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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 22  |  Issue : 4  |  Page : 231-236

Confirmation of previous results of the occlusion effect through auditory steady-state responses in normal-hearing adults


1 Department of Hearing Sciences, Cuban Neuroscience Centre, Havana, Cuba, Cuba
2 Department of Linguistic, Macquire University, Sydney, Australia

Date of Web Publication13-Oct-2016

Correspondence Address:
Alioth Guerrero-Aranda
Street 251, Between 242 and 244, #24203, Boyeros, La Habana
Cuba
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-7749.192133

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  Abstract 

Context: The occlusion effect (OE) is a well-known phenomenon in audiology clinical practice. Hence, some authors recommend the application of a correction factor to compensate for the OE. However, only two studies have assessed the OE using auditory steady-state responses (ASSRs). Aims: The aim of this study is to confirm the findings from previous ASSRs studies of the OE for a larger sample of normal-hearing adults. Settings and Design: Thirty-two normal-hearing adults (32 ears) with a mean age of 21 ± 2 years participated in this study. For each participant, one ear was selected at random, and the first measures to be obtained (occluded or unoccluded) were randomly determined. Subjects and Methods: The stimulus comprised a combination of four sinusoidal carrier tones, 500, 1000, 2000, and 4000 Hz, modulated in amplitude (95% depth) at the following rates: 104.2, 107.8, 111.4, and 115 Hz, respectively. It was presented through bone conduction for each participant under two different conditions (occluded and unoccluded ear). The overall ambient noise was 52 dB sound pressure level. Statistical Analysis Used: Repeated-measures analysis of variances was performed to compare occluded and unoccluded ASSR thresholds and amplitudes at 30 dB hearing level for 500, 1000, 2000, and 4000 Hz. Results: Occlusion caused a significant decrease of bone-conducted ASSR thresholds at low frequencies and a significant increase at 4000 Hz. Mean ASSR amplitudes were significantly higher after occlusion at low frequencies. However, some participants showed no OE at frequencies at which it is expected to be present. Conclusions: Despite the high ambient noise issue, results of previous studies are confirmed here in a larger sample of cases.

Keywords: Auditory steady-state response, Occlusion effect, Thresholds


How to cite this article:
Guerrero-Aranda A, Mijares-Nodarse E, Hernandez-Perez H, Torres-Fortuny A. Confirmation of previous results of the occlusion effect through auditory steady-state responses in normal-hearing adults. Indian J Otol 2016;22:231-6

How to cite this URL:
Guerrero-Aranda A, Mijares-Nodarse E, Hernandez-Perez H, Torres-Fortuny A. Confirmation of previous results of the occlusion effect through auditory steady-state responses in normal-hearing adults. Indian J Otol [serial online] 2016 [cited 2023 Feb 5];22:231-6. Available from: https://www.indianjotol.org/text.asp?2016/22/4/231/192133


  Introduction Top


The occlusion effect (OE) is a well-known phenomenon in audiology clinical practice. It is observed in adults with normal hearing and sensorineural hearing loss but absent when a conductive component to the hearing loss is present. The OE is often defined as the increase in sound pressure in front of the tympanic membrane for low frequencies through 1 kHz, and the subsequent improvement in hearing sensitivity at these frequencies when a bone-conducted (BC) signal is presented to the skull and the opening of the ear canal is occluded. [1],[2],[3],[4],[5],[6] This phenomenon is observed in the clinical practice when masking is necessary or when identifying and quantifying the conductive component for a patient with hearing loss. Hence, some authors recommend the application of a correction factor to compensate for this frequency-dependent variation in thresholds when assessing bone-conduction hearing while the ear canal is occluded. [7]

