|Year : 2015 | Volume
| Issue : 2 | Page : 92-97
Relationship between behavioral measure of ANL and its physiological mechanism in the normal hearing participants
Vishal Kooknoor1, Hemanth Narayan Shetty2
1 Department of Speech and Hearing, Karnataka ENT Hospital and Research Centre, Chitradurga, Karnataka, India
2 Department of Audiology, All India Institute of Speech and Hearing, Mysore, Karnataka, India
|Date of Web Publication||20-Apr-2015|
Karnataka ENT Hospital and research Centre, Chitradurga - 577 501, Karnataka
Source of Support: None, Conflict of Interest: None
Context: In previous studies, the efferent mechanism was speculated on low and high acceptable noise level (ANL) groups. Thus, an attempt is made to measure the auditory afferent and efferent mechanisms indirectly through event related potentials on same individuals of different ANL groups. Aims: To study the relationship between behavioral measure ANL and its physiological mechanism. Settings and Design: One shot partly replicative and correlative research design was utilized. Subjects and Methods A total of 40 normal-hearing individuals were classified into low, high and average groups based on ANL. From each study participant the contra-lateral suppression of otoacoustic emission (CSOAE), auditory brainstem response in different fixed levels (50, 55, 60 and 65 dB HL) and also in 5 dB noise above each fixed level of intensity were measured. Kruskal-Wallis test and Pearson correlation statistical analysis were used. Results: The mean amplitude of CSOAE was larger in the lower ANL group than compared to other groups, but this difference was not significant. Further, it was found that the slope of V-A (quiet) in the low ANL group was significantly steeper than the high ANL group (P < 0.05). Similar result was found in slope of V-A (noise) . In addition, the Pearson correlation coefficient result revealed that there was no correlation between global ANL and CSOAE. However, a moderate negative correlation was found between global ANL and the slope of V-A (quiet) and; in slope of V-A (noise) . Conclusion: The slope of V-A in quiet and in noise conditions was found steeper in low ANL group indicating stronger auditory afferent and efferent auditory pathway at the central level.
Keywords: Afferent system and efferent system, Annoyance, Slope, Suppression
|How to cite this article:|
Kooknoor V, Shetty HN. Relationship between behavioral measure of ANL and its physiological mechanism in the normal hearing participants. Indian J Otol 2015;21:92-7
|How to cite this URL:|
Kooknoor V, Shetty HN. Relationship between behavioral measure of ANL and its physiological mechanism in the normal hearing participants. Indian J Otol [serial online] 2015 [cited 2019 Nov 20];21:92-7. Available from: http://www.indianjotol.org/text.asp?2015/21/2/92/155292
| Introduction|| |
Audiology as a discipline has grown by leaps and bounds in recent years. Technological development has contributed a lot to the audiologist's success with their clients. Hearing aids are one example that have had a tremendous impact on the audiologist's reach and functioning. Though the number of patients who have benefitted from hearing aids has increased, there are still some issues with hearing aid fitting and acceptance by hearing impaired individuals that persist. There are many instances where the hearing loss individual reports dissatisfaction over his hearing aid, even though the audiologist prescribed a hearing aid after much deliberation and following the best fitting protocols. There is a lack of information on the prevalence of dissatisfaction among hearing aid users in India, but western studies report that 23.65%  individuals fitted with hearing aids fall into this group. One of the common areas where the speech perception deficit gets wide attention for hearing impaired individual is when listening to background noise. This is the reason why audiology research is still looking for better clinical tools in the area of hearing aid fitting and outcome measure evaluation.
Previously the improvement in speech recognition was the single most factor important for a hearing aid benefit in the hearing impaired individual. Though we measure speech perception ability in various conditions, it does not accurately reflect the hearing aid benefit. Bentler et al.,  and Humes et al.,  reported that there is no relationship between hearing aid benefit and speech identification scores in noise. Further, it is often difficult to know whether the patient rejects the device due to inadequate benefit or due to annoyance with the hearing aid. In a recent study Kochkin  reported that 23% hearing aid users reject their device though their speech identification scores are better. Among them, 63% of hearing aid users were unwilling to wear the hearing aid because of background noise. Thus, knowing the annoyance level of background noise using acceptable noise level (ANL)  will give information regarding the patient's acceptance of hearing aids while using it in the background noise. Individuals who have lesser scores on ANL are more likely to use hearing aids in background noise listening conditions  and use their hearing aid for the longer term.  Further, the ANL aims at identifying individuals who have an inherent ability to withstand background noise while listening to running speech presented at the most comfortable level. It is a relatively easy measure to obtain among all age groups. In addition, ANL test is a good predictor tool regarding hearing aid usage at the time of prefitting.  Conventional ANL values were not related to age  gender  hearing sensitivity,  or type of background noise.  The most important part is that conventional ANL in unaided condition does not change with the introduction of hearing aid.  Thus, in recent times ANL is gaining popularity in hearing aid practice and research.
