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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 23  |  Issue : 3  |  Page : 162-167

Effects of nonlinear amplification on differential sensitivity measures in individuals with cochlear hearing impairment


1 Department of Audiology, All Institute of Speech and Hearing, Mysore, Karnataka, India
2 Department of Audiology, Rehabilitative Health Sciences, College of Applied Medical Sciences, KSU, Riyadh, Saudi Arabia

Date of Web Publication31-Aug-2017

Correspondence Address:
N Devi
All India Institute of Speech and Hearing, Mysore, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/indianjotol.INDIANJOTOL_2_17

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  Abstract 

Objective: The current study focused on exploring the influence of multichannel amplification with wide dynamic range compression (WDRC) on frequency, intensity, and duration discrimination measures at two frequencies in different channels of amplification. Methods: Discrimination measures were performed on twenty ears with moderate flat sensorineural hearing loss using 2-down and 1-up adaptive procedure to achieve 70.7% response on the psychometric function. All stimuli were generated through AUX script and presented using Psycon version 2.18 experimental software and delivered through the speakers oriented at an angle of 0° azimuth. Discrimination measures were performed in both aided and unaided conditions for two pure tone frequencies of 750 and 1500 Hz with 500 ms duration (on/off ramp duration of 80 ms to avoid spectral splatter) presented at a level of 20 dBSL (with reference to the threshold separately in each condition). The responses were recorded through three interval alternative forced choice paradigm with visual feedback. An average of final four reversals out of eight was considered for discrimination threshold estimation. Results and Conclusions: Results revealed a statistically significant deterioration in intensity discrimination and a significant improvement in difference limen for frequency and duration with nonlinear amplification. The deteriorated intensity discrimination scores were explained in perspectives of WDRC acting in amplification device.

Keywords: Duration discrimination, frequency discrimination, intensity discrimination, just noticeable difference in cochlear hearing loss and amplification affects


How to cite this article:
Devi N, Amritha G, Tanniru K. Effects of nonlinear amplification on differential sensitivity measures in individuals with cochlear hearing impairment. Indian J Otol 2017;23:162-7

How to cite this URL:
Devi N, Amritha G, Tanniru K. Effects of nonlinear amplification on differential sensitivity measures in individuals with cochlear hearing impairment. Indian J Otol [serial online] 2017 [cited 2020 Feb 25];23:162-7. Available from: http://www.indianjotol.org/text.asp?2017/23/3/162/213865


  Introduction Top


Cochlear hearing loss not only reduces the audibility but also alters the perception of speech sounds. Individuals with cochlear hearing loss often complain that the speech being distorted, unclear, and uncomfortably loud. These complaints remain partially unsolved even with appropriate amplification. Amplification increases only audibility, but the natural perception of sounds may not be restored completely.[1] Advanced digital signal processing technology uses many new features such as wide dynamic range compression (WDRC), multichannel compression, noise suppression, and adaptive directionality. Among these, the widely clinically used is WDRC. WDRC feature applies amplification to soft sounds and low gain for louder sounds, thus enhancing the use of narrow dynamic range of an individual with hearing impairment. As dynamic range varies across frequencies, along with WDRC multichannel compression allows the appropriate compression to be applied at different frequencies.[2],[3],[4]

The minimum change in the absolute physical quantity in terms of intensity, frequency, and duration which is required to perceive two sounds as different is accounted by measuring the difference limen or just noticeable difference (JND). In individuals with cochlear hearing loss, with several methods to measure the JNDs, it is found that cochlear hearing loss performed worse than that of normal if the comparison is at a given sound pressure level, but if we are comparing at equal sensation levels, the cochlear hearing loss performs similar or even better than that of normal.[5],[6] However, a significant effect of hearing loss on duration discrimination tasks using difference limen for duration (DLD) when compared to age-matched normal.[7] With the understanding of the consequences of cochlear hearing loss, it is recommended that suprathreshold measures should also be incorporated to track the functional problems, not just the audibility alone.[8]

As WDRC uses different gain values to the incoming signal depending on the level of the signal and temporal characteristics of the signal, the differential sensitivity abilities of hearing aid wearer might vary depending on the compression. Different researchers have explored the effect of amplification, on intensity discrimination and also studied the long-term effect on level discrimination and the effect of compression parameters.[9],[10] However, there are considerable controversy in this aspect and no any clear evidences available in the current literature to substantiate on frequency and duration discrimination.