Auditory steady-state responses (ASSRs) evoked by modulated tones have lately attracted the attention of researchers and clinicians dedicated to pediatric audiology. The brainstem ASSR has been demonstrated to provide reasonable estimates of behavioral thresholds for air-conducted (AC) stimuli in adults and infants with normal hearing. [8],[9],[10] However, it is well established that thresholds for both AC and BC stimuli are necessary to distinguish the type of hearing loss. Currently, ASSRs to BC stimuli are accepted by many researchers/clinicians because of their demonstrated ability to provide reasonable estimates of BC behavioral thresholds in adults and infants with normal hearing or hearing loss. [11],[12],[13],[14]

Only two studies have assessed the OE using ASSRs. [15],[16] In the first case, Small et al. [15] studied a sample of 12 postterm infants, concluding that there did not appear to be a significant OE in young infants; therefore, it was possible to do bone-conduction testing with ears occluded without applying a correction factor. However, in subsequent research, Small and Hu [16] studied 22 young infants and 10 older infants. In young infants, they reported that the OE was small but emerging at 0.5 kHz and negligible at 1 kHz; while in older infants, it was emerging at both 0.5 and 1 kHz. Nevertheless, they confirmed that it is not necessary to compensate for the OE during bone-conduction testing in young infants; however, they suggested either removing insert earphone or compensating for the OE in older infants during bone-conduction testing. In another experiment from the same research, Small and Hu [16] assessed the ASSR thresholds and amplitudes in a sample of 20 adults. They found that the OE was present at 1 kHz in 71% of the cases. At 0.5 kHz, they found no significant difference in either thresholds or amplitudes after EC occlusion. The former was reported as an unclear finding although the percent occurrence of participants who showed decreased ASSR thresholds with occlusion was significant. At 2 and 4 kHz, they found no OE; notwithstanding, they reported that 18% of cases showed a difference between the unoccluded and occluded thresholds, providing an estimate of the false-positive rate. Hence, it is desirable to replicate this finding to confirm the number of individuals who shows, on the one hand, an OE when not expected and, on the other hand, no OE when actually expected. This finding may have some clinical implications.

The aim of this study was to confirm the findings from previous OE-ASSR studies for a larger sample of normal-hearing adults.


  Subjects and Methods Top


Subjects

Thirty-two adults (15 of which were female) participated in this study with ages ranging from 18 to 26 years (mean of 21 ± 2 years). All participants had normal hearing (i.e., pure-tone air and bone conduction thresholds ≤25 dB hearing level (HL) at 0.5-4 kHz) and reported no history of middle ear infections. Informed consent was obtained from all the healthy volunteers.

Auditory stimuli

Stimuli were delivered by the AUDIX system (NEURONIC SA, Havana, Cuba). They comprised a combination of four sinusoidal carrier tones of 0.5, 1, 2, and 4 kHz modulated in amplitude (95% depth) at 104.2, 107.8, 111.4, and 115 Hz, respectively. Each carrier frequency was adjusted in intensity according to the audibility differences between individual tones. Each tone was added to another to create the multiple frequency stimuli. They were presented through bone conduction using a Radioear B-71 bone oscillator (Radioear Corporation, New Eagle, PA, USA) that was coupled to the head with approximately 4.0-4.5 N. The oscillator was held by an elastic headband on the temporal bone slightly posterior to the upper part of the pinna. [15]

Stimuli calibration

The BC stimulus was calibrated in reference equivalent threshold force levels in dB re: 1_N corresponding to 0 dB HL for the mastoid [17] using a Brüel and Kjaer Model 2250 sound level meter and Model 4930 artificial mastoid (Brüel and Kjær Sound and Vibration Measurement A/S, Nærum, Denmark).

Auditory steady-state responses recording

Ag/AgCl disc electrodes were fixed with electrolytic paste at Cz (positive), 2.5 cm below inion (negative), and Fpz (ground). Impedance values were kept below 5 kΩ at 10 Hz. The bioelectric activity was amplified with a resolution of 16 bit (0.012 μV) and analog-filtered between 10 and 300 Hz (high-pass 1 st order: F1 = 10 Hz [−3 dB, −6 dB/octave] and low-pass, Fh = 300 Hz [−3 dB, −18 dB/octave] Butterworth response characteristics). The responses were averaged over 24 and 32 epochs, each 8192 samples long. The averaged time domain waveforms were transformed into frequency spectra by means of the Fast Fourier transform.

