Cross correlation analysis of auditory brain stem response (ABR) by means of the fast Fourier transform (FFT) was performed in normal subjects and its clinical significance was discussed. Cross correlation functions were computed between individual ABRs and each of following three kinds of normative ABR templates (nABR): (1) mean of ABR recordings from 30 normal males (MnABR), (2) mean of recordings from 30 normal females (FnABR) and (3) mean of MnABR and FnABR (MFnABR), because significant difference in wave V latency between MnABR and FnABR was observed. It was, therefore, subsequent that three kinds of cross correlation functions were obtained for each individual ABR. As the cross correlation index, R0 (correlation coefficient at 0msec), RM (maximal correlation coefficient) and DL (latency delay in msec) were adopted. Means and standard deviations of the parameters in normal subjects were compared among three kinds of cross correlation functions. From the results, evaluation of ABR waveform characteristics with cross correlation analysis using nABR of the same gender seems to be more significant than the analysis using MFnABR or using nABR of the other gender.
The binaural interaction (BI) in the auditory brainstem response (ABR) was studied in ten young adults with normal hearing to rarefaction click stimuli. A special care was taken on stimulus presentation; a novel stimulus paradigm was devised to reduce the influence of the acoustic reflex on the ABR and to record each response with almost the same signal-to-noise ratio. For that purpose, sequences of stimuli were periodically presented in the order of right monaural-left monaural-binaural-left monaural-right-monaural-binaural, with an inter stimulus interval of 22ms. BI was obtained by subtracting the sum of the monaurally (ipsilaterally and contralaterally) evoked potentials from the binaurally evoked potentials. The waveform of BI was composed of three positive peaks and two negative troughs, and the major BI appeared in the region of the brainstem response wave V and VI. There was no evidence of the interaction in the early portion of ABR. BI showed a linear dependence on intensity; latency increased and amplitude decreased lineally as the stimulus intensity reduced.
A comparative study was made on electrocochleography by the transtympanic and the ear canal recording technique in normal hearing subjects. The results obtained were as follows: The auditory whole nerve action potential (AP) recorded from the ear canal showed extremely low ampliitude but AP latency, wave form and threshold did not so differ significantly from the transtympanic recording technique. Cochlear microphonics (CM) recorded from the ear canal also showed a low output potential as well as AP. Furthermore, there was a limitation of information from the cochlea because of a higher detection threshold of the CM and mixture of the electromagnetic induction wave, Therefore, the transtympanic recording technique is more efficient than that of the ear canal recording technique.
The JIS diagnostic audiometer is generally used to obtain the mean hearing threshold levels (MHTL), for physically handicapped persons and workers in noisy environment based on the laws concerning welfare services and accidental compensation. Hearing threshold level values obtained by an audiometer are represented only as a set of numbers obtained by multiplying 5dB by integers. HOW-ever, these numbers are discrete and not continuous. Therefore, another set of MHTL given by the calculation called the one fourth or one sixth method by the law in Japan should be consisted of numbers also obtained by multiplying 5dB by integers. From calculation by mathematical binary scale, we can obtain a MHTL which contains no decimal numbers or numbers not wholly divisible by 5dB. Furthermore, the recongnition of the presently used grades far compensation coincides completely with the grade approved by the formerly used calculation which I believe is unreasonable and meaningless.
In this experiment to determine critical bandwidth, the masked threshold of tone by two narrow-band noises on either side of the tone was measured. The noises were of equal spectrum level and were symmetrically separated on a linear frequency scale with respect to tone. When the separation of the two noises was incresed, masked threshold of the tone first remained constant, but dropped after a critical frequency separation was reached. In our experiment, we used the narrow-band noises without bandwidth on the peak and with steep slopes (200dB/oct). Each masker noise had the spectrum level from-10dB to 50dB SPL. Tones at 250, 500, 800, 2000, 4000 and 6000Hz were masked by two noises and each tonal duration varied from 3ms to 350ms. The results were as follows: In this method, the critical bandwidths showed a clear continuing decrease as the signal frequency was decreased below 6, 000Hz than those estimated by Zwicker et al. (1957), Scharf (1970) and, Zwicker and Terhardt (1980). Although the bandwdth values were generally somewhat smaller than those estimated by Moore and Glasberg (1983), the way with unequalized TDH39 earphone that they varied with signal frequency was similar to that described by Moore and Glasberg. If the critical bandwidth is related to the equivalent rectangular bandwidth (ERB) function of the auditory filter proposed by Moore and Glasberg, this may indicate a need to revise the classical critical band function. The width of the critical band in the same signal frequency was independent of sound pressure from 20dB to 80dB SPL. We also found that the masked threshold of short duration signal was independent of critical bandwidth. Moreover, masked threshold at short delays indicated no threshold elevation or overshoot into critical bandwidth of a 15ms, 2000Hz signal by two narrow-band noises.
ABR and EEG were studied in 32 cases of acute anoxia. The ABR showed wave I to wave V in 19 cases and only wave I or no response in 13 cases. However, the EEG was isoelectric in 18 out of 25 cases. The acid-base equilibrium of the cases was inclined to acidosis; therefore, the effect of acidosis on EEG and disturbance of the brain tissue due to anoxic state were suspected. As the effect on the brain stem, a direct effect and secondary effect or brain ischemia arising from increased intra-cranial pressure and a subsequent fall in the brain perfusion pressure were suggested.
ABR recordings were applied for 26 subjects under of anesthesia with enflurane, halothane or thalamonal. Wave III of ABR revealed a prolonged latency about 0.1ms. and wave V about 0.4ms. under enflurane anesthesia. However, its dose-effects were not demonstrated, and consequently latencies of ABR were stable within a certain range of the anesthetic dosages. Similar tendencies were recognized under halothane and thalamonal anesthesia. These results suggest that ABR is a quite effiient tool for monitoring brainstem integrity during surgery under general anesthesia. Taking into consideration of affects of anesthetics, the control recording of ABR seems to be important prior to neurosurgical manipulation.
The masking levels evaluated by band noise (band noise masking level: BNML) and by pure tones (pure tone masking level: PTML) at frequency of the pitch of tinnitus were compared in 343 tinnitus ears without hearing loss or associated with cochlear deafness whose pitch was identified by pure tones. The following results were obtained; 1) The correlation coefficient between BNML and PTML was 0.80 when both masking levels were indicated by sensation level and it was 0.93 when they were indicated by audiometer level. 2) The correlation coefficient between loudness and masking level was 0.53 in BNML and was 0.67 in PTML when both loudness and masking level were indicated by sensation level. It was 0.87 in BNML and 0.89 in PTML when both loudness and masking level were indicated by audiometer level. 3) The regression line between loudness (x) and masking level (y) was calculated. It was y=0.67x+7.61 in BNML and y=0.90x+7.19 in PTML when both loudness and masking level were indicated by sensation level. It was y=0.85x+4.99 in BNML and y=0.88x+11.68 in PTML when both loudness and masking level were indicated by audiometer level. 4) As far as the relationship between pure tone hearing level at frequency of the pitch of tinnitus and loudness, BNML and PTML was concerned, all loudness, BNML and PTML indicated by sensation level became small abruptly when the hearing level exceeded 25dB. However, the PTML indicated by audiometer level was minimally influenced by the hearing level at frequency of the pitch of tinnitus. 5) The PTML indicated by audiometer level could be most useful as an indicator representing the intensity of tinnitus whose pitch was identified by pure tones.