Why does upward spread of masking happen

In contrast, the BM response for a signal frequency of about 0. Compressive and suppressive responses have also been measured in auditory neurons Cooper and Yates ; Delgutte a ; Pang and Guinan A compressive growth of response can be inferred from a comparison of neural rate-level functions for a signal at CF, and a signal well below CF e. Delgutte b measured the level of a CF-tone required to maintain a criterion discharge rate for an auditory nerve fiber, as a function of the suppressor level a suppression growth function.

The amount of suppression was found to average about 40—50 dB; the CF tone had to be increased by about 40—50 dB increase in level from 30 to 80 dB SPL to maintain the discharge rate at criterion level, for an increase in suppressor level from about 60 to 90 dB SPL.

Similarly high amounts of suppression were reported by Javel ; for a suppressor lower in frequency than the CF, the shift of neural response functions based on a measure of the synchrony of neural discharge to higher levels was about 28 dB. Although nonlinear growth of suppression may be the major cause of USM for low-level signals Delgutte a , physiological Robles et al.

Because suppression requires simultaneous presentation of the suppressor and suppressee Arthur et al. In forward masking, however, the masker cannot suppress the signal and a more accurate estimate of compression can be obtained Oxenham and Plack ; Nelson et al. Oxenham and Plack estimated compression by comparing the slopes of on- and off-frequency GOM functions and reported that compression estimates were slightly, but significantly, different for their simultaneous and nonsimultaneous conditions compression exponents of about 0.

The amount of suppression in simultaneous masking can be estimated by comparing the thresholds for a simultaneous- and forward-masked signal Moore and Glasberg ; Moore and Vickers ; Oxenham and Plack ; Sommers and Gehr For example, Oxenham and Plack estimated the amount of suppression by taking the difference between the off-frequency simultaneous and nonsimultaneous masked signal thresholds. There is a possibility that the compression and suppression estimated by some psychophysical studies may not be a true reflection of the actual amounts of compression and suppression as a result of off-frequency listening Leshowitz and Wightman ; Johnson-Davies and Patterson ; O'Loughlin and Moore Off-frequency listening refers to the detection of signal activity from a region of the BM with a CF other than that of the signal.

For example, if the masker frequency is below the signal frequency, then the signal may be detected using a place with a CF above the signal frequency. Because the response to a below-CF signal grows more linearly than the response to a signal at CF, measures of compression without background noise may be underestimates Nelson et al.

As off-frequency listening decreases the estimate of compression, it is possible that it also affects the estimate of suppression. Oxenham and Plack found compression exponents of 0. Indeed, these values are much higher i. The use of a noise to restrict off-frequency listening in measurements of GOM has been evaluated by Nelson et al. Estimated compression was found to be greater with the notched noise than without; the notched noise restricted the use of signal information from areas adjacent to the signal place, where the signal response grows linearly.

In summary, a comparison of GOM slopes measured in the absence of notched noise centered at the signal frequency may lead to an underestimate of the roles of both signal compression and suppression in USM.

The aim of the present study was to extend the findings of Oxenham and Plack by measuring the contributions of compression and suppression to USM in the presence of background notched noise designed to restrict off-frequency listening.

GOM functions were obtained for a 4-kHz signal masked by an on-frequency 4 kHz or off-frequency 2. The amount of signal compression was estimated from the slopes of the off-frequency GOM functions.

Suppression was estimated by comparing the off-frequency simultaneous- and forward-masked GOM functions. Because of the need to keep the signal level constant relative to the level of the notched noise, the present study adaptively varied the masker level, rather than the signal level.

The signal had no steady-state duration and 3-ms raised-cosine on- and off-set ramps. The masker had a steady-state duration of ms and 2-ms raised-cosine on- and off-set ramps. The signal was presented in the temporal center of the masker simultaneous masking or at a masker-signal gap ramp offset to ramp onset of 6 ms forward masking.

A notched noise was added to limit off-frequency masking by Oxenham and Plack In their study, a notched noise was presented with a spectrum level 30 dB below the spectrum level needed to mask the signal for one of the subjects.

In the present study, the notched noise was presented with a spectrum level in the passband 40 dB less than the signal level, although the signal was still clearly audible. The noise was gated on 50 ms before masker onset and gated off 50 ms after signal offset. All stimuli were digitally generated by a PC using a sampling rate of 48 kHz and output via a soundcard with bit resolution.

Antialiasing was provided by built-in filters. The stimuli were presented to the right channel of Sennheiser HD headphones. The headphone input came directly from the output of the soundcard DAC.

Subjects were tested individually while seated in an IAC double-walled sound-attenuating booth. The stimuli were presented to the subjects' right ears. A 3I-3AFC adaptive procedure was used to determine the masker level at which the subject would achieve A block of trials began with the presentation of a light on a computer-simulated response box.