Artifact rejection was set to eliminate epochs where the electrophysiological activity exceeded ±50 μV in amplitude to reduce contributions to the electroencephalographic recording caused by muscle artifact. All cases were allowed to lie comfortably on a bed in a sound-treated room. The test booth interior was dimly lit, and the cases were encouraged to relax and to fall asleep to reduce the residual noise level (RNL). In any event, cases were requested to make no movements.

The average ambient noise levels in the testing room (using one-third-octave-wide bands) were 35, 43.5, 39.2, 39, 30.4, 21.6, and 17.2 dB sound pressure level (SPL) at 0.125, 0.5, 1, 2, 4, 6, and 8 kHz, respectively. The overall noise level in the testing room was 52 dB SPL.

Experimental design

For each participant, one ear was selected at random for testing during the experiment. In the occluded condition, an insert earphone foam tip E-A-RLINK 3A (ETYMOTIC RESEARCH, INC. Elk Grove Village, IL, USA) was inserted into the canal (at ear-canal entrance) on the same side as the bone oscillator. This method of occluding the ear was used to simulate typical clinical audiometric testing conditions. The first measures to be obtained (occluded or unoccluded) were randomly determined. Multiple ASSRs were elicited to BC stimuli beginning at a starting intensity of 40 dB HL. The threshold for each carrier frequency was determined using a bracketing technique. [18] The lower intensity at which a response was present was considered threshold. The stopping criterion was an RNL lower than 2 nV through a maximum number of 32 sweeps. ASSR amplitudes at 30 dB HL for each frequency were also analyzed for both occlusion conditions.

Statistical methods for response detection

The statistical test used for assessing the presence of a significant response is based on a variant of the multivariate Hotelling's T2 statistic. [19]

The test consists of a multivariate comparison (using vectors formed by the real and imaginary part) of the Fourier components for each modulation frequency (Fp ) versus the average of Fourier components for adjacent (no modulation) frequencies (mean Fn ). The latter is computed from 120 frequencies bins in total, 60 above and 60 below the corresponding modulation frequency and is considered an estimate of noise level. The statistic for measuring the deviation of the signal from the noise level at a given frequency component is:

T2H = N (Fp − Mean Fn )' × (Fp − Mean Fn )/Var Fn

The apostrophe represents a transposition of the vectors, and Var Fn represents an estimate of the variance of the noise across all 120 adjacent frequencies. This statistic is proportional to an F-statistic which follows a Fisher distribution with 2 and 118 degrees of freedom, [20] and from which 95% confidence limits can be obtained. These limits allow us to establish which responses at each modulation frequency are significantly different from the noise level.

Statistical analysis

Statistical analysis was conducted using the STATISTICA 10 software (StatSoft Inc. Tulsa, OK, USA). Mean ASSR thresholds were compared across frequencies and occlusion conditions. The percent occurrence of the OE was also determined for each frequency. Bone conduction ASSR amplitudes were compared across frequencies and occlusion condition at 30 dB HL. Repeated-measures analysis of variance (ANOVA) was performed to compare occluded and unoccluded ASSR thresholds and amplitudes at 30 dB HL for 0.5, 1, 2, and 4. Greenhouse-Geisser epsilon adjustments were applied to determine the significance. Individual comparisons were carried out for significant main effects. Cases for whom amplitudes did not reach statistical significance were also included in the analyses. The criterion for statistical significance was P < 0.05 for all analyses.