Before understanding the mechanism of ANL in individuals with hearing loss and in hearing aid users, it is important to know the physiological process involved in ANL in normal hearing subjects. Harkrider and Smith  and Harkrider and Tampas  explored the efferent and afferent mechanism involved in ANL on individuals with normal hearing. Harkrider and Smith  studied efferent auditory system in individuals having different values of ANL. They recorded acoustic reflex and contra-lateral suppression of otoacoustic emission (CSOAE). The result revealed that lesser level of noise acceptance is due to reduced activation of acoustic reflex pathways and/or suppression mechanism of efferent neurons of medial olivary complex (MOC) bundles. In yet another study, Harkrider and Tampas  reported that ANL is not influenced by peripheral structure, auditory nerve and the cochlear nucleus of auditory brainstem. However, at the upper auditory brainstem region, different response was noted among the ANL groups, and this was captured in the amplitude and latency of the V peak of auditory brainstem response (ABR). To be specific, the larger V peak of amplitude was noted in individuals with higher ANL scores (i.e., above 13 dB). They speculated that either more excitation in afferent auditory pathway or less inhibitory mechanism of efferent nerve fibers upon the afferent neurons. However, in their study, the efferent mechanism was speculated on without evaluating the MOC pathway. Additionally, the ABR elicited for tone bursts (500 Hz and 3000 Hz) at low and high intensities was correlated with the ANL obtained from the running passage. There was a discrepancy in the stimulus used in recording ABR and in obtaining the ANL value. Because of procedural variability, the present study is taken up as replication of previous studies to control the confronting variables to explain the physiological mechanism of afferent and efferent mechanisms involved in ANL on the similar group of participants.
The use of speech stimuli in obtaining ANL and electrophysiological measure is in proximity to the natural context. Further, recording complex ABR (ABRc) provides information on accurate encoding of timing and frequency features of speech at the upper auditory brainstem. Complex ABRc to speech stimuli also reflects the aggregate of neural synchrony compared to that for clicks and tone bursts stimuli.  For the speech stimulus, acoustic masking is common due to the iteration of stimuli during the acquisition of ABRc response. In analyzing the peak, V alone of ABRc investigates the synchronous response pertained to onset of speech stimulus. However, in calculating the slope from the amplitudes and latencies of discrete peaks of V and A (ABR slope = amplitude [V-A]/latency [V-A]) reflect synchronization, transmission of neural activity of lateral lemniscal input to inferior colliculus (IC) and the subsequent summation of underlying neural activity.  Thus, it is hypothesized that the slope of V-A measured in quiet using speech stimulus elicited from individuals who accept more noise are different from those individuals who do not accept more noise.
Further, the study of Harkrider and Tampas  compared the ANL test with the event related electrophysiological measure elicted in quite condition. Thus, to have uniformity among the behavioural and electrophysiological tests, the responses were obtained in noise. In the present study, the slope of V-A was measured in the presence of broadband noise delivered to the ear contra-lateral at 5 dB sensational level (SL) with reference to presentation level of speech stimulus. This is because the contra-lateral noise activates the medial olivary complex (MOC) of the auditory brainstem. It has an inhibitory effect on contra-lateral outer hair cells and the subsequent response of afferent auditory neurons.  Thus, it is hypothesized that slope of V-A (noise) might be steeper for those individuals who accept more noise than those who do not accept noise.
Thus, the aim of the study is to determine the relationship between the behavioral measure of ANL and physiological measure at the cochlear and auditory brainstem levels in the normal hearing participants. The following objectives were formulated to compare: (a) The response of global ANL and CSOAEs (b) the behavioral response of ANL and slope of V-A (quiet) ; slope of V-A (noise) and (c) To correlate the response of global ANL and physiological responses (CSOAE; slope of V-A (quiet) ; slope of V-A (noise) ).
| Subjects and Methods|| |
A total of forty normal hearing participants within the age range of 15-30 years were included in the study. All participants had normal hearing sensitivity in both ears (air conduction thresholds of 15 dB HL or less for the pure tones at octave frequencies from 0.5 to 8 kHz); normal middle ear status (i.e., type A tympanogram and static admittance between 0.5 and 1.75) with measurable reflexes in ipsi- and contra-lateral ears from 0.5 to 4 kHz in octave frequencies. No other known otological or neurological deficits were reported by participants. All the participants were native speakers of Kannada.
