Lecture 13

Question 1

What is pitch?

Answer 1

The perceptual quality responsible for melodic changes in music. Is NOT a physical property.
Whether the sound is low or high.
Between 20 and 5000Hz

Question 2

What is the perceptual correlate of a waveform repetition rate?

Answer 2

The waveform of a periodic sound has a repetition rate. The repetition rate usually corresponds to the pitch of such sound. Frequency of a pure tone is represented by the firing rate of specific neurons and by firing pattern of groups of neurons with characteristic frequency that is close to frequency of pure tones.

Question 3

What is the range of musical pitch?

Answer 3

Range corresponds to the range of pitch on musical scale that goes from below 20Hz - higher boundary of pitch is about 5000Hz, doesn’t mean you cannot hear any change in pitch about this - it means pitch is not as well defined.

Question 4

What is rate-place coding?

Answer 4

Because of phase-locking (tendency of neurons to fire at specific phases of the waveform) intervals of successive spikes correspond to period and multiples of period of pure tone.

Question 5

What is temporal coding?

Answer 5

Frequency of a pure tone is represented by interspike intervals of neurons whose characteristic frequency’s close to frequency of pure tone.
More likely than rate-place mechanisms.

Question 6

What is this type of coding:
Present two pure tones, 500 and 502Hz, ask participants to tell whether or not which interval has higher pitch or if two sounds have the same pitch. As you compare the difference in two tones - typically one is fixed (500Hz), the other is changed.
Vary frequency - if you do this, then find the smallest difference listeners can detect.

Answer 6

Rate-place coding.

Question 7

What is described in frequency discrimination limens?

Answer 7

Difference would be ability to detect 0.1dB in excitation level. Use of rate place coding unlikely - listeners can only detect level difference of 1dB or lower.
Pitch perception poor above 5000Hz (phase locking starts to break down)
Need large difference at higher frequencies to notice change.

Question 8

In cochlear coding of complex tones, what are resolved and unresolved harmonics?

Is temporal and rate-place coding possible?

Answer 8

Excitation patterns for complex tones show separate peaks for low frequency harmonics only = resolved = represented by rate place and temporal coding. Each resolved peak represents one harmonic. When you stop seeing individual peaks, harmonics become combined - flat line.

High frequency harmonics are unresolved = produce the BM pattern that is complex, because more than one harmonic stimulates the same region on the BM. Has repetition rate that is equal to repetition rate of complex tone.
Place-rate coding not useful, temporal coding still possible because neurons phase-lock to envelope of complex vibration patterns, caused by interaction of 2 or more unresolved harmonics.

Question 9

How is the repetition rate produced in cochlear coding of complex tones?

Answer 9

Produced by unresolved harmonics, represented by duration of inter-spike intervals.

Question 10

What does the excitation pattern describe for cochlear coding of complex tones?
What is the reason for the difference between the resolved and unresolved harmonics?

Answer 10

Describes the overall output of the cochlea.

Reason for difference between peaks: in pattern you see separate peaks that correspond to low frequency low number harmonics, are visible and equally spaced - why they are resolved.

Question 11

Why does the flat line exist in the representation of harmonic complex in the cochlea.

Answer 11

Auditory filters are wider - if you combine output of all the wide bandwidth auditory filters at high frequencies then you cannot separate individual peaks of harmonics - more than one harmonic going through each auditory filter.

Question 12

In mistuned harmonic series, what is the strongest effect seen?

Answer 12

Mistuned harmonics shift the pitch of a harmonic series - strongest effect for the 2nd, 3rd, and 4th harmonics.
Harmonics whose frequency close to 600Hz have stronger weight (more dominant) for determining pitch of complex tone.

Question 13

Why are resolved harmonics the most important?

Answer 13

F0 discrimination much better when resolved harmonics are present in a complex tone. Unresolved harmonics don’t have a well defined pitch.

Question 14

in F0 discrimination, how could this apply to voice and speech disorders?

Answer 14

Looking at frequency/harmonics, you can apply and explain the perceptual characteristics for speech lexical tones - coded through pitch, F1, voice disorders, speech disorders - people who can’t produce pitch changes accurately.

Question 15

What would you expect to see for someone with a hearing loss for pitch of complex tones?

Why look at F0?

Answer 15

If they have enough residual hearing to about 600-1000Hz, expect good presentation of sounds, pitch perception, may produce intonation.

Looking at F0: Because complex tones. Can have auditory threshold for detecting change in pitch.

Question 16

How does the pattern recognition model work?

Answer 16

Rely on knowledge that info about frequencies of resolved harmonics is coded separately in auditory nerve - pitch derived from pattern of frequencies present. Obtained through harmonic template
Auditory system could determine pitch of complex tone by selecting template that most closely matches frequency components in complex tone.

