Serge M. Petrov
Research Institute of the Ear, Throat, Nose, and Speech, Bronnitskaja str., h. 9,
St.-Petersburg, 190013 Russia senn2001@mail.ru Fax (812)316-79-11
Abstract The scope of parents’ knowledge regarding cochlear implantation (CI) is very limited. How could we graphically demonstrate the problems faced by implanted children to their parents?Based on the parallels between speech perception of implanted patients and speech perception after comb filtration by normal hearing persons, it is possible to evaluate the process of self-training of inexperienced subjects to perceive spectrally deprived speech and to show the complexities of implanted children’s acoustical perception to their relatives. Speech consisted of five bands of the 50 Hz width in the frequency range of 200 to 6,250 Hz. Eight mothers had to repeat words from two word lists. After seven to nine days of examination, all subjects achieved a plateau of intelligibility. The mean values of intelligibility in the first and the last tests were 32 and 84 percent respectively. There was no significant difference between the final result of self-training and the result of control list intelligibility (p>>0.05).The results obtained on the last day of the experiment confirm the successful development of perception of a new sound picture of speech. These results demonstrated to the mothers that high intelligibility achieved by them in the two repeated lists, was strengthened, by analogy with their children, as a new skill. The implanted children’s mothers highly appreciated the demonstration and recommended to all mothers to take part in such assessment.Key words: Speech perception, spectral deprivation, training. Introduction When handling the global problem of cochlear implantation in addition to the specialists’ work dealing with the selection of candidates, surgical procedure, subsequent setting up of the speech processor and child’s hearing habilitation, the parents’ role is quite essential due to the need for their direct involvement in the children’s rehabilitation. However it is necessary to understand the problem more profoundly in order to make this work more effective. Unfortunately in most cases the parents’ knowledge of CI is very limited so they fail to understand why their child cannot start talking and perceiving speech at once. Moreover, when two or three children attend at the same time, their parents, naturally, see differences between the children and also fail to understand the reasons for them. Having heard about the progress of earlier implanted patients, they cannot understand why their child does not have the same results. To be able to provide for a precise physiological explanation of the features of implanted patients’ acoustical perception and the problems arising it requires information about the subject from the basics of acoustics and physiology. But this is hardly possible. How could we demonstrate the problems of implanted children to their parents in a more simple and understandable way?
Earlier we conducted research dealing with the perception of speech processed with a comb filter, i.e. the measurement of intelligibility of a speech signal with deleted segments of a spectrum (i.e. spectrally deprived speech). Example of such transformation is shown in a figure 1 (analyzed by program “COOL” of Syntrillium Software Corporation). This particular signal will be used in further study.  |
1000 3000 5000 7000 FREQUENCY (Hz) Figure 1. Spectrogram of spectrally deprived speech. The comparison between the speech intelligibility of implanted patients and the intelligibility of normal subjects after comb filtering (with large values of the comb filter period) revealed some parallels (Petrov, 2003). 1. The absence of experience of transformed speech perception and almost complete misunderstanding of such speech at the first presentation.2. Similarity between the best final results – high (up to 100%) intelligibility of such defective speech signals achieved after adaptation to new auditory images.3. Another common feature of the two cases is the extremely poor spectral content of the speech signal in a wide frequency range. The frequency range of 300 to 5,500 Hz processed by the C40/40+ implant is converted into 8 or 12 electrical pulses. Only four bands (of the 50 Hz width) within the frequency range of 200 to 6,250 Hz are retained in the speech signal after comb filtration with a filter period of 2,000 Hz.4. Cochlear coordinates of bands’ perception after comb filtration are invariable as in the case of cochlear implant (invariable places of electrodes). 5. The possibility to improve speech perception through training. Based on the parallels between speech perception of implanted patients and speech perception after comb filtration of normal hearing persons, it is possible to evaluate, first, the process of self-training of inexperienced subjects to perceive spectrally deprived speech and, second, to a certain extent to try and show the complexities of implanted patients’ acoustical perception to their relatives. It is necessary to specify that our study demonstrates cochlear implanted patients problems, but does not simulate their speech perception (Loizou et al.,1999). Methods The research was conducted on eight adult subjects (age 23-31). They were mothers of implanted children and they had normal hearing (hearing thresholds are not more than 10 dB, tympanogram -type A). All of them took part in a this procedure for the first time. The speech signal was delivered monaurally, to the right ear. The standard Russian word lists for the speech intelligibility analysis routinely used in speech audiometry were applied. The speech signal passed via an analog-to-digital converter and was saved to the hard drive of a personal computer. Comb filtering of the speech signal was done using special software