Abstract
Purpose
The purpose of the study was to compare the speech recognition capacity between listeners with and without acoustic reflex using different types of noises and intensities.
Materials and methods
We studied 18 women allocated to 2 groups: acoustic reflex present (20 ears) and absent (16 ears). They were presented with 180 disyllable words (90 to each ear), emitted randomly at a fixed intensity of 40 dB above the pure tone average hearing level. At the same time, 3 types of noises were presented ipsilaterally (white, pink, and speech), one at a time, at 3 intensities: 40, 50, and 60 dB above the pure tone average hearing level.
Results
The ages and auditory thresholds were statistically equal between the groups. There was a significant difference in mean number of hits between the 2 groups for the 3 types of noises used. There was also a significant difference in mean number of hits for noise type and intensity when white and pink noise was used at 40 and 50 dB and for all the intensities when speech was used.
Conclusion
Acoustic reflex helps communication in high-noise environments and is more efficient for speech sounds.
1
Introduction
Hearing is the main sense responsible for speech and language acquisition in children. A deficit in this function may compromise not only language development, but also social, emotional, and cognitive aspects .
The hearing mechanism, from the point of view of anatomists and physiologists, is described as comprising 3 divisions: external, middle, and inner ear. Each compartment of the ear has the particular function of allowing sound to be transmitted, amplified, and finally transformed into electric stimuli that travel to the cortex by the auditory nerve. The middle ear structures include the tensor tympanic muscle and the stapedius, the smallest striated muscles of the human body. The contraction of the tensor of the tympanum exerts force on the head of the stirrup bone, pulling it backward. The muscles exert forces in opposite and perpendicular directions to the movement of the ossicular chain .
Most information on the musculature behavior of the tympanum comes from experiments with animals. Several studies showed that acoustic reflex, even when submitted to unilateral stimulation, results in bilateral contraction, a finding subsequently used in clinical applications. Other studies proved the existence of a direct relation between duration of the stimulus and duration of the muscle contraction. Although acoustic reflex has been less studied in human beings, it has been observed that the contraction could be provoked by the simple exposure to intense sound .
The stapedial reflex is defined as a contraction of the middle ear muscles induced by intense acoustic stimulus. Because the acoustic reflex alters the mechanical properties of the inner ear’s transmission system, mechanical resistance can be measured indirectly. This makes it a valuable clinical and auditory research instrument in human beings .
As mentioned before, the tensor tympanic muscle pulls the handle of the malleus inward; and the stapedius exerts a backward force on the stirrup bone causing greater rigidity in the system and reducing sound transmission, mainly those from low frequencies, that is, those less than 1000 Hz. Thus, the changes in middle ear impedance, due to the above-mentioned contractions, have little or no effect on frequencies greater than 2000 Hz .
Both muscles seem to contract at practically the same time; and although the tensor tympanic seems to exert more force, the stapedius seems to be the more efficient muscle. In human beings, the acoustic reflex is generated at intensities of around 80 or 90 dB above the auditory threshold . Some studies, however, prove the subclinical activation of this reflex, that is, with much smaller intensities than expected, mainly for noise .
Although some authors maintain that the main function of the acoustic reflex is to protect the auditory system, several limitations to this theory have long been known. The reflex is not efficient for attenuating sounds at frequencies higher than 1000 Hz and even less so at frequencies greater than 2000 Hz. Another limitation is the latency of muscular contractions, given that there is a determinate interval between the moment at which sound reaches the tympanum and the onset of muscular contraction. Thus, intense sounds may reach the auditory system and cause damage before the muscles can contract. These latencies, according to earlier studies, are approximately 0.06 second for the stapedius and 0.15 second for the tensor tympanic .
In addition, prolonged exposure to excessively high sound intensities, generally produced by man, may result in a diminished attenuating effect caused by fatigue. With constant sound stimulation, a continuous contraction is initially observed, followed by a gradual decrease until a state of rest is reached. Only one new contraction, altering substantially the stimulus frequency, will be generated .
Once the inefficiency of the acoustic reflex in protecting the auditory system was observed, several studies showed that the most important function of this reflex may be to improve speech discrimination, mainly in high-noise environments. However, one effect described in the literature is antimasking , defined as the reduction of low sound frequencies, be they environmental or from the individuals themselves. This mechanism allows the person to listen to higher-frequency sounds, that is, in the spoken communication range, by attenuating possible noise .
For low- to moderate-intensity sounds, the cochlea functions as a set of band-passing filters, that is, for intensities approaching the auditory threshold; however, the bandwidth is very narrow. As the intensity of the stimulus increases, the bandwidth gradually widens up to around 70 dB sound pressure level. The cochlea then behaves like a set of high-pass filters. This means that high-intensity stimuli start to stimulate auditory nerve fibers tuned to much higher than nominal frequencies. If the noises were not attenuated by the antimasking effect, high-frequency components of this noise, generated by high-pass filters, would mask speech frequencies and compromise discrimination .
