The document discusses a study investigating how the medial olivocochlear (MOC) reflex affects decrement detection in auditory perception. It outlines the effects of decrement location and pedestal level on detection thresholds, demonstrating that thresholds improve when the decrement is at the temporal center of a pedestal compared to its onset. Moreover, findings indicate that cochlear nonlinearity and off-frequency listening influence these detection capabilities at various levels of sound input.

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Improved Decrement Detection with Decrement Location may Result from Efferent Processing
Jessica Chen, Skyler Jennings
Department of Communication Sciences and Disorders - The University of Utah, Salt Lake City, UT 84112
• Speech is characterized by a highly fluctuating temporal
envelope. A faithful neural representation of these
fluctuations is essential to robust speech understanding[1]
.
• The contrast between the peaks and valleys of the
speech envelope may be reduced by basilar membrane
compression[2]
(Fig. 1A).
• The medial olivocochlear (MOC) reflex may improve the
neural representation of speech by enhancing the contrast
between peaks and valleys. This reflex reduces outer hair
cell (OHC) gain, leading to a better post-cochlear signal-to-
noise ratio (SNR)[3]
(Fig. 1B).
• When elicited by sound, the MOC reflex has a short onset
delay (~20 ms) and reaches full strength by ~100 ms[4]
.
• The current experiment tested the hypothesis that detection
of a short decrement in a steady-state pedestal will improve
as the decrement location moves from pedestal onset to the
temporal center of the pedestal.
• This hypothesis is based on the assumption that, for
decrements temporally close to the pedestal onset, gain
reduction from the MOC reflex will be minimal. Conversely,
for decrements closer to the temporal center of the pedestal,
gain reduction from the MOC reflex will be relatively larger.
• Since compression depends on input level, this experiment
measured decrement detection over a large range of levels.
• The primary goals of the study were to determine the
effects of (1) pedestal level on decrement detection and (2)
decrement location on decrement detection.
Participants
• Young normal hearing listeners, n = 6
»» Audiometric thresholds < 20 dB HL
»» Average age = 25 years old
Stimuli (Fig. 2 and 3)
Pedestal Background Noise
Duration 500 ms (gated together)
Ramp 5 ms 5 ms
Level Independent variable
-40 dB
(relative to pedestal spectrum
level)
Type NBN
(1950-2050 Hz)
BBN
(20-8000 Hz)
Statistics Low Noise Noise (LNN) Gaussian Noise (GN)
Decrement
Location
2% or 47.5%
of total pedestal duration
N/A
Decrement
Depth
Dependent variable N/A
Decrement
Duration
10 ms N/A
Decrement
Inner Ramps 2 ms N/A
500 ms
10 ms
500 ms
10 ms
METHODS
40 50 60 70 80
0
5
10
15
20
25
Pedestal level (dB SPL)
DecrementThreshold(∆L)
2%
47.5%
Psychophysical Data
40 50 60 70 80
0
5
10
15
20
25
Pedestal level (dB SPL)
DecrementThreshold(∆L)
2%
47.5%
Modeling Data
40
60
80
100
A. Decrement at onset
40
60
80
100
B. Decrement at temporal center
Output(dB)
20 40 60 80
Input (dB)
PD
PD
Breakpoint
post-
cochlear
contrast
contrast
improved
Figure 1:The effects of compression & MOC feedback on peak-
valley contrast. A) When the input signal falls on the compressive
portion of the input-output (I/O) function, post-cochlear contrast
between the pedestal and decrement is small. B) When the (I/O)
function is linearized, the post-cochlear contrast between pedestal
and decrement is improved.
Figure 2: Schematic of stimuli used. 47.5% condition (top,
pink). 2% condition (bottom, blue)
Figure 3: Spectrogram of the stimuli and background noise.
47.5% condition (top),
2% condition (bottom)
Figure 4: Experimental Data -
Individual data (dashed lines), mean data (solid lines).
Figure 5: Auditory Model Simulations -
Predicted mean data.
The authors would like to thank Kayla Hirschmugl for help with data collection and Ali Almishaal for help with statistical analysis
and editing. We would also like to thank Judy Dubno and Jane Ahlstrom for helpful comments on this work. Model simulations
were conducted using resources from the Center for High Performance Computing at the University of Utah.
• Mean thresholds for the 47.5% condition were consistently better than those for the 2% condition, except for at the
highest levels where thresholds were roughly equal across conditions (Fig. 4).
• In the 47.5% condition, mean thresholds improved monotonically with pedestal level.
• Mean thresholds for the 2% condition grew non-monotonically with level. Specifically, as the pedestal intensity
increased, thresholds first worsened, and then improved.
• Data analysis showed a significant main effect of level on decrement thresholds [F(4,20) = 19.72, p < 0.05].
• There was also a significant main effect of decrement location [F(1,5) = 94.196, p < 0.05].
• The interaction between level and decrement location was significant [F(4,20) = 36.777, p < 0.05].
»» This effect can be seen by how the difference between 2% and 47.5% is dependent on level.