The following procedure was utilized to assess the physiological auditory efferent and afferent auditory processing in individuals having varied annoyance level. The study was carried out in two phases. In Phase-I, behavioral ANL procedure was carried out. In phase-II, physiological responses were obtained from each participant. All the measurement was carried out in an acoustically treated room, where the level of ambient noise is well within the permissible limit (ANSI 1999).
Phase-I: Acceptable noise level behavioral procedure
Acceptable noise level was obtained at four fixed presentation levels 50, 55, 60 and 65 dB HL.  At 50 dB HL fixed presentation level, running standardized Kannada passage  was delivered. Each participant was instructed in the following manner: "You will listen to a Kannada passage through an insert earphone. The level of Kannada passage will be constant throughout the testing." The broadband noise was presented ipsilateral at 30 dB HL and gradually the level of noise was increased to obtain background noise level (BNL): The following instruction was provided to obtain BNL. "Inform the level of background noise that is, the most you would be willing to accept or "put up-with" without becoming tense and tired while following the Kannada passage." The ANL was calculated by subtracting the BNL from 50 dB HL fixed presentation level. A similar procedure was carried out at different fixed levels of presentations (i.e. 55, 60 and 65 dB HL).
Phase-II: Physiological procedure
Contra-lateral suppression of otoacoustic emission
Otoacoustic emission was collected for condensation click of 100 μs duration at 60 dB equivalent SPL. The presentation rate of the clicks was 10/s. Clicks of 250 sweeps were delivered to the test ear using Etymotic Research-2 insert earphone. The response window to the set of 4 clicks (linear manner) was summed with the previous set of clicks in that sequence. If the noise level exceeded 45-52 dB P SPL, there was a pause in the recording which continued once the noise level reduced below the rejection level. OAE data were considered if the accepted number of sweeps was above 85%. Three trials were obtained in the test ear on the same participants to check the reliability of OAEs. A similar procedure was carried out in the presence of broadband noise delivered at 65 dB SPL (i.e. 5 dB above the click level) in the contra-lateral ear through an insert earphone.
Auditory brainstem response
Before acquiring ABR, the threshold for synthetic speech stimulus/da/  and also for noise were obtained. Each participant was made to sit in a reclining chair. The electrode site was cleaned using skin preparing gel. The silver chloride electrodes were placed using conduction paste. The ABR was recorded by placing noninverting electrode on the vertex (Cz), inverting electrode (Ai) on the test ear mastoid and ground electrode on the forehead. Participants were instructed to relax throughout the test without any body movement. The synthetic stimulus/da/was presented in alternating polarity to the test ear through the insert receiver at 50 dB HL (reference: SL) at a repetition rate of 7.1/s till 1500 sweeps. The ABR was captured by assigning prestimulus recording window of 5 ms and poststimulus recording window of 15 ms within the filter setting of 100-3000 Hz. A gain of 1 lakh was provided with an artifact rejection of ± 29.7 μV to each epoch. The average epoch should be considered for analyses if the accepted number of sweeps is 90% (i.e. 10% artifact rejection was acceptable for considering further analysis). For reliability, the recording was repeated. Further, the ABR was obtained in the presence of noise delivered to the contra-lateral ear at 5 dB SL with reference to a fixed presentation level. A similar procedure was carried out at each fixed presentation level of 55, 60 and 65 dB HL (reference to SL). In addition, the ABR obtained at each fixed level in the presence of broadband noise was delivered to the ear contra-lateral (5 dB SL with reference to fixed presentation level).
Acceptable noise level was determined by subtracting the BNL from each fixed level, that is, ANL = fixed presentation level − BNL. The global ANL was calculated by averaging ANL obtained at the fixed presentation levels from each participant. 
The Average OAE response was noted from each participant. Similarly, the average OAE response obtained in the presence of broadband noise delivered to the contra-lateral ear was registered. The contra-lateral suppression was calculated by taking the transient evoked OAE (TEOAE) average response without the noise minus the TEOAE average response to noise.
