Hearing is one of the five ordinary senses, inherent to people and some other vertebrates; it is the ability to perceive different sounds by means of such an important organ as the ear. Hearing is generally performed by the auditory system, the process, when the ear detects certain vibrations and transduces them into the necessary nerve impulses, which can be analyzed by the brain. “The auditory system can be quite sensitive to the spectral shape, however, as in the differences among whispered vowels.” (Manley et al., 2004) Nowadays, many studies deal with different hearing processes, which call some problems with hearing and the perception of information. One of such processes is hearing loss and auditory suppression. “Hearing loss caused by damage to the cochlea (inner ear) is probably the most common form of hearing loss in the developed countries.” (Moore, 1995) In this paper, we are going to study and analyse several types of auditory suppression, their interactions, and their effect on other parts of the body. Cochlear nonlinearity and two-tone suppression are considered to be two of the most frequent processes, which happen in the inner ear and provoke certain troubles. To find out the necessary treatment and help people to restore their hearing abilities, it is better (1) to study the articles by such writers as Abbas, Hall, or Javel, Geisler and Ravidran and examine the research, conducted by Duifhuis, (2) to analyze the described by them researches concerning two-tone suppression in auditory nerves and the suppressions in the basilar membrane, and, finally, (3) to compare the symptoms of two-tone suppression and cochlear nonlinearity.
The basilar membrane is considered to be one the most important elements of the inner ear, which aims to separate the liquid-filled tubes and provide a vertebrate with an opportunity to hear. If something serious happens in this part of the ear, it is quite possible to not only get some problems with information perception but also become deaf. The cochlea is another section of the inner ear that moves according to the vibrations coming and helps to divide the vibration sound and make them comprehensible to the brain. One of the most important functions of the basilar membrane, which may be developed in some mammalian species’ cochlea, is the ability to disperse all incoming sound waves.
The damage of the cochlea is probably the most frequent form of hearing impairment. One of the common symptoms of cochlear nonlinearity is a disability to recognize weak sounds. General complaints lie in the difficulty of understanding speech that is accompanied by some background noises. (Moore 1995) Investigations of cochlear nonlinearity help to grasp not only the major idea of hearing impairment but also clear up what exactly influences the decreasing of such abilities as detecting and comprehending sounds. Many types of research prove that loss of hearing or wrong interpretation of the information may be caused by numerous biological, psychological, and environmental factors. Cochlear nonlinearity happens when the input to the BM (the basilar membrane) is doubled, and the output is less doubled. Brian Moor speculates upon the psychological and perceptual aspects, which are crucially important for cochlear nonlinearity and two-tone suppression. He mentions that hearing loss may be caused by aging. However, it is not the only reason. Damage to the cochlea may happen by many other different things and growth slopes.
Another form of basilar membrane damage is two-tone suppression. Two-tone suppression is one of the general characteristics of the normal ear and turns out to be a result of the process when “the tone-driven activity of a single fiber in response to one tone can be suppressed by the presence of a second tone.” (Moore, 1995) It is necessary to remember that such characteristic is inherent to normal ears only, the damaged ear is deprived of the two-tone suppression. In simple words, two-tone suppression occurs when a second tone is added at a different frequency and the first tone is decreased. With the help of the work by Duifhuis, we can realize that suppression may be rather a dependant on the subject, however, the general form of the suppression remains to be constant (Duifhuis, 1980).
Duifhuis admits that “Suppression is a non-linear phenomenon so that one should not expect the effects of 2 suppressors to add up” (Duifhuis, 1980) In his research, he chooses an interesting way – to use suppression frequency as a primary independent variable. He measures suppression monaurally in order to limit the number of alternative techniques. First, two-tone suppression was set at 1 kHz, further experiments were characterized by other frequencies (0.5, 2, and 4 kHz). During these researches, it was mentioned that suppression is much more prominent at higher levels, and at lower levels, suppression is considered to be less prominent. It was proved that suppression is rather dependent on different subjects: suppressor and suppresses frequencies and levels. With the help of his investigation, Duifhuis calls into question the already existing facts and proves that frequency is not the only issue that influences two-tone suppression. The experiments of this very scientist are considered to be useful because the research, described in this paper, has lots in common with the one by Duifhuis.
Two-tone suppression can easily arise from cochlear nonlinearity. In this case, the major cause of two-tone suppression is the basilar membrane’s mechanical measurements. This is why to clear up the true nature of two-tone suppression, it is better to concentrate on both physiological and psychophysical investigations. Duifhuis makes a wonderful attempt to describe all the measurements of two-tone suppression on the psychological level. He proves that the effect of two-tone suppression may be decreased only in a noisy environment. Duifhuis distinguishes two levels of the stimulus components, which become the major variables of the investigation; they are L1 – a suppressee, and L2 – a suppressor. The quantity of suppression in the given stimulus is dependant on both the suppressee and the suppressor. It is also necessary to point out that, in their article about the effects of suppressors, Yasin and Plack suppose that the increase of suppression as the major function of suppressor level is much greater for the suppressor that is below the signal frequency. (Yasin and Plack, 2007) The increasing of suppression turns out to be the major theme for discussion in this article, they also use temporal masking curves in order to measure the chosen levels.
The work by James Kates shows that “two-tone rate suppression is a nonlinear property of the cochlea in which the neutral firing rate in the region most sensitive to a probe tone is reduced by the addition of a second (suppressor) tone at a different frequency.” (Kates, 1995). The physiological investigations on two-tone suppression can easily prove that suppression may be presented not as a neural response to the cochlear partition, but also as its mechanical motion. To analyze this very point, it is necessary to remember the theory offered by Hall in 1977. He explains that “neural response is related directly to the first of second spatial derivative of the travelling wave along the basilar membrane.” (Hall, 1977).