Subjects started a block of trials by pressing a start key. The length of each observation interval was indicated by a light on the response box in one of two rectangles.

The interstimulus interval was ms. On each trial, the masker was presented in all three intervals. The signal was presented at random in one of the intervals. The task within each trial was to select the signal interval. Subjects responded by pressing the appropriate response key. After a response, visual feedback was provided by the presentation of a colored light.

In each block of such trials, the masker level was decreased after an incorrect response and increased after every two consecutive correct responses.

A reversal was counted every time the masker level changed direction. The masker level was varied in steps of 4 dB for the first four reversals. For the following 12 reversals the step size was reduced to 2 dB and the levels for the last 12 reversals were averaged to obtain the threshold value.

In this way, an estimate of threshold was obtained from each block of trials. Data were collected after 2 h of practice. Five estimates of threshold were obtained for each condition.

The most deviant threshold was discarded and the mean was calculated from the remaining four estimates of threshold. The maximum masker spectrum level that could be produced by the system without clipping was 72 dB. If a masker spectrum level of 72 dB was reached within a block of trials, the estimate of threshold from that block was discarded. Temporal simultaneous and nonsimultaneous and frequency on- and off-frequency conditions of the masker were presented as a randomized set.

Four subjects were tested. One was the author I. Absolute thresholds for the 4-kHz 6-ms signal used in the experiment were The individual GOM functions for both forward- and simultaneous-masking conditions are shown in Figure 1.

Each panel presents a set of GOM functions for an individual subject. Filled and open squares represent the on-frequency simultaneous- and forward-masked GOM data, respectively.

Filled and open circles represent the off-frequency simultaneous- and forward-masked GOM data, respectively. Dashed lines represent threshold values for the signal alone. Within each panel, the on-frequency simultaneous and forward-masked GOM data were fit with linear functions and the off-frequency simultaneous- and forward-masked GOM data were fit by third-order polynomial functions.

In both cases, the fitting procedure minimized the root mean square rms deviations between the experimental and predicted values. The solid thick and thin lines fitted to the data represent the functions fitted to the forward- and simultaneous-masked data, respectively.

Consistent with the results of Oxenham and Plack , , for all subjects the increase in the on-frequency masker level with increasing signal level is close to linear mean gradient, 0. On average, there was a slight tendency for the on-frequency forward-masked functions to be higher than the simultaneous functions mean of 2. The off-frequency simultaneous- and forward-masked functions diverge for all subjects, with generally higher masker levels required for threshold with an off-frequency forward masker than an off-frequency simultaneous masker.

The range of signal levels over which this off-frequency divergence occurs and the extent of the divergence differ across subjects. For A. For P. GOM functions for individual subjects. Each panel shows masker levels as a function of signal level for on- and off-frequency simultaneous and forward maskers.

On-frequency simultaneous- and forward-masked GOM data are represented by filled and open squares respectively. Off-frequency simultaneous- and forward-masked GOM data are represented by filled and open circles, respectively. Dashed lines represent absolute threshold for the signal alone.

Solid thick and thin lines represent functions fitted to the forward- and simultaneous-masked data, respectively. Error bars smaller than the data point symbols are omitted for clarity.

Following Oxenham and Plack , the GOM function for the off-frequency masker may be assumed to be an estimate of the shape of the BM response function for the signal at the place tuned to the signal. The slope of the off-frequency GOM function is an estimate of the compression exponent, c. The value of c was calculated by taking the first derivative of the third-order polynomial fits to the off-frequency simultaneous- and forward-masked GOM data.

The derived values of c , plotted as a function of signal level, are shown in Figure 2. Each panel of Figure 2 presents two compression functions for a given subject, with thin and thick lines representing values of c for a simultaneous or forward masker, respectively.

Compression estimates are also low for high-level signals with a simultaneous masker for all subjects and a forward masker for I. The estimates of c greater than 1, as seen in the high-level off-frequency data for C.

Variability may be partly responsible: The polynomial fits were pushed into the final steep phase largely by the highest one or two data points.

Compression functions for individual subjects. Each panel shows the value of c as a function of signal level. Thin and thick lines represent values of c for a simultaneous Sim and forward masker For , respectively. For all subjects, compression is greatest for mid-level signals 35—60 dB SPL , for either a simultaneous or forward masker.

For mid-level signals, maximum forward-masked compression exponents average 0. Over the same range, the simultaneous-masked compression exponents average 0. However, there are differences between individuals. The estimates of signal compression are greater for a forward than a simultaneous masker for A.

For I. However, in the case of the off-frequency forward-masking thresholds for this subject, the slope of the best-fitting polynomial seems to be slightly greater than the slope inherent in the data at mid levels. To estimate the amount of suppression as a function of signal level, the third-order polynomial functions fitted to the off-frequency data were used to generate values of masker levels for a range of signal levels.

