Pitch perception

Question 1

What is pitch?

Answer 1

• “That auditory attribute of sound according to which sounds can be ordered on a scale from low-to-high”
• Corresponds to frequency in the case of pure tones
• usually corresponds to f0 for complex periodic tones

Question 2

What does pitch do for us?

Answer 2

• Sound scene analysis
• distinguishing sound sources
• conveys information in vocalisations
• conveys melody and harmony

Question 3

state the types of sounds that can evoke pitch perceptions

Answer 3

-pure
-complex periodic
-sam

Question 4

describe pure tones

Answer 4

freq; 400 Hz
period; 2.5 ms

Question 5

describe Complex periodic tones

Answer 5

f0 = 400Hz
period = 2.5ms
f1 = 800Hz
period = 1.25ms
-harmonic tone contains several freqs (each with own period) but summed waveform repeats at period of f0
periodicity = 2.5ms

Question 6

describe Sinusoidally amplitude modulated tone (SAM)

Answer 6

-pure tone at high freq (carrier freq 5kHz)
-modulate the amplitude of this tone at lower freq (modulation freq 400Hz)
-periodicity of 2.5ms in time domain but no spectral power at 400Hz in freq domains

Question 7

Explain the relationship between frequency, period and periodicity

Answer 7

• Periodicity is the repetition rate of a waveform
• In the case of pure tones this is the same as the period (1/f)
• Complex periodic tones contain multiple frequencies (integer multiples of f0) and periods but the compound waveform repeats at an interval equal to period of f0
  
  • irregular period in spectral domain and freq domain → harmonics multiple of each other
  • ⇒ regular periodicity equal to period of fundamental freq
• Complex aperiodic sounds also contain multiple freqs but these are not harmonics and there is little or no regular repetition in the time domain
  
  • Freqs equal present and powerful ⇒ no periodic freq bands
  • Aperiodic sounds - spectral domain but no harmonics also aperiodic in temporary domain

Question 8

Describe Tonotopy

Answer 8

• location on cochlea corresponding to f0
• place coding

Question 9

Phase locking

Answer 9

• AN afferents fire with interstice interval equal to periodicity
• Temporal coding

Question 10

how do you decode pitch

Answer 10

Tonotopy
Phase locking

Question 11

Spatial and temporal filtering in the cochlea

Answer 11

• Complex periodic tone (f0 = 440 Hz) time (a) and frequency (b) domains
• frequency tuning along cochlear 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 12

Place coding for pitch

Answer 12

• Pitch could be ‘read off’ from the f0
• However: missing fundamental
  
  • Harmonics tone in which f0 is missing but other harmonics present - same pitch is perceived
  • Pitch could be read off from spacing between harmonics (spectra periodicity) which is equal to f0 even if f0 is missing

Question 13

Temporal coding for pitch

Answer 13

• 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
  • 200Hz = pitch for 400, 600, 800Hz

Question 14

How could the pitch of a SAM tone be coded?

Answer 14

Temporal coding

Question 15

Can non-human mammals perceive pitch?

Answer 15

ferrets are good at freq discrimination but poorer at pitch judgements

Question 16

Neural coding of pitch

Answer 16

• For complex periodic sounds neurons could phase-lock to periodicity
• limit of phase-locking (~4kHz) 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 information
• however, neurons in auditory cortex only phase lock to freqs up to low 100s
• temporal code represented by phase locking in early auditory system converted into rate-based code in cortex

Question 17

Pitch-selective neuron in marmoset A1

Answer 17

• Complex periodic tone with f0 of 180 Hz and versions with missing (a)
• Neuron with best frequency of 180 to pure tones (c) continues to respond to the pitch-evoking tones missing this spectral component (b)
• Suggest this response is correlated with periodicity
• RATE CODE!

Question 18

Compare and contrast the processing of spectral information in the auditory and visual system

Answer 18

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