In ABR, the slope of V-A was calculated by subtracting the amplitudes difference of peak "V" and peak "A" divided by its latencies difference of peak "V" and peak "A." The slope was calculated for each fixed presentation level. The global slope of V-A (quiet) was obtained by taking the average of slopes from each fixed presentation level. Similarly, the global slope of V-A (noise) was obtained by taking the average of slopes from each fixed presentation level in the presence of noise.
| Results|| |
The data of CSOAE, slope of V-A (quiet) and slope of V-A (noise) obtained from low, average and high ANL groups were statistically analyzed. The Statistical Package for Social Science (SPSS) software (version 17) was utilized for the statistical analyses. The analyses performed under each objective of the study are reported as follows:
To compare the response of global acceptable noise level and contra-lateral suppression of otoacoustic emissions
Descriptive statistical analyses were performed to document the mean and standard deviation of OAE, OAE (contra-lateral noise) and CSOAE in three groups of ANL [Table 1]. It was found that mean OAE amplitude was larger in high ANL group compared to the average and low ANL groups. Similar results were noted for the amplitude of OAE in the presence of noise delivered to the contra-lateral ear. In addition, the mean amplitude of CSOAE was larger in the lower ANL group than in average and in high ANL groups. A Kruskal-Wallis test was performed on the mean amplitude of CSOAE obtained from three ANL groups (low, average and high ANL groups). The result revealed no main effect of group on the mean amplitude of CSOAE (H (2) =1.66, P > 0.05).
|Table 1: Mean and SD of OAE, OAE (contra - lateral noise) and CSOAE from the high, average, and low ANL groups|
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To compare the behavioral response of acceptable noise level and slope of V-A (quiet) ; slope of V-A (noise)
The mean and standard deviation of the slope of V-A (quiet) and slope of V-A (noise) obtained from the three groups of ANL are tabulated in [Table 2]. The mean slope of V-A (quiet) was found steeper in the low ANL group followed by the average and then the high ANL groups. To know if there was any significant difference between groups in the slope of V-A (quiet) , a Kruskal-Wallis test was performed. The result revealed a main effect of group on the slope of V-A (quiet) (H (2) =11.51, P < 0.05), such that the steep slope of V-A (quiet) was found in the low ANL group compared to their counterparts. In addition, the post-hoc Duncan test was carried out to see which groups might have caused significant differences in the slope of V-A (quiet) . It was found that the slope of V-A (quiet) in the low ANL group was significantly steeper than the high ANL group (P < 0.05). In addition, the slope of V-A (quiet) in the average ANL group was not significant, neither with a low ANL group (P > 0.05) nor with a high ANL group (P > 0.05).
|Table 2: Mean and SD of slope of V-A(quiet) and slope of V-A(noise) from the high, average, and low ANL groups|
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Further, a Kruskal-Wallis test was performed on the mean slope of V-A (noise) . The result revealed that the slope of V-A (noise) in the low ANL group was significantly steeper (H (2) =10.23, P < 0.05) than their counterparts (i.e. average and high ANL groups). The post-hoc Duncan test revealed that the slope of V-A (noise) in the low ANL group was significantly steeper (P < 0.05) than the high ANL group. In addition, the slope of V-A (noise) in the average ANL group was not significant either with a low ANL group (P > 0.05) or with a high ANL group (P > 0.05).
Correlate the response of global acceptable noise level with the physiological responses
The average ANL group was not included in the correlation analyses. As mentioned earlier, the slope of V-A in quiet and in noise from the average ANL group was not significant either with the low ANL group or with the high ANL group. The correlation coefficient [Table 3] was computed between global ANL and CSOAE; slope of V-A (quiet) ; slope of V-A (noise) and CSOAE and slope of V-A (noise) using Pearson product-moment correlation.
The Pearson correlation coefficient result revealed that there was no correlation between global ANL and CSOAE; and CSOAE and slope of V-A (noise) . However, a moderate negative correlation was found between global ANL and the slope of V-A (quiet) , indicating that the steeper slope of V-A was noted in individuals having low ANL values [Figure 1].
|Figure 1: Steeper slopes of V-A(quiet) noted with lesser acceptable noise level values|
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Further, a moderate negative correlation found between global ANL and the slope of V-A (noise) , indicated that [Figure 2] subjects with low ANL values had a steeper slope of V-A (noise) in two-tailed Pearson correlation coefficient.
|Figure 2: Steeper slope of V-A(noise) in the low acceptable noise level group|
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| Discussion|| |
This study was designed to investigate how the aggregate of response from the cochlea and in auditory afferent and efferent pathways vary on participants having different values of ANL. There are studies that thoroughly examined the peripheral structure till auditory cortex , on participants having varying values of ANL. However, in their studies the finding was speculated on the efferent pathway. A consistent finding in ANL research is a large inter-subject variability. In addition, these studies concentrated on the response elicited from particular generation sites through measuring discrete peaks. In the present study, the responses at different generation sites were utilized to compute its successive synchronization, transmission and summation of underlying neural activity of the upper auditory brainstem (lateral leminscus [LL] and IC) on the variability of ANL obtained by study participants.