Studies on patients with a section of the stapedius muscle tendon poststapedectomy and on individuals with facial paralysis, who did not have acoustic reflex, confirm the existence of this facilitating effect on speech recognition because they performed poorly on voice discrimination tests when compared with individuals with normal acoustic reflexes .
The contraction of the pupil as a response to intense light exposure and the contraction of the intratympanic muscles seem analogous. The purpose of the pupil is not to protect the eye from bright light because closing the pupil increases visual field depth. Similarly, the purpose of the acoustic reflex is not to protect the auditory system, but rather to accentuate speech perception by attenuating the low frequencies .
According to some authors , acoustic reflex alterations would cause more harm in terms of central auditory processing because this stapedius muscle mechanism seems to be directly related with the ease of capturing speech sounds, which would create better information-coding conditions and, therefore, speech intelligibility. According to some studies , individuals with auditory processing disorders may find their speech, reading, writing, language, and social behavior compromised.
In general, the acoustic reflex is important for separating the auditory signal from other internal or environmental noises and for controlling the attenuation of low-frequency speech sounds, thus favoring the perception of high-frequency sounds, the attenuation of voiced sounds, and the recognition of strong-intensity speech .
In light of the important limitations presented for the protective function of the acoustic reflex, given that there is still no consensus as to the communication function of this reflex, the purpose of the present study was to assess the role of the acoustic reflex in high-noise environment communication.
2
Materials and methods
The protocol of this study is based on Resolution No. 196/06 of the National Health Council of the Ministry of Health for research with human beings and was approved on November 3, 2006, by the Research Ethics Committee of the Universidade Estadual de Ciências da Saúde de Alagoas (protocol no. 603).
The study was conducted in the Universidade Estadual de Ciências da Saúde de Alagoas’ Laboratory of Audiology. A sample of 18 women was studied, with a total of 36 ears. The participants were allocated to 2 groups: the acoustic reflex–present group (20 ears) and the acoustic reflex–absent group, idiopathic reasons (16 ears). The number of subjects was defined by a calculation of sample size, which will be subsequently described.
The following inclusion criteria were adopted: auditory threshold less than or equal to 20 dB hearing level (HL) and age between 18 and 55 years. The exclusion criteria were exposure to occupational noise or recreational noise, previous ear surgeries, more than 3 ear infections within the last year, use of ototoxic drugs, and hereditary cases of deafness. The inclusion and exclusion criteria were considered for both groups, and the only difference between them was the absence or presence of acoustic reflexes at the frequencies studied (0, 5, 1, 2 and 4 kHz).
For subject selection, a questionnaire was applied, after which the informed consent form was read, explained verbally, and signed by the study participants. Data were then collected using the following procedures: otoscopy, pure tone audiometry, immittance audiometry, and the speech discrimination test. The otoscopy was conducted with sterilized specula to observe the integrity of the tympanic membrane. The pure tone audiometry was performed in an acoustic cabin. The psychoacoustic limit method, using the descending technique with 10-dB steps, was used to study auditory threshold; and response confirmation was determined by the ascending technique with 5-dB steps. Frequencies were assessed at one-eighth intervals between 0.25 and 8 kHz. The acoustic cabin followed ANSI 3.1-1991 recommendations. Immittance audiometry verified middle ear conditions and more specifically, those of the ossicle-tympanic system; acoustic reflexes were also studied at frequencies of 0.5, 1, 2, and 4 kHz. According to the immittance audiometry used, the intensity threshold for the study of ipsilateral and contralateral acoustic reflexes was 105 and 110 dB HL, respectively. Finally, a speech discrimination test was carried out in an acoustic cabin.
A total of 180 disyllable words (90 for each ear) were used for the discrimination test. These were randomly emitted at a fixed intensity of 40 dB above pure tone average hearing level (500, 1000, and 2000 Hz). The noises used were white, consisting of 10- to 10 000-Hz frequencies, where frequencies up to 6000 Hz at equal intensity and energy were efficient; pink, which filters white noise and consists of frequencies of 500 to 4000 Hz, the band where it is most effective; and speech sounds . The speech sounds of various individuals speaking at the same time were recorded to simulate a noisy environment. The frequencies ranged from 0 to 4.5 kHz. The noises used were at intensities of 40, 50, and 60 dB above pure tone average hearing level. That is, for each type of noise, 30 words per ear were provided; and after every 10 words, noise intensity was increased by 10 dB. Noise aspects were analyzed, and the fast Fourier transform of each one is shown in Figs. 1-3 .
To be able to analyze the ears individually, the speech discrimination test was applied. The words and noises were emitted ipsilaterally. Only the hits were considered, that is, the words repeated correctly. The responses characterized as distortions were stored in a databank and will be analyzed in future studies. If the subject did not respond or responded incorrectly, the word was repeated once more after the next word on the list was presented.
The speech discrimination in noise test was inspired by the test developed by Santos and Schochat and was performed with monosyllables or sentences at different intensity levels.
2.1
Data analysis
Sample size was calculated by the difference between the means, as follows:
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