  Results Top


Threshold

Mean bone conduction ASSR thresholds for unoccluded and occluded ears were significantly different (F{1,31} = 7.96; P = 0.008), and that the size of the OE was frequency dependent as shown in [Figure 1].
Figure 1: Comparison of mean auditory steady-state response thresholds (Y-axis) for ears occluded (dotted line) and unoccluded (straight line) for 0.5, 1, 2, and 4 kHz (X-axis) bone-conducted stimuli. Vertical bars denote 0.95 confidence intervals. An asterisk indicates a significant difference in thresholds between occlusion conditions

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[Table 1] shows the results of repeated-measures ANOVA for ASSR thresholds between occlusion conditions across frequencies (0.5, 1, 2, and 4 kHz). The effect of varying frequency was significant (F{3, 93} =10.48; P < 0.0001). In addition, the frequency × occlusion condition showed a significant interaction (F{3,93} =38.18; P < 0.0001; G-G epsilon = 0.87; P < 0.0001). Individual comparisons indicated that thresholds for occluded ears were significantly lower compared to those for unoccluded ears at 0.5 kHz (P < 0.0001) and 1 kHz (P < 0.001) but were not significantly different at 2 kHz (P = 0.9). At 4 kHz, occluded thresholds were significantly higher than unoccluded thresholds (P = 0.002).
Table 1: Comparison of bone - conducted auditory steady-state response thresholds

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Amplitude

ASSR mean amplitudes for stimuli presented at 30 dB HL are shown in [Table 2] for unoccluded and occluded ears across carrier frequencies. On average, cases displayed a larger increase in mean amplitude at 0.5 kHz compared with 1 and 2 kHz when ears were occluded, while at 4 kHz the mean amplitude was similar after occlusion as shown in [Figure 2]. A repeated-measures ANOVA showed that at 30 dB HL, the main effects of the occlusion condition and frequencies were significant (F{1, 31} = 12.04; P = 0.002 and F{3, 93} = 5.34; P = 0.002; respectively). In addition, the frequency × occlusion condition showed a significant interaction (F{3, 93} = 4.13; P = 0.008; G-G epsilon = 0.82; P = 0.01). Individual comparisons indicated that amplitudes for occluded ears were significantly higher compared to those for unoccluded ears at 0.5 kHz (P = 0.001) and 1 kHz (P = 0.02) but were not significantly different at 2 kHz (P = 0.07) nor at 4 kHz (P = 0.9).
Figure 2: Comparison of mean auditory steady-state response amplitudes (Y-axis) for ears occluded (dotted line) and unoccluded (straight line) for 0.5, 1, 2, and 4 kHz (X-axis) bone-conducted stimuli at 30 dB hearing level. Vertical bars denote 0.95 confidence intervals. An asterisk indicates a significant difference in thresholds between occlusion conditions

Click here to view
Table 2: Comparison of mean bone conduction auditory steady - state response amplitudes between occlusion conditions across frequencies for stimuli presented at 30 dB hearing level

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Percent occurrence

[Figure 3] shows the percent occurrence of the OE for the frequencies explored. This represents the percentage of cases who showed a difference in ASSR thresholds before and after occlusion (threshold for occluded condition subtracted from that for unoccluded condition); note that this difference was >0 for 0.5 and 1 kHz but <0 for 2 and 4 kHz. At 0.5 kHz, the OE was present in 90.6% of cases. A similar finding was noted at 1 kHz but to a lesser extent, 68.8% present. Although an OE was not observed in a large number of cases at 2 kHz, 20% showed the effect of occlusion. For 4 kHz, 62.5% of cases presented a change after occlusion.
Figure 3: Comparison between percentage of individuals (Y-axis) who showed occlusion effect (black bars) and who did not (gray bars) at 0.5, 1, 2, and 4 kHz (X-axis)

Click here to view



  Discussion Top


To date, only one OE-ASSR study has been carried out in adults using a sample of 20 cases. [16] Our goal was to confirm the findings from this study in a larger sample of normal-hearing adults. However, our results cannot be directly compared to those from Small and Hu because of a high ambient noise difference in testing conditions. Our test room ambient noise was high and particularly at low frequencies. We know this is critical when making threshold measures for BC stimuli in unoccluded ears; notwithstanding, we found some results worthy to be discussed.