Hence, it is important to study the differences in intensity, frequency, and duration discrimination thresholds on individuals with cochlear hearing loss with amplification device, and it is also important to know the extent to which the current amplification technology can restore normal hearing. Thus, the present study aimed at exploring the impact of digital hearing aid on frequency, intensity, and duration discrimination for individuals with cochlear hearing loss.


  Methods Top


Participants

A total number of twenty ears were included in the study. All the participants were native speakers of Kannada and had no previous experience with any amplification devices. The participants were aged between 18 and 55 years (mean: 38.27 years and standard deviation [SD]: 10.11). Equal numbers of male and female participants were included in the study. The participants were selected if they have moderate sensorineural hearing loss (SNHL) with flat configuration for the frequency range of 250 Hz to 8 KHz in the better ear (air conduction thresholds in the range of 41–55 dBHL and air-bone gap of ≤10 dBHL) and their speech recognition scores ≥70%. Testing was carried out in a sound-treated room.[11] Before the evaluation and experimental procedures, written consent was obtained from all the participants for their willingness to participate in the study.

Test procedure

Audiological assessment

Detailed Audiological assessment was carried out before the experimental procedures for all participants. Pure tone thresholds were estimated using GSI 61 Clinical Audiometer for both air conduction and bone conduction thresholds using modified Hughson and Westlake procedure.[12] Immittance audiometry was carried out using GSI TympStar to rule out any middle ear pathologies on the day of testing.

Hearing aid programming

A digital nonlinear four channel behind the ear programmable hearing aid was used for the study. The hearing aid was connected to a personal computer through Hi Pro. All the settings in the hearing aids were made with NOAH software (NOAH version 4.3.0, HIMSA: Denmark). At the initial fitting session, the gain values were set based on the NAL-NL1 nonlinear fitting prescriptive formula.[13],[14] The acclimatization level was set to two out of the five levels. The compression threshold was 55 dB and compression ratio was 2:1 with short rise and fall time and soft knee point were the WDRC parameters for all the participants.

Verification of fitting

Real ear measurements were performed using calibrated FONIX 7000 real-ear analyzer and digi speech as stimulus. Participants were positioned at a distance of about 0.5 m from the loudspeaker, and the probe tube was inserted within 5 mm of the tympanic membrane to avoid standing waves. The real ear measurement procedure was carried out.[15] The hearing thresholds of the participant were manually entered into the database of real-ear analyzer, and the hearing aid gain values are increased to match the target values specified by NAL-NL1.

Aided and unaided speech recognition scores

Unaided and aided speech recognition scores were obtained for the participants. Twenty-five words from phonetically balanced word list [16] were used as the stimuli for speech recognition scores. Those participants, who scored ≥70% in aided speech recognition scores with the optimum gain according to the prescriptive procedure NAL-NL1, were considered for the study.

Difference limen measures

During experimental procedures, measures to find difference limen for frequency, intensity, and duration were carried out. All the measures were performed with the use of Psycon software experimental version 2.18 installed on to personal laptop (Sony Vaio, model-SVE14IJ11W).[17] The stimuli were generated by the Psycon software through AUX script and presented at 20 dBSL through the speakers oriented at an angle of 0° azimuth with reference to the nasion of the participant. For the participants with diagnosed cochlear hearing loss, the stimuli were presented at a level of 20 dBSL with reference to aided threshold at the desired frequency for aided difference limen measurements.

All the differential sensitivity measures (difference limen for intensity [DLI], frequency, and duration) were estimated at two different frequencies (750 and 1500 Hz). Stimuli were pure tones of 500 ms duration with 80 ms of symmetrical ramp duration to avoid spectral splatter at onset and offset. These stimulus frequencies were selected as these do not interfere with the crossover frequencies of the channels in the multichannel aid used for this study. Three interval alternative forced choice (3IAFC) method was used for all three measures with 2-down and 1-up procedure to achieve 70.7% response on the psychometric function.[18] The adaptive procedure was selected to obtain a precise measure. The session was limited to 100 trials. The final four reversals out of eight were considered to obtain the mean score and SD. If the SD was more than 2.5, the run was repeated.

