Frequency selectivity and masking

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Frequency selectivity and masking

• Lecture 4

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Review Structure of the cochlea
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Review Structure of the cochlea
(a) The spiral duct of the cochlea is divided
into two channels by the flexible basilar
membrane and is filled with fluid (yellow). The
inner hair cells, which sit on the membrane,
connect directly to the auditory nerve. (b) When
sound enters the cochlea (here shown uncoiled) it
agitates the fluid, causing a ripple to travel
along the basilar membrane. This movement (which
is grossly exaggerated here) is detected by
sensory hair cells, supported on the membrane.
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Review job of the hair cells
Hair bundles are the ear's detectors. (a) The
bundle in this hair cell from a turtle is a
pyramidal structure composed of stereocilia,
which are connected by tip links. (b) When the
bundle is displaced, the tip links get stretched
and pull open the transduction channels, thereby
generating an electrical signal due to positively
charged ions. Myosin motor proteins attached to
the channels may be involved in active
amplification. (Picture credit Carole M
Hackney/Keele University see Fettiplace et al.
in Further Reading)
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How do the OHCs increase responses at low
levels? Blue input (displacement) Red output
(hair bundle) Note the greater gain (output minus
input) for low-level sounds
a) When a frog hair bundle is shaken using a
microneedle, its response (red) is characteristic
of a noisy Hopf oscillator. The applied force,
which is related to the amplitude of the
displacement of the needle (blue), progressively
increases down the figure. When the bundle is
shaken gently, the Hopf oscillator's gain - its
response divided by the input stimulus - is
large. (b) The Fourier transform of the bundle
displacement has a peak at the stimulus
frequency. The height of this peak grows as the
cube root of the applied force, indicating that
the gain increases as the force decreases (data
courtesy of Pascal Martin and Jim Hudspeth).
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Frequency selectivity on the basilar membrane
iso-level curves showing BM movement at a
single place on the BM, as a function of tone
frequency level. Note the basal shift in the
best frequency at high levels
I/O function for a single place on the BM. Note
the compression at/near the best frequency
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Frequency selectivity on the basilar membrane
iso-level curves (like Fig. 5.2 in your text,
but plotted in gain instead of output on the
BM) Note that the best freq to excite a single
place shifts basally at high levels
The response of the basilar membrane at a
particular location can be measured using laser
Doppler interferometry. These data collected by
Mario Ruggero and co-workers at Northwestern
University show that the resonance is nonlinear
the gain and the sharpness of the tuning both
increase as the level of the sound diminishes
(see Robles and Ruggero in further reading).
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Inner hair cell tuning follows the BM tuning.
Shows the sound level that will elicit a minimum
HC response, as a function of frequency
Cody Russell 1987
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Auditory nerve tuning. Shows level of pure tone
needed to get a just detectable increase in
neural firing rate. Each curve shows response of
a different neuron
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Tuning curve BW is larger at high fs
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Auditory nerve tuning. Shows neural firing rate
(in spikes/sec) for a single fiber at different
tone frequencies and levels.
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What happens to the AN fiber response when the
OHC is damaged? Effect of kanamycin on nerve
fiber
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Psychophysical method 1 PTCs. Each curve shows
a different tone frequency at a fixed (10 dB
above threshold) tone level. Within the curve,
the masker frequency is changed. The dB SPL
value is how much masker (at that masker f)
shifts the tone to a barely audible (threshold)
level
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Psychophysical method 2 Notched-noise measure to
estimate auditory filter. Measure the signal
threshold as a function of how wide the gap is.
The smaller the gap, the more noise within that
auditory filter, and the higher the signal
threshold. The bigger the gap, the less noise in
the auditory filter, and the lower (better) the
signal threshold.
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Psychophysical method 2 Notched-noise measure to
estimate auditory filter. Measure the signal
threshold as a function of how wide the gap is.
The smaller the gap, the more noise within that
auditory filter, and the higher the signal
threshold. The bigger the gap, the less noise in
the auditory filter, and the lower (better) the
signal threshold. (from Patterson 1976) Graph
shows the width of the masker gap on the x axis
and the signal threshold on the y axis. The
wider the gap, the lower the threshold.
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Masking Illustration of critical band theory.
The R curve shows the bandwidth of a masking
noise centered at the signal frequency. As
masker BW increases, signal threshold goes up
(gets worse) , up to the critical BW (red arrow).
Above that point, making the masker BW wider has
no effect on tone threshold. (The M line is
for a modulated masker ignore that one for now)
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Psychophysical method 3 masking patterns.
Threshold in presence of a fixed 410 Hz masker.
Different curves show different masker levels.
Note the shallower high-freq slope.