Portuguese Version

Year:  2004  Vol. 70   Ed. 2 - (9º)

Artigo Original

Pages: 200 to 206

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The facilitating stimulus effect in the acoustic reflex threshold

Author(s): Renata M. M. Carvallo 1,
Jordana C. Soares 2

Keywords: acoustic reflex, auditory sensitization, facilitating stimulus, normal hearing

Abstract:
The auditory sensitization, a tool used in the research of the acoustic reflex, allows the decrease of the acoustic reflex threshold from a facilitating stimulus. It may be presented before or simultaneously at the elicitor tone. The thresholds after and before the facilitating stimulus are compared and it is expected to get the decrease of the threshold. From the study of the acoustic reflex it is possible to get much information about the auditory pathways, like structures of the brainstem, since the acoustic reflex pathway is related to the auditory nuclei in this site. They are also involved in the auditory processing. Thus, alterations of the acoustic reflex could be reported to deficits in auditory processing abilities. Aim: This study aims to research the acoustic reflex sensitization from a high-frequency facilitating tone (6 KHz) Study design: Clinical study with transversal cohort. Material and Method: normal hearing young women. Results: The acoustic reflex threshold decreased between 6,71 and 17,23 dB in the ears that presented sensitization. Conclusion: A high-frequency facilitating tone presented simultaneously produces a decrease of the acoustic reflex threshold in normal hearing people.

INTRODUCTION

Significant tool for diagnostic assessment in audiology, the study of acoustic reflex allows investigation of the afferent system (sensorial) and the efferent system (motor) of the stapedial arch reflex, in addition to the tympanic ossicle system (Carvallo & Albernaz, 1997)1.

The acoustic reflex involves auditory nuclei of the brainstem related to auditory processing activities 2, 3. Therefore, acoustic reflex abnormalities can indicate changes in some of these nuclei and deficits in the skills involved in auditory processing, such as localization, selective attention, speech recognition in noise and frequency selectivity 1.

Carvallo et al. (2000)4 found a significant affection in acoustic reflex of children with mean age of 12.25 years with auditory processing disorder. To Colletti et al. (1992)2, subjects without acoustic reflex, with stapedial muscle damage, have worse performance in frequency selectivity tasks and in speech recognition in noise. To these authors, acoustic reflex would improve speech discrimination in the presence of ipsilateral masking, owing to the effect of the reflex in attenuating sound energy in low frequencies.

Stapedial arch reflex encompasses the auditory nuclei located on the superior olivary complex. The path taken by the sound information up to contraction of stapedial muscles was described by Borg (1973)5 in a study with rabbits. Ipsilateral path, with enough intensity and duration in the stimulus, is activated by impulses of cochlear sensorial cells that are transmitted through the acoustic nerve to the ipsilateral ventral cochlear nerve. Part of these axons of the ventral cochlear nucleus which are involved in the ipsilateral acoustic reflex goes through the medial portion of the motor nucleus of the facial nerve, through the trapezoid body. Thus, these axons go down through the facial nerve to the ipsilateral stapedial muscle. Some fibers pass through the ventral cochlear nucleus, through the trapezoid body, to the medial superior olivary complex. Some impulses are transmitted to the medial portion of the ipsilateral facial motor nucleus from the medial superior olivary complex.

In the contralateral reflex, impulses are transmitted to the medial superior olivary complex, after stimulation of the acoustic nerve and cochlear nucleus. Thus, the impulses are directed to the contralateral facial motor nuclei, reaching the contralateral stapedial muscle. Both the ipsi and the contralateral pathways are simultaneously triggered by uni or bilateral stimuli.

The lowest intensity of acoustic stimuli that triggers the reflex, that is, the acoustic reflex threshold is found between 70 and 100dB in normal hearing people 4, 6, 7. For white noise and narrow band noise, the acoustic reflex is 20dB lower 8.

A tool used to study acoustic reflex is auditory sensitization procedure, which allows acoustic reflex thresholds' reduction based on a facilitating stimulus. This stimulus can be presented before or simultaneously to the prompting tone of acoustic reflex. They are compared to pre and post-exhibition facilitating stimuli, and it is expected to have a threshold reduction, as reported in many studies in the literature.