All the participants were given three practice trials and demonstrated the correct response at 40 dBSL which was clearly audible. Once the participant was familiarized with the task, the actual testing began at 20 dBSL. The sampling rate was set to 44100 Hz and interstimulus interval was maintained at 500 ms and intertrial interval at 400 ms. Aided scores were considered to be of prime importance for the study; hence all measures such as threshold and difference limen were performed in free field. The participants were instructed that “three blocks will be appearing on the screen and you will be hearing tones in all the three blocks, but one among them is different in terms of either its pitch or loudness or duration, you have to select the block which has a different tone among the three blocks presented.” Visual feedback regarding their correctness of the response was provided after each response. The absolute difference limen scores for intensity (dB SPL), frequency (Hz), and duration (ms) were documented.


  Results Top


The data obtained were tabulated and analyzed using Statistical Package for the Social Sciences (SPSS) version 17 software (SPSS Inc. Released 2008. SPSS Statistics for Windows, version 17.0. Chicago: SPSS Inc.). The Shapiro–Wilk test of normality was done which showed nonnormal distribution. Hence, Wilcoxon signed-rank test was administered to compare the different parameters within the group. The measures obtained were T750 - threshold at 750 Hz (dB SPL), ΔI750 - absolute DLI (dB SPL), ΔF750 - absolute difference limen for frequency at 750 Hz, and ΔT750 - absolute DLD or time at 750 Hz, in ms. Similar measures were obtained for 1500 Hz also. The following comparisons were done using appropriate statistical measures.

Comparison of aided and unaided absolute difference limen scores

In unaided condition, the ΔI ranged from 1 to 4 dB SPL; ΔF ranged from 45 to 100 Hz at both the test frequencies, whereas the ΔT ranged from 80 to 198 ms at 750 Hz and 80–120 ms at 1500 Hz. The aided absolute difference limen scores were obtained using a four channel digital behind the ear hearing aid programmed and optimized according to the loss of the participant. The aided absolute difference limen scores were obtained at 20 dBSL for the test frequencies of 750 and 1500 Hz with reference to the aided threshold at the frequency. The mean and SD of the scores are depicted in [Table 1].
Table 1: The mean and standard deviation of unaided and aided difference limen scores

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The results revealed that the aided scores were less than the unaided scores for absolute difference limen for frequency and duration both at 750 and 1500 Hz which indicates an improvement in frequency and duration difference limen with amplification. However, the absolute DLI in aided condition is greater than that of unaided condition showing an increase in the magnitude of intensity difference limen at 750 and 1500 Hz in other words, DLI deteriorated with amplification. Wilcoxon signed-rank test was used to test the significance between aided and unaided difference limen scores within the group. The results of Wilcoxon signed-rank test is as depicted in [Table 2].
Table 2: Results of Wilcoxon signed-rank test for aided and unaided difference limen scores

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It can be inferred from [Table 2] that there is a significant difference between aided and unaided conditions for absolute DLI, frequency, and duration at 750 and 1500 Hz (P < 0.05) using Wilcoxon signed-rank test, i.e., there is a significant improvement in frequency and duration difference limen; however, there is a significant deterioration in intensity difference limen.

Comparison of difference limen scores between 750 and 1500 Hz

The differences between 750 and 1500 Hz in terms of the difference limen scores were studied in unaided condition. There was no much difference in magnitude between 750 and 1500 Hz in terms of threshold, difference limen for frequency, and DLI; however, there is a great difference between 750 and 1500 Hz in terms of DLD at 750 and 1500 Hz. Wilcoxon signed-rank test was used to test the significance between 750 and 1500 Hz. The results of Wilcoxon signed-rank test is represented in [Table 3].
Table 3: The results of Wilcoxon signed-rank test of unaided difference limen scores between 750 and 1500 Hz

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The Wilcoxon signed-rank test of significance revealed that there was a significant difference between 750 and 1500 Hz only in terms of DLD (P < 0.05). However, there is no significant difference between 750 and 1500 Hz in terms of threshold, DLI, and frequency (P > 0.05). A comparison of results obtained for different parameters is shown in [Table 4].
Table 4: Comparison of results obtained for different parameters considered for the study

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Overall, the results of the study revealed that there is a significant difference between aided and unaided differential sensitivity with an improvement in frequency and duration discrimination and deterioration in intensity discrimination at 20 dBSL with the multichannel nonlinear hearing aid. There was no significant difference between the frequencies tested for intensity and frequency difference limen. However, a significant difference was noted in duration discrimination between the test frequencies.


