Pitch Perception

Question 1

What is pitch?

Answer 1

• ‘that auditory attribute of sound according to which sounds can be ordered on a scale of low to high’
• corresponds to frequency in the case of pure tones
• usually corresponds to f0 for complex periodic tones

Question 2

Why is pitch useful?

Answer 2

• sound scene analysis - identifying the diff sounds from a ‘babble’ of acoustic info reaching our ears
  ◦ identifying different voices
• distinguishing sound sources
• conveys info in vocalisations
  ◦ in speech - some languages are tonal and the pronunciation of the word can change its meaning
• conveys melody and harmony

Question 3

What is the frequency and period of pitch-evoking sounds?

Answer 3

freq 400HZ, period of 2.5ms
- f0 = 400Hz, period 2.5ms
- f1 = 800Hz, period 1.25ms

Question 4

What do harmonic tones consist of?

Answer 4

Consists of several frequencies that have their own period but the summed waveform repeats at a period of f0

= periodicity

Question 5

What is periodicity?

Answer 5

The repetition rate of a waveform

Question 6

What is the period of the pure tones?

Answer 6

1/f

Question 7

What is the period of complex tones?

Answer 7

They contain multiple frequencies but are usually multiples of f0 and periods but compound waveform repeats at an interval equal to period of f0

Question 8

What is the period of complex aperiodic sounds?

Answer 8

also contain multiple frequencies but these are not harmonics and there is little to no regular repetition on the time domain

Question 9

What are pitch-evoking sounds?

Answer 9

• sinusoidally amplitude modulated tones (SAM)
• pure tone at high freq (the carrier frequency, 5kHz)
• modulate the amplitude of this tone at a much lower freq (400Hz, modulation freq)
• periodicity of 2.5ms in time domain but no spectral power at 400 Hz in frequency domain
• the high freq goes through a cycle of waxing and waning every 2.5ms, it has a periodicity of 2.5ms but in the frequency domain there is no freq corresponding to the period of 2.5ms
  ◦ instead see carrier freq
  ‣ have regular repetition in the time domain but no corresponding frequency in frequency domains
  ‣ periodicity with no frequency

Question 10

What is tonotopy?

Answer 10

location on cochlea corresponding to fo, place coding

Question 11

What is phase coding?

Answer 11

AN afferents fire w interspike interval equal to periodicity, temporal coding

Question 12

Describe temporal and spatial filtering in the cochlea

Answer 12

• complex periodic tones (f0 = 440Hz) time (a) and frq (b) domains
• freq tuning along cochlea length (c)
• low numbered harmonics are resolved but high numbered harmonics are ambiguous (f- tuning is broader than spacing between harmonics)
• for high harmonics, there is high f vibration but this is modulated by the periodicity of f0 (e)

Question 13

Place coding for pitch

Answer 13

• pitch could be ‘read off’ from the f0
• however missing fundamental:
  ◦ harmonic tone in which f0 missing

Question 14

Temporal coding for pitch

Answer 14

• afferent spikes phase lock to periodicity
• seems both temporal and place coding could account for pitch of complex periodic tones
  ◦ if harmonics resolvable
  ◦ if f0 below limit for phase locking

Question 15

is the pitch for SAM tones coded by temporal coding or place coding?

Answer 15

Both but usually temporal coding as frequency is coded but cannot see place coding

Question 16

Can non-human mammals perceive pitch?

Answer 16

Yes but not as accurately 30% difference

Question 17

Neural coding

Answer 17

• For complex periodic sounds, neurons can phase lock to periodicity
• Limit of phase locking is similar to upper limit of human pitch perception
• Neurons in the AN and ascending auditory brainstem system represent full details of acoustic stimuli, including precise timing info
• However, neurons in the auditory cortex only phase lock to frequencies up to low 100s
• Temporal code represented by phase locking in early auditory system converted into rate based code in cortex