Figure 1 shows an example of acoustic reflex sensitization. It is possible to notice that there is reduction of thresholds after presentation of simultaneous facilitating stimulus of 6kHz at 84dB intensity (slightly below the acoustic reflex in 500Hz without facilitation). The reflex threshold without facilitating stimulus was identified at 86dB, being reduced to 72dB after the new investigation of thresholds was conducted with simultaneous presentation of facilitating stimulus.

Hughes (1954)9 studied the effect of auditory sensitization in adult subjects, noticing the reduction of absolute auditory thresholds after exposure to low frequency stimuli. Ipsilaterally, after exposure of 3 minutes or facilitating tone of 500Hz at 100dB HL, there was average reduction of 6dB in auditory thresholds for 500Hz, in 16 out of 17 subjects in which the procedure was conducted. There was also reduction of thresholds when the facilitating stimulus of 500Hz was provided for 1 to 6 minutes. Contralaterally, there was also threshold reduction, but it was minor compared to the ipsilateral condition. To the author, the existence of contralateral sensitization suggested central contribution, but there was the possibility that the efferent tracts would act when the stimulation of one ear interfered peripherically in the other. However, total effects of sensitization did not necessarily occur in the same site.

Hughes & Rosenblith (1957)10 conducted an electrophysiological experiment with cats to reach evidence of electrical activity in the auditory nervous system during sensitization. The effect of pure tone stimulus was investigated in the first neural response (N1) recorded close to the round window of the cat's cochlea. The sensitization effect described here as an increase in N1 amplitude after exposure to a 300Hz tone was found ipsilaterally. There was no contralateral effect. The author related sensitization to theory of action potential (AP), that is, an increased reaction of the nervous system after some stimuli. It can be a property of all regions of the junction 11. The only region of the junction whose activity could contribute to the first neural response (N1) is among hair cells and the 8th cranial nerve. Thus, to some authors, sensitization can be an example of the AP theory, occurring in the junction of hair cells with the 8th cranial nerve.

Simmons (1960)12 in an experiment with adult cats, presented additional evidence to the AP theory, referred by him as increase sensitivity in the action potential of 8th cranial nerve in other parts of the auditory system. The author has surgically implanted some electrodes in the tympanic tensor muscle and in the stapedial muscle to measure the potential of each muscle, as well as in the round window to study the reduction of amplitude produced by muscle contraction. Sound presentation was single or bilateral. After acoustic reflex thresholds were studied, a white noise stimulus of 50dB was provided above the threshold collected for 1kHz during 30 seconds and later the thresholds were investigated again. There was reduction of tympanic tensor muscle thresholds or increase in sensitivity of 24dB on average. Similar results occurred with the stapedial muscle. The main change in threshold occurred when the stimulus was presented bilaterally, but there was significant change when the contralateral ear was stimulated and then the threshold measures and the stimulus were presented to contralateral non-implanted ear. The author concluded that the theory of increase of action potential sensitivity of 8th cranial nerve (AP) extends to the brainstem, such as the cochlear nerve and the superior olivary complex, which participates in the acoustic reflex, an activity of the auditory efferent system.

In 1973, Deutsch13 found reduction of acoustic reflex threshold after stimulation of white noise in normal hearing adult subjects, in three different experiments. First, in 9 subjects, they detected the acoustic reflex thresholds for three times using stimulation with white noise, reaching an average reduction of 2dB after each threshold study, totaling reduction of 3.7 dB. In the second experiment, with other 9 subjects, there was average reduction of 5dB after stimulation of 60 seconds with white noise at 115dB SPL. In the last experiment, conducted with other 7 subjects, white noise was provided at three different intensities (20, 30 and 40dB SL) during 60 seconds each. The acoustic reflex thresholds were studied before and after noise exposure in three different situations. There was reduction of up to 6.4 dB with independent reduction of intensity to stimulus. The author referred to Simmons (1960)12 and to the theory of increased sensitivity of action potential (AP) of 8th cranial nerve after stimulation. He also alluded to the possibility of a stimulus causing the effect of fatigue resulting in auditory recruitment (reduction of acoustic reflex thresholds). Recruitment would explain the 2nd and 3rd experiments, but hardly the first one, on which the levels and the duration of the stimulus would be enough to cause the effect of fatigue.