  Discussion Top


In individuals with SNHL, the perception of sound is being altered, and this is not only limited to the reduced audibility. Hence, it has been suggested that the normal perception of sound cannot be completely restored even with an adequate amplification.[1] The threshold measures would not always replicate the functional status of cochlea; hence discrimination measures which would provide much more information than just audibility.[8] The smallest perceivable difference between two signals is termed as difference limen or JND.[19] It could be either in terms of frequency, intensity, or duration. However, this ability to discriminate depends on the characteristics of the reference signal and also on individual's ability.[20] The literature proved that this ability to discriminate small changes will be adversely affected in those with SNHL where the resolving power of the ear is compromised.[21] Hence, the present study was designed to check whether an adequate amplification, using a multichannel programmable digital hearing aid, would restore the normal discrimination in postlingual adults with moderate SNHL.

Twenty ears with moderate SNHL were considered for the study. Routine procedures including pure tone audiometry, immittance audiometry, otoscopic evaluation, and speech audiometry were done before the difference limen measures for participant selection. The difference limen measures were carried out using Psycon software version 2.18 using a forced choice paradigm. There could be a difference in thresholds obtained using the software and audiometer because the former is calibrated in dB SPL and latter in dBHL. Hence, thresholds were obtained using the Psycon software using an adaptive procedure, i.e., 2–down and 1-up 3IAFC method. The adaptive procedure was incorporated targeting on the precision. The stimuli used were 750 and 1500 pure tones presented through loudspeaker oriented at 0° azimuth. These frequencies were selected such that the target frequencies should be in the mid-frequency range and should not overlap with the crossover frequency of the hearing aid channel separation.

For both the groups, the difference limen measures were administered for both desired test frequencies of 750 and 1500 at 20 dBSL (re: Threshold using adaptive procedure). The individuals were fitted with a digital programmable hearing of four channels, and the hearing aid was selected based on its electroacoustic characteristics to suit moderate degree of hearing loss. The hearing aid was programmed for each individual, optimized the gain, and verified using real ear measures. The hearing aid was kept constant for all the participants in the group. Unaided and aided difference limen scores were obtained for the participants. The absolute difference limen scores for frequency, intensity, and duration were obtained for the group at 750 and 1500 Hz. Statistical analysis was done using Statistical Package for the Social Sciences version 17. Two within group comparisons were done for the obtained scores, i.e., unaided versus aided and 750 versus 1500 Hz. Wilcoxon signed-rank test was administered.

Comparison of aided and unaided difference limen scores

The aided and unaided difference limen scores were compared using Wilcoxon signed-rank test. The results revealed that there was a significant difference between unaided and aided difference limen scores for intensity, frequency, and duration (P < 0.05). There was a considerable improvement in the difference limen scores for frequency and duration between both the conditions and a decrement in intensity discrimination with aid.

The performance for these individuals ranged from 1 to 4 dB SPL. This better performance is because of the recruitment or abnormal loudness growth in them which is more evident in low sensation levels.[5],[6],[22] There was a significant decrement in the DLI, i.e., aided DLI showing poorer scores than unaided condition. This could be because of the WDRC acting on the hearing aid. This change in perception with the hearing aid of WDRC is supported by the fact that with the compression, the difference in level between two sounds will be lessened which could be one of the explanations for the increased absolute DLI with the amplification. This viewpoint is that there is an increased JND for level discrimination with increasing the compression ratios. The authors found a clear effect of compression ratio and attack time on JNDs for interstitial lung disease.[10]

The three major characteristics of the compression which is acting on hearing aids are compression ratio, attack time, and release time. There should be further evidence required to justify that the detrimental effect of DLI with aid is due to compression because the current study did not focus on the effect of compression parameters on intensity discrimination. With the current knowledge, this effect of compression on level discrimination is inconclusive because of the scarcity of literature and contradicting studies that an effect of compression on level discrimination of words and sentences which was justified that it could be because of the low compression ratios acting on current hearing aids.[23] However, there is an improved level discrimination for high-frequency complex at high sound pressure level for normally aided ears and which was not found for the low-frequency complexes and at low SPLs at high frequencies in bilateral sloping SNHL individuals who were fitted monoaurally.[9] These findings were attributed to perceptual learning with the hearing aid. Putting all these together and with the findings of the current study, it can be expected that when changing the amplification from linear to nonlinear can have an effect on the discrimination of level differences.

