signals from both ears.

– Coincidence detectors fire only when signals arrive from both ears simultaneously (sensitive to the different when the sound hits one ear and the other).

– If signal reach both ears at the same time, then ITD is zero.

– If a signal comes in on the right earlier, it will travel father along these cells and meet the left signal
at a different location .