Two-tone suppression may produce another effect if the frequency of the excitatory tone is a bit higher than the frequency of the suppressor tone. The relations between the suppressor and the suppressee turn out to be rather important, however, the role of suppressee, taken alone, is not that significant in the whole process of two-tone suppression. (Javel et al., 1978) Even if the object of the experiment are the cats, it is possible to achieve certain results and, with the help of a thorough examination of cats’ auditory nerves, explain how exactly the suppressee interacts with the suppressor. The discharge rate of the auditory nerve fibres of cats should be measured by several stimuli: the suppressor tone and the excitatory tone.. One of them causes an increase of the rate, and another suppresses it. Comparing these two reactions, it is quite possible to analyze whether the suppressee may behave and influence the other functions of the ear alone.
The last but not less important issue, which we are going to consider in this paper, is the use of the pulsation threshold method in order to induce two-tone suppression. Duifhuis is one of those scientists, who are eager to analyze the impact of this very method and its further impact. The pulsation threshold method is used in order to evaluate the basilar membrane reaction to two-tone suppression. During the investigation, Duifhuis uses a stimulus time pattern: a repetition frequency (4 Hz) is characterized for the masker stimulus and the probe. This very method was chosen because of one simple reason: “day-to-day variability is essentially equal for toward masking and pulsation threshold… the effects of suppression measured in terms of threshold differences are greater.” (Duifhuis, 1980).
The peculiar feature of such a method is a simultaneous usage of the suppressee and the suppressor. In order to prove the effectiveness of this method, it is necessary to compare it with some other methods. In this work, we will use forward masking as the major opponent of the pulsation threshold. The results, demonstrated by Duifhuis, will help to prove that the effects of suppression are greater in the pulsation threshold. The frequency in the probe should be the same as the one in suppressee during the pulsation threshold methods. If these assumptions will be correct, the effectiveness of the method we call pulsation threshold will be proved. “Suppression is not merely an effect of suppressor – suppressee amplitude ratio but that it also increases as the overall level increases.” (Duifhuis, 1980). Another important method that has to be mentioned is direct masking. Hartman admits that “auditory bandwidths determined by direct masking are wider than those determined by forward masking or pulsation threshold.” (1997) To get a clearer understanding of the effectiveness of the methods chosen, it is also necessary to admit the boundaries of the results of each method.
Hearing becomes an important sense for any human and other vertebral. In a moment, many problems may appear with hearing and the perception of the necessary information. The auditory system is crucially important and helps to define different types of vibrations and make them available for evaluation. Any damage, whether it is cochlea nonlinearity or two-tone suppression, should be analyzed thoroughly on different levels. Suppressive interactions may be studied only by means of numerous investigations and theoretical backgrounds. The works by Abbas, Hall, Javel, and Duifhuis, of course, help to get a clear understanding of such notions as “pulsation threshold”, “auditory suppression”, and “two-tone suppression within a cochlear model”. Cochlear nonlinearity causes certain troubles for humans and other living beings to perceive the information on the necessary level. Two-tone suppression is another process that may arise from cochlear nonlinearity and causes some other problems with hearing. The peculiar feature of two-tone suppression is that its effect does not call any neural inhibition. This very phenomenon is analyzed by means of presenting one tone at the characteristic frequency of a neuron, and another tone is presented later with its certain frequency and intensity. The comparison of such methods as forwarding masking and pulsation threshold helps to see and analyze the pros and cons of both and get a clear understanding of which one is more effective for such a phenomenon as two-tone suppression. To study this phenomenon, it is also necessary to clear up the difference between the suppressee and the suppressor, learn more about their relations, and present the information that may prove which of them does play a significant role in the process, and which of them does not really crucially important, but still, its presence obligatory.
Works Cited
- Abbas, Paul, J. And Sachs, Murray, B. Two-Tone Suppression in Auditory-Nerve Fibers: Extension of a Stimulus-Response Relationship.” Journal of the Acoustical Society of America. 59, 1 (1976): 112-122.
- Duifhuis, Hendrikus. “Levels Effects in Psychophysical Two-Tone Suppression.” Journal of the Acoustical Society of America, 67.3 (1980): 914-27
- Hall, J. L. “Twp Tone Suppression in a Non-Linear Model of the Basilar Membrane.” Journal of the Acoustical Society of America, 61 (1977): 930-39.
- Hartmann, William, M. Signal, Sound, and Sensation. Springer, 1997.
- Javel, Eric, Geisler, C., and Ravidran, A. “Two-Tone Suppression in Auditory Nerve of the Cat.” Journal of the Acoustical Society of America. 64, 81 (1978): S135
- Kates, James, M. “Two-Tone Suppression in a Cochlear Model.” IEEE Transactions on Speech and Audio Processing. 3, 5 (1995): 396-406.
- Manley, Geoffrey, A., Popper, Arthur, N., and Fay, Richard, R. Evolution of the Vertebrate Auditory System. Springer, 2004.
- Moore, Brian, C. Perceptual Consequences of Cochlear Damage. Oxford University Press, 1995.
- Yasin, Ifat and Plack, Christopher, J. “The effects of Low- and High-Frequency Suppressors on Psychophysical Estimates of Basilar Membrane Compression and Gain.” Journal of the Acoustical Society of America, 121, 5 (2007): 2832-41.