DLF was adversely affected or the absolute value of DLF was abnormally large for all individuals. This could be because of the reduced frequency resolution in individuals with SNHL due to widened auditory filters as a consequence of structural damage.[24],[25],[26],[27],[28] The data showed a wide range even with participants of similar degree of hearing loss [25] who reported that there is no one-to-one correlation between threshold and DLF and this could be differences in the filter bandwidth. However, there is no one-to-one correlation even with filter bandwidth and DLF.[21] It could also be due to changes in phase locking information for large DLF.[29] The abnormally large DLF of 100 Hz or above was observed specifically in individuals with asymmetrical hearing loss. This could be due to diplacusis seen in them which became evident when presented in free field. There was a considerable improvement in DLF with the aid which was statistically significant. This could be either due to the increased audibility due to the gain provided by the hearing aids or due to the effect of multiple channels. However, further evidence is required to substantiate this viewpoint and due to scarcity in literature. The individuals required large differences in duration to discriminate between the stimuli.[25] This could be due to poor phase locking and defective channel capacity to separate onset and offset of stimuli by hearing impaired. Again, this could be because of the increased audibility.

Comparison of differences in the difference limen scores between 750 and 1500 Hz

The comparison of difference limen scores between 750 and 1500 Hz showed a significant difference only in terms of duration difference limen and not in terms of intensity and frequency difference limen. The DLF at 750 and 1500 Hz did not show any statistically significant difference (P > 0.05) due to the widened filter bandwidth which is supported by the literature. There could be various other factors which would have influenced the study such as heterogeneity of the participants in terms of onset and cause of hearing loss, recruitment, and diplacusis; other nonaudiological factors such as motivation, attention, and memory. Further studies are required to validate the results of the study.


  Summary and Conclusions Top


The present study focused on exploring whether an appropriate amplification will restore the normal discrimination in a group of individuals with moderate SNHL and also checked whether there is any effect of frequency on difference limen measures between 750 and 1500 Hz. The results revealed that there was a significant difference in aided and unaided difference limen with an improvement in frequency and duration discrimination and deterioration in intensity discrimination which could be due to the WDRC. A significant difference was obtained between 750 and 1500 Hz in terms of duration discrimination. However, there was no significant difference in DLF between the frequencies which could be widened filters. The decrement in intensity difference limen was attributed to the compression acting in the hearing aid used. However, the current data are not sufficient to substantiate this. For this, we need to validate the results at different compression thresholds and compression ratios and compare the results with linear hearing aids. The scores of frequency and duration difference limen improved which was attributed to the use of amplification device. However, with the use of just one hearing aid, one cannot generalize this finding. Hence, the results have to be validated using different multichannel aids varying in the number of channels and different types of aids, etc. Considering the time constraints, the study was conducted only in two mid-octave frequencies, i.e., 750 and 1500 Hz and only in one intensity level, i.e., at 20 dBSL. This study has to be extended to check the frequency and intensity effects. This study was conducted only in one group of participants with moderate SNHL and only in one age group. Hence, the study has to be extended to different degrees, configuration of hearing loss, and to different age groups. The study considered a basic pure tone as the stimuli, which should be extended to different types of complex stimuli such as modulated signals, noise, and speech.

Clinical applications

  1. Counseling tool: Creating realistic expectations in patients and family is a difficult task because most often there is a misconception that with the hearing aid, the normal perception would be restored. This study can be presented as an evidence to show that even with amplification, the normal perception could not be completely restored; however, there is a benefit from amplification and an overall improvement
  2. Hearing aid benefit: In hearing aid trials, to check whether a particular hearing aid is beneficial or not we do speech tests, aided thresholds, or real ear measurements. If the participant is nonverbal and speech tests could not be done, along with pure tone aided thresholds, these measures can be done so that it gives more information rather than just audibility. If there is an improvement in the discrimination abilities, this could be even reflected in real life situations
  3. Monitoring of hearing aid use: Many studies have shown that there will be an overall improvement in the discrimination scores in frequency domain with acclimatization. If we are using a pure tone to quantify this, there may not be a significant change in the aided thresholds and is difficult to monitor it, if we are using a speech material to monitor, the participants can get used to the material used and shows better scores. However, with this measure of discrimination, the chance factor of false response and the chance to adapt to the material is minimal. Hence, it serves as a good measure to account for the effect of acclimatization.


Acknowledgments

The authors would like to thank the Director, All India Institute of Speech and Hearing, Mysore, for granting permission to carry out the study and the participants for their cooperation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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