Techniques for scaling human cochlear frequency selectivity were studied and validated. Twelve ears of normal adults were examined by means of compound action potential tuning curve (CAP-TC). CAP-TC was reduced from the pure-tone masking profile of CAP recorded by transtympanic electrocochleography. CAP was evoked by 2 or 4kHz tone pip with an intensity 20dB above threshold, then a pure-tone masker was presented at various frequencies and the amount of CAP amplitude suppression was plotted as to make a CAP-TC. Q10dB and half-power bandwidth were measured in CAP-TC as frequency selectivity parameters. These values corresponded well with the published results in animals and humans. However, some differences from animals were found: (1) Frequency of the CAP-TC tip was not necessarily equal to that of the tone pip, (2) Low- and high-frequency slope of CAP-TC showed no statistical difference at 4kHz. Measurements with an acoustic manikin suggested that the difference (1) was caused by the transfer function variation of the external canal. It was concluded that electrophysiological evaluation of human frequency selectivity by CAP-TC is reliable and deserves clinical application.
A total of 202 cases (404 ears) of low birth weight infants (27-36w) were divided into nine groups by post-conception week, and their ABR waves latency and the interpeak latency were studied by using 90dB nHL and 70dB nHL stimulations. The results showed that an earlier completion of the response from the VIII nerve occurred in the development of the auditoy conduction tract among low birth weight infants. As regards the development of the brain stem involving the cochlear nucleus and the colliculus caudalis, the process involving the whole areas proceeded in immature infants, while gradual and staged development seemed to take place as the infants grew toward young childhood. The current study suggested that ABR hearing screening could be most rewarding when it was done at 58-70 weeks by determining the level of maturity in infants, and if warranted, it must be repeated at 84-95 weeks for further work-up.
Slow components of ABR during natural sleep were measured in 7 normal subjects ranging from 23 to 31 years of age. The response threshold of the slow component was not changed during sleep state, but minor changes were observed in the slow component latency. To compare the stability or reproducibility of the slow phase component during awake and sleep state, standard deviation of the latencies of 10 slow components which were averaged every 200 times and filtered (0-300Hz) using the fast Fourier transform, was calculated. No significant changes were observed in standard deviation of latencies of 10 slow components between sleep and awake state. Therefore, the slow component of the ABR is available for estimation of hearing threshold even during sleep.
The simultaneous recording of auditory evoked potentials with logarithmic time scale were tried in order to simplify display of the successive components of the potentials in infants. The subjects were 16 normal infants aged from 2 months to 5 years old. The results were as follows; 1) The logarithmic scale display of the auditory potentials in infants revealed the same modality with those of adults. (Audiology Japan, 26, 735-739, 1983) Each component of potentials mainly generated from the 8th nerve up to the auditory cortex could be identified clearly without missing its charactristics. 2) The discrepancy of the pattern of the potentials between adults and infants was the presence of a deep negative potential following the wave 2 of ABR in infants, especially under the age of 12 months. 3) The logarithmic time scale display showed the possibility of the duplication between slow positive ABR and Po of MLR, and nextly Pa, Pb of MLR and P1 of SVR, and furthermore, Nc of MLR and N1 of SVR. These modalities of duplications were the same with those of adults.