In all cases, good answers would be in the form of coherent essays,with informative figures, 
including the specified points:

<br><br>1) For each of the following 4 sounds, describe the cues that are available in auditory nerve fibres to signal their pitch. Ensure that you explicitly describe the acoustic structure of each of these sounds, in time and frequency:<ul><li>A sinusoid at 500 Hz<ul><li>a simple periodic wave, consisting of a single spectral line <li> contains both place cues (variations in amount of firing across the auditory nerve fibre array) and time cues (phase locking)</ul><li>A sinusoid at 8 kHz<ul><li>a simple periodic wave, consisting of a single spectral line <li> contains place cues only as there is no phase locking at such a high frequency</ul><li>A train of narrow (10 &micro;s) pulses at a fundamental frequency of 200 Hz, filtered at 900 Hz with an ideal low-pass filter<ul><li>a complex periodic wave, but with only 4 harmonics in its spectrum.<li>Such low-order harmonics will be mostly resolved, so contain strong place and time cues to each harmonic frequency.<li>There will also be time cues to resolved harmonics (through phase locking) as the maximum frequency of any harmonic is 800 Hz. </ul><li>A train of narrow (10 &micro;s) pulses at a fundamental frequency of 200 Hz, filtered at 4 kHz with an ideal high-pass filter<ul><li>a complex periodic wave, with harmonics at 4 kHz and above. <li>Auditory filters get wider as frequency increases, so these harmonics will not be resolved. Hence there are no place cues to harmonic frequencies.<li>Time cues will be available through phase locking to unresolved harmonics beating together in wider auditory filters. </ul></ul><br>2) Discuss the three main ways in which outer hair cell damage is reflected in human psychoacoustic measurements.What changes in physiological functioning are supposed to underly these changes? Use simple figures to illustrate your answer as much as possible.
To what extent can acoustic hearing aids ameliorate each of these deficts?<ul>
<li>OHCs as 'motors' feeding back energy into the basilar membrane, and thus amplifying BM motion at low levels.Thus responsible for low thresholds, compression of input/output functions, and 'exquisite' frequency selectivity.<li>Three main effects of OHC damage:<ul><li>Raised thresholds ('hearing loss') due to lack of active feedback. Hearing aids amplify sounds.<li>Reduced dynamic range/loudness recruitment due to linearisation of I/O functions. Hearing aids have compression.<li>Reduced frequency selectivity due to broadening of auditory filtering on the BM. Hearing aids do nothing for this.</ul></ul><br>3) Describe, in detail, the following psychoacoustic techniques for assessing frequency selectivity. Explain the rationale behind them,and use appropriate diagrams to illustrate the typical results that would be obtained.<ul><li>psychophysical tuning curve <ul><li>fixed low-level sinusoidal probe; narrow-band noise maker; find the level of masker,as a function of its centre frequency, that just masks the probe<li>low-level sinusoidal probe taps responses of a small group of auditory nerve fibres<li>V-shaped with very steep high-frequency sides and shallower low frequency sides <li>extra points: similar shapes to physiological tuning curves; cannot be used at high levels; subject to off-place listening</ul><li>notched-noise masking<ul><li>sinusoidal probe tone; relatively wide-band masker with a spectral notch; find the probe threshold in the presence of noises with spectral notches varying in width and asymmetry<li>probe tone controls what auditory nerve fibres are being 'listened to'<li>can estimate frequency response of auditory band-pass filters; steeper upper slopes than lower<li>extra points: changes with level and frequency; control of off-place listening</ul><li>masked audiogram<ul><li>fix a narrow-band masker or sinusoid at a particular frequency<li>determine the change in threshold for sinusoidal probes at a wide variety of frequencies<li>probe tone controls what auditory nerve fibres are being 'listened to'<li>hence threshold changes reflect the amount of activity across the BM due to a masker <li>related to excitation patterns on the BM</ul></ul>