Our results clearly show a decrease of ASSR thresholds at 0.5 and 1 kHz when the ear canal is occluded. While this is in line with previous studies that have used behavioral audiometry, [21],[22],[23] it only partially agrees with the ASSR study of Small and Hu. Although decreased ASSR thresholds were observed individually at both 0.5 kHz and 1 kHz in a significant proportion of the test group, Small and Hu [16] found that the OE was present at 1 kHz but not at 0.5 kHz when the analysis was performed across all 20 cases; nevertheless, the number of participants who showed decreased ASSR thresholds with occlusion was significant. We believed that this partial disagreement is primarily due to the ambient noise difference across studies. When we compared our unoccluded ASSR thresholds to those reported in Small and Hu, [16] our mean thresholds are about 8 and 12 dB higher at 0.5 and 1 kHz, respectively; however, our occluded thresholds are only about 3-7 dB higher. It means that at least about 5 dB of our OE is likely to be accounted for by elevation of our unoccluded thresholds due to high noise levels (particularly at low frequencies) in the sound booth rather than occlusion of the ear canal alone. This would likely explain how we obtained a larger and more stable OE than Small and Hu. Our results are also consistent with previous studies showing a decrease in the size of the OE as stimulus frequency increases to approximately 1.5 kHz before the effect disappears. [22],[23],[24] However, in addition to this, we found the opposite phenomenon at 4 kHz where ASSR thresholds increased after ear canal occlusion. Some authors have found similar results where the OE tends to invert at high frequencies whether it is measured as ear-canal sound pressure level [25],[26],[27] or as behavioral and ASSR thresholds. [15],[16],[28],[29],[30]

Regarding the amplitudes, it is important to clarify that we decided to use a stimulus intensity of 30 dB HL as it allowed us to compare our results with the only other ASSR study of the OE. [16]

ASSR amplitudes showed significant changes after the occlusion at low frequencies. This partially agrees with Small and Hu, [16] who found significantly larger mean ASSR amplitudes with occluded ears at 1 kHz but not at 0.5 kHz. This disagreement at 5 kHz could be explained by our high ambient noise, particularly at this frequency. According to this, we found higher mean amplitude for unoccluded ears than Small and Hu at 5 kHz (43 vs. 34 nV). This suggests that our more stable OE regarding amplitudes could be reflective of our test room conditions. At high frequencies, we found no significant changes after occlusion, and this is in line with the results reported for Small and Hu. [16]

Here, we confirmed that the OE is not present in every individual. This agrees qualitatively but not quantitatively with the results of the Small and Hu study. [16] While they reported that OE was observed in 71% of the cases at either 0.5 or 1 kHz, we found a larger percent occurrence at 0.5 kHz (90.6%) but a similar one at 1 kHz (68.7%). We think this difference at 0.5 kHz could be primarily due to our high levels of ambient noise at this frequency as explained above. At 2 kHz, the OE was not present; however, 20% of cases showed a change in ASSR thresholds after the ear canal occlusion while at 4 kHz, the percent occurrence was 62.5%. This is a similar percent occurrence (18%) to that reported by Small and Hu at 2 kHz but not at 4 kHz. In their opinion, these values provide an estimate of the false-positive rate for reporting an unexpected OE.

These results of OE percent occurrence could also have clinical implications, for instance, when deciding to use a correction factor to compensate for the OE (i.e. when testing bone conduction and the ear canal is occluded) in a hearing-impaired case in which the OE is actually not present or vice versa.


  Conclusions Top


Despite the high ambient noise issue of the present study, results of the previous researches are confirmed here in a larger sample of cases, suggesting that ASSR constitutes an objective technique for the study of the OE. The OE is a phenomenon that, despite its replicability, is not present in every individual; furthermore, in some individuals, it is present when unexpected. We think this may have some clinical implications in audiological practice. It is important to stress that our findings are only applicable to our testing conditions, i.e., in the presence of high ambient noise levels.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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