The amount of suppression for a given signal level was calculated by subtracting the interpolated masker level for the off-frequency simultaneous condition from the masker level for the off-frequency forward condition. The values of suppression calculated in this manner indicate the decrease in the BM response to the signal at the signal place. Because the off-frequency masker threshold can be taken as an estimate of the BM response to the signal Oxenham and Plack , the decrease in the physical level of the off-frequency masker from a forward- to simultaneous-masking condition is assumed to be equivalent to the decrease in the BM response to the suppressed signal.

The amount of suppression as a function of signal level is shown in Figure 3. The maximum amount of suppression ranges from about 6 to 17 dB across subjects. The amount of suppression as a function of signal level. Lines of differing thickness represent individual subject data. Despite individual differences, there are two main consistent features of the data. First, the on-frequency simultaneous- and forward-masked GOM functions are linear. Previous studies with a brief signal and a short masker-signal gap have also reported linear on-frequency forward-masked GOM functions Oxenham and Moore ; Oxenham and Plack , ; Plack and Oxenham Linear masking functions can be explained by the equivalent rate of BM response growth for both the on-frequency masker and the signal at the signal place Plack and Oxenham ; the BM response to both the masker and signal grows either linearly for low-level signals or compressively for mid-level signals.

The linear increase in level of the simultaneous on-frequency masker with increase in signal level was similar to that reported by Oxenham and Plack ; ms 4-kHz signal, Hz noise masker. However, Oxenham et al. Similarly, in van Klitzing and Kohlrausch's study, for a 2-ms signal masked by a simultaneous frozen-noise masker, a nonlinear response was particularly evident for mid-level maskers; a greater increase in masked threshold was observed for an increase in masker level from 30 to 50 dB than for an increase in masker level from 50 to 70 dB.

He was also very interested in modeling, or, in other words, creating formulas to describe sensorineural hearing loss. Very importantly in that paper, Plomp talked about the A and D components of sensorineural hearing loss.

The A and D components refer to the attenuation that is created by the threshold change plus the distortion that occurs above threshold. In other words, what sort of distortion does the peripheral auditory system add to the signal? Distortion can happen in a lot of places in the auditory system within the ascending tracks in the central auditory system, but I am going to the limit my comments today very specifically to the distortion that is added by the peripheral auditory system.

There is a lot of distortion added by the peripheral auditory system. The nature of that distortion and the amount of that distortion varies significantly from person to person with sensorineural hearing loss, and it can even vary significantly from one side of the head to the other.

I am doing another talk in this series, specifically focusing on asymmetrical hearing loss. Therefore, I am not going to spend a lot of time talking about asymmetries in the auditory system today. I do want to quickly point out that these distortional aspects that happen in the auditory system can even vary from one side of the head to the other. Plomp was very interested in modeling the effects of distortion and trying to understand if we could predict those effects. One of his great conclusions Plomp, was that the distortional aspect that comes along with sensorineural hearing loss is not very well predicted by the amount of threshold hearing loss that a person has.

It is true that as you have more threshold hearing loss, the nature of the distortion and the amount of distortion in the auditory system has a tendency to be greater, but the relationship between threshold and the amount of distortion is really quite weak.

Importantly, Plomp talked about the distortional aspect of hearing loss, or the D component, as a measured speech-in-noise ability. It is important to keep in mind that aided sound field thresholds are performed with relatively low-level narrow band test signals.

Thus, aided threshold will tell us something about the lowest level of sound that a child can detect in the aided condition as a function of frequency. However, because we are now for the most part using multi-channel compression technology, the aided threshold obtained to low-level narrow band test signals is of very limited utility in predicting how the hearing instrument will operate in response to real speech inputs. Both gain and frequency response characteristics will likely change as the type and level of the input signal changes.

They can be particularly helpful to us in studying the relative levels of low versus high frequency output from multi-channel WDRC instruments and thus assist us in evaluating the appropriateness of the electroacoustic fitting. Reference Skinner M. For the past 25 years, Dr. Seewald's work has been focused on issues that pertain to the selection and fitting of amplification in infants and young children and is known internationally for his work in developing the Desired Sensation Level DSL Method for pediatric hearing instrument fitting.

Recorded Webinar. This course will provide an overview of cortical auditory evoked potentials, current research, benefits and limitations to using CAEPs in a busy clinic, and several case studies. Course Details. Information from these studies will be presented. Although remote microphone systems are primarily recommended for use in school settings, recent studies have shown potential benefits related to their use in the home environment.

Issues that maximize educational and communication outcomes for school-aged children will be highlighted. It will discuss means for verifying settings in hearing assistive technology and means for improving patient-provider communication.

Jorgensen, AuD, PhD.