• 2% delay condition
»» The initial worsening of thresholds with increasing pedestal level may be due to cochlear compression[6]
.
◊ Compression is expected to reduce the effective peak-to-valley contrast between the pedestal and the
decrement, requiring relatively larger decrement depths at threshold.
• 47.5% delay condition
»» In general, thresholds in the 47.5% condition were better than the 2% condition.
◊ This may be explained by a linearization of the input-output function (see Fig. 1B), which may increase the
contrast between the pedestal and the decrement, allowing decrement depth to be decreased at threshold.
»» Thresholds at the lower pedestal levels were slightly worse than at higher pedestal levels. This may be due to
near-threshold effects[8]
.
◊ Model simulations are roughly consistent with this assumption. However, thresholds from the model were
generally lower than the experimental data.
»» Thresholds improved at the highest pedestal levels.
◊ This improvement may be due to insufficient
masking of spread of excitation by the
background noise, resulting in off-frequency
listening[7]
.
Procedures
• 3AFC paradigm (pedestal level fixed, decrement depth adjusted)
converging on 70.7% on psychometric function
• Each decrement threshold based on the average of 4 measures
Data Analysis
• Two-way repeated measures ANOVA with pedestal levels (5 levels)
and decrement location (2 levels) as within subject factors.
• Significance set at p < 0.05
40 50 60 70 80
5
10
15
20
25
Pedestal level (dB SPL)
DecrementThreshold(∆L)
JC
PG
MW
40 50 60 70 80
5
10
15
20
25
Pedestal level (dB SPL)
DecrementThreshold(∆L)
JN
CS
KH
A. B.
1000
2000
3000
Time (ms)
Frequency(Hz)
100 200 300 400 500
1000
2000
3000
Decrement
Decrement
• Modeling methods (Fig. 6)
»» The effects of absolute threshold were
modeled as a constant noise floor set to
the mean detection thresholds of a 10-
ms, 2-kHz pedestal (θ). (θ = 24.33 dB SPL)
»» For levels ≤70 dB, spread of excitation
was assumed to be minimal or
blocked by background noise, thus the
decrement was assumed to be detected
through the auditory filter centered on 2
kHz.
Figure 6: Block diagram of model stages
Figure 7: Comparison of 2% delay condition thresholds for
subjects with inital worsening of threshold (A) and with subjects without intial
worsening of threshold (B).
◊ Subjects JC, PG, and MW showed inital
worsening of threshold (Fig. 7A).
◊ Subjects JN, CS, and KH did not show initial
worsening of threshold (Fig. 7B). This may be
due to near threshold effects resulting in poorer
thresholds at the lowest pedestal level.
40 50 60 70 80 90 100
65
70
75
80
85
90
95
100
Input dB SPL
OutputdB
Near Threshold
Noise Floor
Gammatone
Filters
Stimulus
Cochlear
Nonlinearity
Envelope
Extraction
Decision
Device
(Constant SNR)
Broken-stick function
»» For pedestal levels > 70 dB SPL,
apprecible upward spread of excitation
was expected, and it was assumed that
listeners could detect the decrement
through off-frequency auditory filters.
◊ Off-frequency listening was simulated
by selecting the best threshold
between the filter centered on the
pedestal frequency & a filter centered
1 ERB above the pedestal frequency.
Parameters of the broken-stick function[5]
2% 47.5%
Gain (G) 50 G = 50 + ∆G**
Compression (c) 0.33 0.33
Breakpoint 1 (BP1
) 39 BPnew
= (G+BP1
-(c*BP1
)-Gnew
)/(1-c)
Breakpoint 2 (BP2
) 90 90
**∆G=BP1
-Pedestallvl
, for all Pedestallvl
>BP1
**∆G=0, for all Pedestallvl
<BP1
• Modeling results (Fig. 5).
»» Overall, model predictions capture the general trends seen in the experimental data.
»» For the 2% condition, the model predicts that thresholds first worsen then improve as pedestal level increases.
»» For the 47.5% condition, the model predicts that thresholds will continue to improve as level increases.
»» The model predicts slightly better performance at low levels than observed in the experimental data.
• Subjects typically exhibited better thresholds when the decrement was located at the temporal center of the
pedestal than at pedestal onset.
»» These threshold differences are largest at mid-levels, where thresholds in the 2% delay condition
worsened while thresholds in the 47.5% delay condition improved.
• Off-frequency listening, due to upward spread of excitation, may explain the improvement in thresholds at
the highest pedestal levels.
• Modeling suggests that cochlear nonlinearity may play a large role in the worsening of thresholds at mid-
level pedestals.
• Preliminary modeling suggest that the improvement of decrement thresholds when located at the temporal
center of a pedestal may be explained by gain reduction from MOC reflex.
◊ Alternatively, at high pedestal levels, basilar membrane response growth could become linear. However, in
model simulations, a very low (and physiologically unrealistic) second breakpoint (~60 dB SPL) was needed to
predicted the data.
INTRODUCTION DISCUSSION
CONCLUSION
PRELIMINARY MODELING
REFERENCES
ACKNOWLEDGEMENTS
RESULTS