for spectral conversion of speech (Petrov, 1999). This program divides the whole signal spectrum into a set of spectral bandwidths and selectively removes certain parts of the spectrum, as defined by the experimenter.The speech spectrum was analyzed in the frequency bandwidth of 200 to 6,250 Hz. After comb filtration of the speech signal and numbering of the frequency bandwidths, the frequency bands with odd numbers were summed up and their sum was used as the test signal for measuring speech intelligibility. The width of the bands with odd numbers was 50 Hz and the centre frequencies of five test bands were selected in such a way that the coordinates of a basilar membrane vibration were equally spaced at 5.2 mm distance from each other. These calculations were made in accordance with the W. Hartmann formula (1996) regarding the relation of the characteristic frequency of the neuron to the coordinate of a point on the basilar membrane. Thus the test speech signal consisted of five 50 Hz bands in a frequency range of 200 to 6,250 Hz in accordance with the cochlea tonotopical organization. As the characteristic of bandpass filters we used the width of test bands at a level of 20 dB lower than the maximum intensity level in every band. All of the pass-bands at this level varied from 170 to 200 Hz, i.e. the computer program used provided for a steepness of decay of fronts of bandpass filters from 0.3-0.35 dB/Hz both for high and low frequencies. The converted speech signal was transmitted via a digital-to-analog converter to an amplifier and then to a standard TDH-39 audiometric headphone. The heterogeneity of the frequency characteristics was 5 dB within 200 to 6,250 Hz. The measurements were conducted using a 4153 artificial ear (Bruel and Kjer). During the study, the subjects adjusted only the most comfortable level of the speech signal. At the beginning of testing the subjects were trained to participate in the experiment. All the subjects were instructed to repeat spectrally transformed words from two daily shown word lists (30 words in one list). Participants did not receive visual representation of any of the words for paired associations with the auditory stimuli. Interword interval was 5 sec. The duration of each session was 5 minutes. When the subject identified the word correctly, s/he was informed by the experimenter. It is possible to say that subjects "mastered" the sounding of 60 words. Investigations were carried out every working-day. The research was carried out until the results of word intelligibility measurements achieved a plateau level. In this case the difference in the number of correctly iterated words in three last measurements did not exceed one word. At the end of the experiment the subject had to listen and to repeat a new (third) list of words treated with a comb filter of the same parameters. Results and discussion
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As a result of the research it was found that there was an increase of intelligibility in sequential daily investigations of converted speech perception, i.e. the self-training of spectrally deprived speech perception. After seven to nine days of examinations all the subjects reached their own maximum of intelligibility (plateau level). The results of an experiment on one subject are shown in Fig. 2. Figure 2. Word intelligibility in sequential tests The mean values of intelligibility in the first and last tests were 32±5 and 84±11 percent accordingly and they were significantly different (p<0.01). The mean value of intelligibility at a single hearing of the third control list of words was 83±9 percent. No significant differences were revealed between the final result of self-training and result of control list intelligibility (p>>0.05). The following observations were made during the experiments: Right at the beginning of testing the mothers were trained to perform the examination procedure. We used words with a full speech spectrum and subjects had no difficulty in repeating the words. Hearing on the first day a speech signal treated by a comb filter they found it is difficult to repeat words. Subjects commented that it was hard to believe that the stimuli were real words. The introductory briefing on the essence of the research helped them to get a clearer idea of the challenges faced by their children and draw the conclusion that they had cherished excessive expectations hoping for immediate results once the implant was fitted for the first time. It should be noted that in sequential tests 90 to 100 percent of words from the previous test were correctly identified by subjects. The subjects’ vocabulary kept “enriching” from test to test. The word intelligibility grew to the plateau level. It should be noted too that these levels varied depending on specific subjects (from 60 up to 93 percent). The 100-percentage intelligibility mark was not reached by any of the mothers.The differences in the final (level of a plateau) results of the mothers’ word intelligibility clearly demonstrate diverse human capacities for perceiving spectrally deprived speech. Based on the parallels with CI the audiologist can explain to the mothers the differences in the results of their children’s rehabilitation. In the absence of the 100% word intelligibility for all subjects it becomes possible to explain to their mothers that special audiological methods should be applied in order to develop the perception of new sound pictures. It should be emphasized that the work done involved self-training only, so as to make their children’s rehabilitation a success special training sessions are required as well as the parents’ direct participation in the rehabilitation process. It should be noted that at the end of the research all the subjects changed their initially negative evaluation of the speech quality. Moreover, they