Chobot & Wilson (1977)14 observed auditory sensitization as a result of frequency in 6 young adult women with normal hearing. First, they detected the acoustic reflex threshold. Then the stimulating stimulus with white noise was presented for 90 seconds at 110dB SPL. Thirty seconds after the end of stimulation, a new study of reflex thresholds was made. The reflex prompting stimulus and the facilitating stimulus were presented to the right ear and all the measures were made on the left ear, or contralaterally. There was average reduction of 5dB for the frequency of 500Hz, 4dB for 1kHz and there was no significant reduction for 4kHz. The authors also agreed on the AP theory 10, 12, in which there is increase in sensitivity of action potential for the 8th cranial nerve after stimulation.

Sesterhen & Breuninger (1976)7 found average reduction of 30dB of acoustic reflex threshold after the presentation of 8kHz tone simultaneously to the 2nd investigation of acoustic reflex threshold in 25 adult ears. In a later study (1977)15, the same authors studied the reduction of acoustic reflex stimulus thresholds in normal hearing people and in hearing people with hearing loss. There was reduction of 20 to 30dB in threshold after simultaneous presentation of a facilitating tone of 6 or 8kHz at the same intensity of the prompting stimulus of the acoustic reflex. The authors believe that this technique can integrate the audiological assessment of babies and children, a population that is difficult to assess based on pure tone audiometry.

In a study with normal hearing subjects (9 subjects aged 18 to 31 years) and with subjects with sensorineural loss (9 subjects aged 17 to 34 years), Blood & Greenberg (1981)16 also studied auditory sensitization. They found acoustic reflex threshold reduction by simultaneous presentation of a facilitating tone of 4kHz, in a monotic situation (4kHz facilitator and reflex prompting tone on the same ear) and dichotic situation (4kHz facilitator on one ear and reflex prompting tone on the contralateral ear). In monotic situation, subjects with normal hearing presented average reduction of 20dB in thresholds, whereas those with hearing loss presented average reduction of 5 to 6 dB. In monotic situation, reduction was significantly greater than in dichotic situation for normal hearing subjects. This difference was not significant in the group of subjects with hearing loss. The authors mentioned that possible cochlear damage is responsible for lower reduction of thresholds in subjects with sensorineural hearing loss.

Stelmachowicz & Gorga (1983)17 studied facilitation of acoustic reflex in 3 normal hearing adults using facilitating stimuli of different frequencies (500 Hz, 1, 2, 4 and 6 kHz). The results revealed that facilitation does not depend on frequency of facilitating stimulus, since the phenomenon occurred in difference frequencies of facilitator, suggesting that the tonotopic organization of the cochlea would not interfere in this process. The frequency-independent results indicated that facilitation of acoustic reflex is not primarily mediated by afferent mechanisms. The authors suggested that facilitation would occur in the efferent portion of the acoustic arch reflex.

Jeck et al. (1983)18 observed reduction of acoustic reflex thresholds in 10 normal hearing subjects aged 20 to 30 years for frequencies of 500Hz, 1 and 2kHz, based on facilitating stimulus of 6kHz presented simultaneously. The reduction of the acoustic reflex threshold was 10 to 12 dB. The authors suggested that sensitization could improve the signal-to-noise ratio in complex situations of hearing, attenuating low frequencies.

Soares & Carvallo (2003)19 investigated auditory sensitization of term neonates without risk of hearing loss. The subjects were neonates that had passed the universal neonate universal screening with transient evoked OAE. We conducted tympanometric screening with 226Hz probe and acoustic reflex in 1kHz at 100dB. The acoustic reflex thresholds in frequencies of 1, 2 and 4kHz were compared before and after simultaneous presentation of the facilitating tone of 6kHz. There was reduction of acoustic reflex thresholds for all frequencies in both genders. The differences between ears, frequency and gender were not significant. The authors believe that this study could be useful as a tool to investigate the afferent tract of neonates, children and adults. They suggested that the procedure be conducted in other populations, such as for example subjects with auditory processing abnormalities.

Fielding & Rawool (2002)20 found reduction of acoustic reflex thresholds using a different artifact in the presentation of the facilitating stimulus. The speed of the click of the immittance probe was increased from 50 to 100 clicks per second. After the increase in speed, there was reduction of 10.5dB in threshold of acoustic reflex. The survey was conducted with children aged 6 to 10 years, both male and female, with normal hearing and without history of neurological disease, without diagnosis of abnormal auditory processing and no chronic diseases. Smaller reductions than 5dB indicated temporal processing disorders. The authors believe that the threshold reduction phenomenon based on increased speed of probe click probably has the involvement of codification of each click as a separated auditory event and addition of energy through the clicks. They also stated that difficulties in temporal processing have been found in children with auditory processing disorders.