Question 17

Can pattern recognition model explain weak pitch that is obtained from unresolved harmonics?

Answer 17

No

Question 18

How does the temporal model work?

Answer 18

Could be that auditory system relies most strongly on temporal info that is conveyed by neurons whose characteristic frequency is close to frequency that corresponds to the interspike period.
Because for unresolved harmonic, interspike period represents repetition rate that is much lower than characteristic frequency of relevant nerve fibres - resulting pitch would be weak.

Question 19

Can temporal models explain pitch that is obtained from unresolved harmonics?

Answer 19

Yes, but have trouble accounting for why unresolved harmonics are important.

Question 20

What is the missing fundamental?

Answer 20

Complex sounds with frequency components that are resolved have a pitch that corresponds to the fundamental frequency even if there is no energy at the fundamental frequency.
Pitch of a tone does not change without energy at the fundamental frequency.

Question 21

How does the dominance region work?

What is the most important frequency for unresolved harmonics?

Answer 21

Harmonics around 600Hz are the most dominant in determining pitch of complex tones. Why:
F0 of 100, remove first two harmonics, you then have harmonics at 300 and up, but also contains multiples of 300Hz, and harmonics of 100Hz. At fundamental of 300Hz, sound would have pitch corresponding to 100Hz fundamental. Harmonics based on 100Hz intervals - perceive pitch corresponds to lowest common denominator.

Question 22

In rate-place and temporal coding, how is it represented in the auditory system?

Answer 22

Rate place coding - firing of hair cells that is compiled into excitation pattern. Info coming from auditory filters on BM - characteristic frequency is grand total in auditory nerve.
Temporal coding - in the BM and from hair cell activity - equally spaced in time, locked in place to BM phase.

Question 23

In a sawtooth wave with a fundamental frequency of 250Hz, how are harmonics of this waveform represented in the pattern of the hair cell activity/excitation pattern?

Answer 23

Pattern on BM displacement is highly complex - several harmonics, see unexpected pattern. Phase locking info to about 4000Hz, then you lose info at higher frequencies, therefore don’t see any resolved harmonics. Below 3000Hz, see about 12 harmonics in excitation pattern. See resolved and unresolved harmonics in hair cells.

Question 24

How is waveform repetition rate represented in a sawtooth with a missing fundamental?

Answer 24

Miss part of info, but still have info from phase locking of harmonics. Periodicity is represented at the level of the unresolved harmonics - when you combine several harmonics into an auditory filter - audibility shows the repetition rate.

Question 25

In the pitch of complex tones, why is perceived pitch 104Hz?

Experiment

Answer 25

Start with harmonic series (400-900Hz) F0 of 100Hz. Start at 400Hz, add increment of 25Hz to each harmonics; The frequency difference between is 100Hz for harmonics. From difference - match to pitch at 104Hz fundamental so pitch is slightly higher than original 100Hz.
At 104Hz, repeat same process of F0 of 103, 105, 106, will find is that F0 of 104Hz on average will have smallest deviation/difference from frequencies of these harmonics.

Question 26

In the melody of resolved and unresolved harmonics, which has the clearest pitch?

Answer 26

One resolved, one unresolved with melody - both cases with unresolved, can recognize melody but there is a difference in clarity of definition of the pitch.
Obvious difference in overall quality related to distribution of energy at different frequencies.

Question 27

Is temporal coding necessary for pitch perception?

At 5000Hz***

Answer 27

Perceived pitch corresponds to missing F0. Sound synthesized contained only harmonics that are resolved likely to be coded through temporal info; temporal info based on the phase locking properties of BM response and auditory nerve fibres.
5000Hz = temporal coding
It is possible to perceive pitch corresponding to complex sound even though not lots of info from phase locking.
Below 5000Hz, can use rate place coding.

Question 28

In pitch perception of complex tones, temporal vs. pattern recognition models, is the pitch of the missing fundamental derived by the auditory system from the repetition rate information?

Answer 28

Pitch is being perceived even though there is no periodicity in the waveform, purely based on spectral distribution of energy.

Question 29

For someone who has a cochlear implant, what information is missing, what temporal info is there?

Answer 29

Relatively impoverished info about frequency of sounds, fewer frequency channels than normal hearing. Poor pitch perception
Get temporal info that is coded depending on AM within each frequency channel - don’t really have good temporal of frequency/place info.

Question 30

Which type of harmonics can weaker pitch be heard?

Answer 30

Unresolved harmonics

Question 31

When is pitch heard?

Answer 31

When first harmonic is missing (missing fundamental)