In order to solve the confusion of Japanese vowels, experiment of perception on vowel /i/, /e/, /u/ was conducted by emphasizing the first and second formant. In acquired deafness confusion was reduced by the emphasis of second formant. Improvement by emphasis of the first formant was observed only at vowel /u/. In four hearing impaired children, two of them had audiometric configurations of “flat” type and the others had “sharp cut” type. The results in four hearing impaired children were followings: 1) In the subject with the “flat” type hearing loss, improvement of vowel confusion was observed by emphasis at the second formant. 2) In the subjects with “sharp cut” type, improvement of vowel confusion was observed by emphasis at first formant. In the subjects with “flat” type, the effects of up-ward spread masking to the second formant was considered as the results of emphasis of the first formant. But those effects couldn't explain the results from the first formant emphasis in the subjects with “sharp cut” type.
To check the FM hearing aid the following tests were performed. 1) Word comprehension test on five children with hard of hearing 2) Audition on normal adult. 3) Test on four children with very hard of hearing (two children could distinguish the voice and two could not). Through these tests it was found that in an ordinary classroom the FM hearing aid does not give very good results but in a larger space better results are obtained. The limits of the FM hearing aid were also checked, and further improvement of FM hearing aid is expected.
“DRF-SSM fitting” is a program of a “hearing aided threshold” to speech range. In subject with normal hearing high tone is emphasized by resonance in the external ear canal, and speech discrimination in advantageous in almost any listening environment. Therefore. this program is to match the “hearing aided threshold” to speech range like that in subject with normal hearing, and to limit a maximum out-put under the UCL. As the result hearing aid's out-put is suitably fitted to the user's dynamic range of hearing. In addition, these procedures have an “insertion gain” and an “ear -mold's influence” and the others. Therefore, this fitting provides clear and comfortable hearing. Actually, only three regulations (gain, tone, and maximum power out put) were given to hearing aided users for several times, and the fitting time was very short. From experience over 200 examples this program was proved as effective to almost all types of hearing loss.
Thirty-two kinds of hearing aid, 17 box type and 15 B. T. E.-type, were tested according to Japanase Industrial Standard (JIS C5512-1981) whether they fulfil or not the requirments given by the welfare law for physically disabled persons. Four did not meet to the requirments and they had no maximum control of the out put of sound. Five had no tone control, and it was supposed that they had difficulty to fit to the patients' ears according to their audiograms. The requirments of the welfare law seemed unsatisfactory, and the authors concluded that the requirments should be revised.
For the development of a better procedure to fit hearing aid for young children with hearing loss, the reliablities and varidities of the procedure using ‘fitting aid’ (used 1/3 octave band noise for test sound and patient's dynamic range for the guideline) and our clinical procedure using audiometry by sounds from a loud speaker and mono-syllable intelligibility test are examined in 20 hearing impaired children and adults. Results are as follows; 1) In case of ‘sharp cut’ hearing loss, their auditory thresholds measured by ‘band noise’ tended to differ from that measured by pure tone. 2) By adjusting the output sound pressure level of loud speaker, even by the sound of speaker in a free field, the reliable measurements were obtained. 3) For getting better hearing discrimination ability of speech, the threshold with hearing aid was required to be less than 60dB (S. P. L.). 4) A mono-syllable intelligibility test had many problems for evaluation of hearing aid.
Auditory brainstem response (ABR) consists of fast and slow components. In this study, relationship between the two components and stimulus frequency was examined. ABR recordings were made in 13 normal hearing adults using tone pips of 0.5, 1, 2, 4 and 8kHz with the intensity level of 40dB nHL. Two ways of rise-decay time settings for the tone pips were applied; 4ms at all frequencies and 2 cycles of every chosen frequency (2-0-2 pip). The slow component and wave V of the fast component showed almost the same latency change: a prolongation of latency with decreasing stimulus frequency. When the rise-decay time of the tone pips were set into 4ms throughout the frequencies, the amplitude of the slow component decreased with increasing stimulus frequency. On the other hand, there was no such a change in slow component amplitude when 2-0-2 pips were used. The amplitude ratio of wave V to slow component decreased with decreasing stimulus frequency, and remained relatively constant at each frequency regardless of rise-decay time of the stimuli. These amplitude changes of the slow component can be explained by the effective areas of stimuli for evoking the responses. Being fixed the rise-decay time to 4ms, the effective stimulus intensity for evoking the response decreases with increasing stimulus frequency because it is only initial half or one cycle of the tone pips which contributes for evoking the response. Those results suggest that the ever mentioned audiometric difficulty for low frequency regions in ABR is mainly related to the fast component being of low amplitude in the frequency range below 1kHz, and the slow component with relatively large amplitude in low frequency region seems to be highly recommendable for audiometric purposes.
The acoustically evoked 40Hz ERP (event related response) was recorded from 16 children aged 6 months old to 7 years, and 5 adults with normal hearing, for validating the audiological utility of the response. The results were as follows: (1) The effects of the depth of sleep on waveform, the threshold and detectability of the response were found in children and even in adults. In addition, a little test-retest variation was also noted in both groups. (2) The response threshold of 40Hz ERP to 500Hz tone burst was 30-40dB (nHL) in children and was 0-10dB (nHL) in adults. The threshold of the response to 2000Hz tone burst tends to be higher than that to 500Hz tone burst in adults. (3) In children, there was little difference between the threshold of 40Hz ERP to 500Hz tone burst and that of ABR (auditory brainstem response) to 500Hz tone pip. (4) If 40Hz ERP was used for hearing evaluation in children, the further studies, such as filter condition or effects of sleep on the response etc., should be necessary.
The auditory middle latency responses (MLR) and the auditory event related potentials (ERP) were recorded in the guinea pigs with chronically implanted electrodes. The tip of electrodes was located at the epidural space above the auditory area of the cortex. In the awake animals, the dominant frequency components of MLR were found at 50 to 60Hz. When the animals were stimulated with tone bursts of 1000Hz at rates around 50 per sec, the ERP resembled to 50Hz sign waves, and the ERP amplitude exhibited a peak. Under ketamine anesthesia, both the dominant frequency component of MLR and the peak of ERP shifted to lower frequencies as 40Hz. These results suggested that the optimal stimulus rate for ERP depends on the dominant frequency component of MLR.
Brain electrical activity mapping of 40Hz ERP was studied in normal hearing subjects by computed topographic recording. The scalp distributions of 40Hz ERP components were as follows, 1) The highest electrical activity of the positive components distributed on the parietal to the frontal area. 2) The distributions of the highest electrical activities of the positive components of 40Hz ERP and those of MLR showed the fairly same tendency. However, the ones of the negative components of them were not uniform in these studies. 3) The highest electrical activity of the positive wave of 40Hz ERP have the extreme stability in normal hearing subjects. These topographical evaluations of the acoustically evoked 40Hz ERP can be applied to the studies of the brain electrical activity.
This paper reviews the literature on the auditory middle latency response (MLR) which has received a growing attention in recent years. Informations are presented on the general characteristics of the response including its power spectrum, scalp topography, effects of filtering, effects of sleep and sedation, effects of stimulus parameters, etc. Widely discrepant results among investigators on the stability of the MLR in infants and neonates are also reviewed. Two incompatible opinions on the neural generators of the MLR are briefly described. The audiological and neurological applications of the response are summarized and their future direction is discussed.