OBJECTIVE

This study aimed at investigating the sensitization of acoustic reflex to a facilitating stimulus of 6kHz in young women aged 20 to 25 years, without audiological complaints and with hearing thresholds within the normal range. We were specifically trying to:

 Check the ipsilateral acoustic reflex thresholds in dB HL in frequencies of 500 to 4000Hz;

 Check the ipsilateral acoustic reflex thresholds obtained with simultaneous facilitating stimulus of 6kHz in frequencies of 500 to 4000Hz;

 Check the difference in thresholds obtained with and without facilitating stimulus in the same frequencies.

MATERIAL AND METHOD

Sample

We analyzed the data of acoustic reflex with auditory sensitization in 47 ears of 25 female subjects aged between 20 and 25 years, without auditory complaints. They all presented normal tympanometric curve of acoustic admittance (Ymt) and pure tone thresholds, from 250 to 8kHz, up to 25dB HL. The subjects were selected from the patients of the Clinical Audiology Center, Centro de Docência e Pesquisa em Fonoaudiologia - FMUSP. They voluntarily agreed to participate in the study by signing the Free Informed Consent Term approved by the Ethics Committee for Research Studies - CAPPesq, Clinical Board, Hospital das Clínicas and FMUSP, under protocol number 213/01.

Equipment

Audiometer GSI 61- Grason Stadler. The device allows the conduction of audiogram in frequencies of 250 to 12,000 Hz, in compliance with the following standards: ANSI S3,6-1989; ANSI S3,43-1992; IEC 645-1(1992); IEC 645 - 2 (1993); ISO 389; UL 544. It has two independent channels with the accessories for speech audiometry and headsets TDH-50.
Middle Ear Analyzer GSI 33 - Grason Stadler Version 2. Microprocessed and counting on three frequencies of tone for immittance probe: 226Hz, 678Hz and 1000Hz. The equipment automatically conducts tympanometric measurements at 50 daPa/s, being that the results are recorded in the graph by the coupled printers. It uses thermal-sensitive paper to print the results. The Middle Ear Analyzer was calibrated for the conditions of altitude of the city of Sao Paulo, and all necessary measures were taken for electrical installation so as to meet the technical specifications of the manufacturer.

Procedures

The subjects were submitted to inspection of external acoustic canal to identify the presence of wax or other abnormalities that would prevent the performance of the tests; pure tone audiometry from 250 to 8000Hz, speech reception threshold (SRT) and speech recognition index (SRI), admittance tympanometry (Ymt) with probe frequency of 226Hz and ipsilateral and contralateral acoustic reflex, with stimuli of 500Hz, 1, 2, and 4 kHz and Broad Band Noise.

In the special immittance mode, we investigated the acoustic reflex threshold for frequencies 1, 2 and 4 KHz, from 2 to 2 dB. After the first investigation of acoustic reflex threshold in the frequencies above described, the sensitization study was made for each frequency separately.
The facilitating stimulus was an ipsilateral tone of 6kHz. For each frequency, the intensity of the facilitator was adjusted to the intensity of the acoustic reflex threshold. The threshold search was made from 2 to 2 dB. It is important to point out that the facilitating stimulus was presented simultaneously to the tone in which the acoustic reflex threshold was being tested for the second time. For example, if the acoustic reflex threshold for 1kHz was 96dB HL, then the sensitization search would present the facilitating tone of 6kHz at 96dB HL simultaneously to the stimulus of 1kHz at 86dB HL (10 dB below the previous threshold). If the reflex was not detected at this intensity, it was elevated or reduced up to the definition of the threshold reflex. If the threshold for 2kHz was 92dB HL, it was necessary to modify the intensity of the facilitator for 92dB and then check the threshold for 2kHz again and so on and so forth, for all frequencies and both ears.

The statistical analysis of the issue was performed using ANOVA. For the comparison of each result we used p-value in which the significance level was 5%. If p-value was greater than 5%, there would be no significant difference between the average of the groups. Significant values were marked with an asterisk (*).