kept wondering why they were finding it difficult to understand words at the beginning of the tests. Once again one can draw parallels with implanted postlingual patients i.e. those patients who lost their hearing after developing speech skills. After the first fitting they find it difficult to believe that later they will perceive this unclear sound picture as speech. As they gain acoustical experience they have increasingly fewer difficulties understanding speech. Once the processor is set up most postlingual patients when asked: "How do you hear now?" would answer: "Just like before". This means that they somehow complete the new unusual (and greatly distorted!) sound picture to make it similar to the one they had before. The patients themselves believe they perceive speech as before. They are convinced about it being able to communicate by phone. The result produced on the last day of the experiment is another proof of the success attained in developing the perception of the new sound picture of speech. The single hearing of the third monitoring word list processed with the comb filter of the same parameters as the training lists did not show any significant differences between the monitoring result and the final result obtained at listening to the two lists. Such a result convinces the mothers that they have not simply learnt the repeated words but have developed new skills. And once again they understand the problem of development of speech perception of their implanted children. Interestingly, after several days of examination some of the mothers already realized that this experiment is not quite the same as their children’s perception since those children who were born deaf do not have any auditory memory. Since this research does not simulate children’s perception through a cochlear implant. As a matter of fact implanted children are perhaps exposed to a harder situation - unlike their mothers they do not have any points of reference to complete or rearrange the speech sounds heard for the first time. However, firstly, such questions and reasoning of the mothers show that they have started to understand the problem and, secondly, it allows us to explain and highlight the challenges faced by the child so that the mothers get a better understanding when listening to explanations of implanted patients’ problems. Sometimes mothers reported that they did not have enough time to analyze the audible sounds. The interword time interval was 5 seconds. By analogy with CI it is possible to explain to the mothers why their children ask to repeat words several times for complete understanding. In the course of the experiment the mothers developed a better understanding of the CI problem shown by the fact that their questions become more and more specific. The mothers have shown that they comprehend the CI problem. The implanted children’s mothers who took part in the experiment appreciated the demonstration very positively and recommended to all mothers to conduct such examinations. Other mothers, after having heard from the parents tested, have expressed their willingness to become involved. Conclusions 1. In the course of sequential daily presentation of spectrally deprived speech there occurs the self-training of transformed speech perception. The mean values of intelligibility in the first and last tests are 32 percent and 84 percent respectively and they differ significantly (p<0.01). During the first tests mothers could not believe that they heard words. They immediately began to understand their children’s problems better. 2. During sequential tests subjects correctly repeated from 90 to 100 percent of the words from the previous examination i.e. from test to test the vocabulary is “enriched”. 3. The absence of significant differences between the final results of self-training and the results obtained from the third monitoring word list (p>>0.05) is a sign of a new perception skill and by analogy with CI clearly proves to the parents that it is possible to improve their children’ speech perception through training. 4. The differences of individual results of the mothers’ word intelligibility obtained in the final test clearly demonstrate diverse human capacities for perceiving spectrally deprived speech. Owing to these different results obtained on normal hearing adults the audiologist can explain (demonstrate) to mothers the different rehabilitation results of their children. Again the analogy can be drawn over to CI. 5. On the last day of the experiment the mothers very dramatically changed their first impression of the quality of perceived words. They were surprised by their initial negative assessment of the words’ clarity during the first test. The increased understanding of the parents during the experiments serves as a graphic demonstration of how the implanted patient is trained to perceive transformed speech and what results s/he can achieve. 6. The implanted children’s mothers who took part in the experiment have appreciated the demonstration very positively and strongly recommended that all mothers take part in such examinations. Other parents have requested a demonstration of transformed speech. 7. The results of the research confirm certain similarities in the ability to improve perception of spectrally deprived speech of normal hearing persons and the speech perception of implanted patients.
REFERENCES
Hartmann W (1996) Pitch, periodicity and auditory organization. Journal of Acoustical Society of America 100: 3491-3502.
Loizou S, Dorman M, Tu Z (1999) On the number of channels needed to understand speech. Journal of Acoustical Society of America 106: 2097-2103.
Petrov SM (1999) Method for the Processing of the Speech. Russian Federation Patent № 2121242.
Petrov SМ (2003) The perception of spectrally deprived speech signal. Human Physiology 29: 17-20.