Audition Pt3 PBS5

  1. Outline the 2 major theories about how the BM functions?
    • 1. Spatial codes: different locations of BM resonate with different frequencies and certain axons respond to certain frequencies.
    • 2. Temporal codes: rate at which action potentials are produced that determine sound frequency information.
  2. Who and what was the study that got the breakthrough for understanding how the basilar membrane works?
    • von Bekesy
    • Travelling wave vibration
    • each portion of membrane vibrates maximally for particular frequency of sound (tonotopy)
    • Experiment: measured response of BM in post-mortem cochlea
    • microscope, silver particles and stroboscopic illumination
    • very loud (140dB) pure tones
    • Fourier based linear systems approach that predicted complex tone responses
  3. What were the weaknesses in von Bekesy's work that was refined later?
    • Use of laser or radioactive sources to study movement
    • at less intense sound levels
    • dead BMs show lower amplitude movements and so appear less specific to given frequency
  4. Spectral decomposition. von Bekesy's study provide evidence that BM acts like a __ __ __. Tonotopy is approaximately ___ with the frequency of stimulation. And just a few notes about this...
    • mechanical frequency analyser
    • logarithmic
    • NB:
    • like Fourier analysis, but brandwidths (freqnecy range) are quite broad
    • BM is continuous sheet, so responses at two locations are not independent of each other
    • at other locations, BM still respond to a given frequency, just with smaller amplitude
  5. What major role do outer hair cells play? Explain how it works.
    • Active amplification
    • Adds mechanical energy to weak sounds, and little to strong sounds (non-linear)
    • When stereocilia is stimulated, they contract, pulling tectorial membrane with them and adding vibration energy to organ of corti.
    • Because of motor protein prestin - very fast (70,000 contracts/second!)
  6. Mention this scientist when talking about amplification and hair cells. Also research on basilar membrane.
    James Hudspeth
  7. Give one reason why we say hearing is 'approximately' linear. Related to outer hair cells.
    • Because outer hair cells amplifies lowe intensity sounds much more.
    • This big change of gain (about 50dB) is a signficant non-linearity in cochlea sensing.
    • This active process can introduce new frequency info on BM.
    • Therefore input and output relationship is compressive and non-liner.
    • (however, over limited range a linear approximation is v useful)
  8. This active process of outer hair cells is useful diagnositcally. Why?
    • Because of this amplification process, outer hair cells intorudce new frequencies onto BM.
    • Therfroe, when we stimulate ear with pure tone, if hair cells working correctly, sound coming out of ear should contain frequencies that weren't in the input.
    • otoacoustical emisions
    • (damage to outer hair cells can reduce this active amplification and create hearing problems)
  9. What is the critical function of inner hair cells?
    Fast transmission of sound info to the brain
  10. What's the difference between how inner hair cells are connected and how outer hair cells are connected to neurones.
    • Outer hair cells: many-to-one connection to ganglion cell
    • Inner hair cells: one-to-many connection to ganglion cells (usually 20 per hair cell). These ganglion cells send fast axons through auditory nerve to cochlea nucleaus.
  11. Orgnisation of choclea nerve is tonotopic how?
    • individual axons responding most to particular frequency (characteristic frequency)
    • Hair cells are band pass filters: they only repond to a limited range of frequencies
    • successive hair cells differ in freq by 0.2% (piano note is 6%)
  12. What are the different types of ganglion cells/axons and why are there different types?
    • 1. High spontaneous rate: respond to quiet sounds (20-50 spikes per sec at rest), but response saturates at 40dB
    • 2. Medium spontaneous rate (<18 spikes at rest)
    • 3. Low spontaneous rate (<1 spikes/s)
    • Medium and low will not respond until 20-30dbPSL, and will continue increasing until 80dB.
    • These different response gains contribute to our perception of loudness.
  13. But what about temporal coding? What is the relationship between how ganglion cells/chochlea nerves fire action potentials and the sound waves?
    • Phase locking
    • Ganglion cells fire in synchrony with the peak sound amplitude.
    • However, action potentials are stochastic events - so their occurance and timing vary from cycle to cycle.
    • Occurance and temporal regularity increase with sound level.
    • This suggests useful temporal code for sound frequency.
  14. Explain what the volley principle is.
    • Neurons can't fire more than 1000Hz and we can hear sounds up to 20kHz
    • Wever and Bray (1937) - small group of axons could signal much higher rates if their outputs were pooled (volley principle).
  15. Evidence for the volley principle?
    • Subsequent recordings from ganglion cells show precise info about timing can be seen in responses of aggregated axons - a clear temporal signal.
    • However, this ability to show phase locking to stimulus disappears once freq exceeds 4-5kHz (so no temporal code for 5-20kHz)
  16. What are the American Standards institute defunitions for pitch and loudness?
    • Pitch: aspect of auditory sensation in terms of which sounds can be ordered on a musical scale from low to high.
    • Loudness: aspect of auditory sensation in terms of which sounds can be ordered on a scale from quiet to loud.
  17. Describe two studies which suggest that temporal rate code signals may be crticial in perceiving and discriminating pitch.
    • Moore (1973)
    • Listeners' ability to discriminate between 2 pure tones changed with frequency
    • they were best at 0.5-2kHz, but once sounds exceeded 5kHz, listeners got much worse
    • Given that phase locking/temporal coding does not occur for frequencies above 4/5kHz, this suggest that temporal rate code is important in frequency discrimination
    • Attneave & Olson (1971)
    • Listeners' performance in identifying melody made up of pure tones decreased above 4kHz.
  18. For complex tones consisting of multiple harmonics, what would the listener hear?
    • The lowest frequency - the pitch is determined by the Fundamental frequency
    • if, that is, the harmonics are all integer multiples of the first harmonic
    • eg. 100, 200, 300 Hz; then listener will heaar 100Hz
  19. However, is the lowest harmonic always the pitch the listener hears?
    • No - if the harmonics in the complex tone are not all integer multiples of the lowest harmonic, the lowest harmonic will not define the pitch.
    • eg. 200, 400, 600 --> hear 200
    • BUT 200, 300, 400 --> hear 100
    • The auditory system will resolve the odd input by perceiving instead the largest valid Fundamental (highest common devisor).
    • This is called the pitch of the missing fundamental.
  20. Why does pitch of the missing fundamental pose a problem for a pure space-based account of pitch?
    • Because it would mean the pitch can be heard for locations on the BM for which there is no significant vibration energy.
    • Thus the key detemrinate of pitch (at least below 4kHz) is porbably conveyed by temporal signals.
  21. However, it is unlikely that this temporal information is frequency per se. Why?
    • It is the periodicity of sound
    • frequency is tight constraint - in nature, noises often produce vibration energy that repeats over time, but is not exactly same on every repetition
    • the brain seeks to identify similarity across time rather than absolute identity
    • this points us away from the cochlea as the location that encodes pitch.
    • ASK!! CHECK
  22. What are the two hypotheses for encoding loudness?
    • Firing rate
    • Number of neurons
    • both of these encoding schemes have merit
  23. Psychophysical loudness __ experiments demonstrate that loudness is bothe a function of __ and __ __. The relative contributions of __ __ and __ __ __ may well change at different __, just like pitch.
    • matching
    • frequency and sound pressure
    • firing rate
    • number of neurons firing
    • freuqncies
  24. Note: Not everything is encodined in the choclea. Further processing in subcortical areas and the cortex is clearly important and may incorporate what?
    prioir knowledge about pitch and loudness, rather than relying solelhy on bottom-up sensory information.
Card Set
Audition Pt3 PBS5
Lecture 3 - Perceiving sound