In the present study, the mean amplitude of OAE for low ANL groups is reduced, compared to their counterparts (average and high ANL groups). This difference found statistically not significant. The result is in accordance with the research report of Harkrider and Tampas,  who opined that activity in the cochlea, are not influenced by variables in the ANL. Therefore, the medial superior olivary complex (MSOC) was stimulated by presenting the narrow band noise to the ear contra-laterally while recording OAE in the test ear. The results of CSOAE revealed that the activity of outer hair cells caused reduction in the amplitude of OAE among all the groups of study participants. It infers that the activity of the MSOC on the contra-lateral outer hair cells is similar among subjects obtaining varying values of ANL. Degrees of ANL do not influence the suppression of OAE response differently.
At the auditory brainstem level, the slope of V-A was found steeper in the low ANL group compared to the average and high ANL groups. The synchronous stimulation, transmission and summation of underlying auditory structures at the LL and IC are significantly enhanced in individuals who accept more noise than those who do not accept the noise. It suggests higher neural conduction time at the LL and IC structures of the upper auditory brainstem. The steeper slope of V-A in low ANL conjecture stronger afferent pathway. The reason could be the lower transmembrane threshold in a low ANL group than average and high ANL groups. The lesser membrane threshold generates greater depolarization and increases the production of action potential and then travels along the axon of a neuron. Though the response captured from the far field scalp electrode, the slope of V-A generated was determined by the action potential generated by the neurons of LL and IC. The present study is in accordance with the research report of Hemanth et al.  Further, there was a moderate negative correlation obtained between the slope of V-A (quiet) and the global ANL. That is, the individuals who accept more noise tend to have a steeper slope of V-A. Thus, the group difference in the slope of V-A (quiet) suggests that the difference in the activity levels of the upper auditory brainstem may account for dissimilarities in accepting noise while listening to the speech.
Further, the slope of V-A (noise) was steeper in a low ANL group than the average and high ANL groups. The difference in mean values was found to be significant between low and high ANL groups. The result of slope of V-A (noise) suggests a stronger efferent pathway in the participants who accept more noise (i.e. low ANL group). In addition, a moderate negative correlation was present between slope (noise) and ANL. That is, the individuals who accept more noise tend to have a steeper slope of V-A (noise). It infers that efferent activity has fine-tuned the neural response of LL and IC at the upper auditory brainstem in those individuals who had accepted more noise.
Harkrider and Smith  reported that those participants who accepted more noise performed better in recognition of phonemes. It implies that stronger efferent pathway inhibit the response thereby fine-tuning the afferent auditory neurons. It also helps in selective attention.  The findings obtained from the participants of normal hearing infer that stronger afferent and efferent auditory pathway are present in those individuals who accepted more noise (low ANL group). For those individuals who obtained higher ANL that is, inability to accept more noise is due to weaker efferent auditory activity. It was known from the literature that there was no difference between normal and hearing impairment in the ANL measure.  Those hearing impaired individuals having higher ANL should be recommended to wear a hearing aid having an option of advanced technology such as a directional microphone and noise reduction algorithms. These strategies in the hearing aid provide less gain to the noise by which annoyance level will be within their acceptable limit.
By measuring ANL before fitting hearing aid or at the hearing aid selection, one can infer the participant's acceptability toward the hearing aid. Individuals who obtain lower ANL values will perform better in listening situations where in competing signals are present. The study highlights the neurophysiological correlate among the individuals having different ANL values. In addition, a similar mechanism is postulated for hearing impaired individuals.
| Conclusion|| |
The greater synchronization and faster transmission of afferent neural activity from LL input to IC are seen in those individuals who accept more noise (low ANL group). In addition, the auditory efferent activity fine tunes the neural response of LL and IC at the upper auditory brainstem. Thus, it is recommended that hearing aids having option of digital noise reduction circuit for those individuals who are annoyed with the noise be prescribed.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]