RESULTS

All subjects complied with the inclusion criteria of absence of audiological complaints, normal tympanometric curve in acoustic admittance (Ymt) with 226Hz probe, pure tone thresholds of 250 to 8kHz up to 25dB HL. The subjects should also have had changes in acoustic reflex thresholds in at least one frequency after sensitization. Only then it was possible to compare the acoustic reflex thresholds before and after the facilitating stimulus of 6kHz. The acoustic reflex thresholds without facilitating stimulus (SF) and with facilitating stimulus (CF) to the right and left ears are respectively shown in Tables 1 and 2.

On the right ear, as shown in Table 1, there was reduction of acoustic reflex thresholds after presentation of facilitating stimulus (CF). Findings were statistically significant in all frequencies, since p<5%. The same applied to the left ear, as shown in Table 2.

To compare both ears, we created a variable which is the difference between SF and CF. Even though apparently the reduction of threshold in each ear had been different, there was no statistically significant average difference between right and left ears, considering that p>5% in all frequencies, which can be seen in Table 3 that follows. The progression of the differences in each ear can also be seen in Graph 1.

It is still possible to analyze the differences found between SF and CF in frequencies for each ear, significant or not. As shown in Table 4, there was a decrease in average of differences as we increase the frequency in both ears. However, this difference was not statistically significant since p>5%.

DISCUSSION

The purpose of the present study was to investigate auditory sensitization by comparing the acoustic reflex thresholds with and without presentation of a simultaneous 6kHz stimulus. It was only possible because all subjects presented some change in acoustic reflex thresholds after sensitization in at least one frequency. Only then it was possible to compare the acoustic reflex thresholds before and after facilitating stimulus of 6kHz. However, not all subjects presented acoustic reflexes in all frequencies.

By presenting a facilitating stimulus, there was significant reduction of acoustic reflex thresholds for all frequencies in both ears of the studied subjects, which is in accordance with other studies in the literature 16-18. The effect detected in this study allowed us to agree with the reported studies concerning independence of frequency, since sensitization occurred in the population regardless of the studied frequency. It is not known whether the same would occur with other facilitating stimuli, but to get to know it, it would be necessary to conduct other studies.

Based on the results obtained in this study, we can say that in addition to conventional investigation of stapedial reflex, it is possible to conduct a search for reduction (sensitization) of acoustic reflex thresholds. If the authors agree that this effect is resultant from the auditory efferent tract 12, 17, it would be incorporated in the routine procedure of the audiological center, complementing the investigation of the efferent auditory pathway.

The importance of the investigation of auditory sensitization in populations that have efferent tract abnormalities, such as subjects with auditory processing disorders, for example, is evident as far as scientific study is concerned. It is believed that it will then be possible to be sure about the auditory site in which there is sensitization, or reduction of acoustic reflex thresholds, which would support the battery of tests of the auditory processing assessment.

CONCLUSION

The simultaneous presentation of a facilitating stimulus of high frequency produced significant reduction of acoustic reflex thresholds for both ears of the studied population. The reduction found was between 6.71dB and 13dB for the right ear and between 14.35 and 17.23dB for the left ear. Even though the reduction of acoustic reflex thresholds has been apparently higher on the left ear, it was not statistically significant. There were also differences in reduction of thresholds throughout the different frequencies in each ear. Once again, this difference between frequencies was not statistically significant.

Table 1. Descriptive analysis of acoustic reflex thresholds in dB HL with and without facilitating stimuli on the right ear.



Table 2. Descriptive analysis of acoustic reflex thresholds in dB HL with and without facilitating stimuli on the left ear.



Table 3. Descriptive analysis of the difference in dB HL of acoustic reflex thresholds with and without facilitating stimuli to the right ear and left ear.



Table 4. Descriptive analysis of the differences in dB HL of acoustic reflex thresholds with and without facilitation of stimulation frequency.

REFERENCES

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1 Full Professor of Speech and Hearing Therapy, FMUSP.
2 Speech and Hearing Therapist, Scholarship FAPESP.
School of Speech and Hearing Therapy, FMUSP.
Address correspondence to: Renata Mota Mamede Carvallo - Rua Cipotânea 51 Cidade Universitária 05360-160 Sao Paulo SP
Fax (55 11) 3064-3654 - E-mail: renamaca@usp.br
Study presented at 17º Encontro Internacional de Audiologia, March 2002, Bauru/SP.

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