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A tone on tone simultaneous masking paradigm was used to determine tuning curves of ABR in rats. Probe stimuli were 0.5, 0.8, 1.0, 2.0, 4.0-kHz short tone bursts. ABR tuning curves were constructed to define masker intensity that resulted in a 50% reduction in probe-elicited wave-V amplitude. The frequency specificity of each probe stimulus was evaluated by Q10 and the maximum masker frequency (MMF) calculated for the tuning curves.Q10's were gradually increased with the increase in rise time. However, a prolongation of the rise time beyond 2 cycles of probe frequency yielded little improvement in Q10. Maximum masker frequencies (MMF) of the tuning curves were not necessarily equal to the frequency of probe without the 2.0-kHz. For the 0.5, 0.8, 1.0-kHz probe, the MMF of the tuning curves showed higher values than the frequency of probe. For the 4.0-kHz probe, the MMF of the tuning curves showed lower values than the frequency of probe.These rusults indicate that the short tone burst stimuli may enable to assess certain auditory function at different frequencies, and they have wider frequencies' spread of the cochlear area around stimulus frequencies.

In order to establish the application of action potentials (AP) of the cochlear nerve and brain stem auditory evoked response (BSR) in clinical audiology, the frequency specificity of them was studied using the special masking techniques.The change of AP and BSR latencies and amplitudes of white noise evoked responses were observed using the high and low pass filtered masking noise. Secondaly the band stop (reject) noise was produced during the tone pip stimuli by the frequency selective masking.According to the first examination, AP responses depended on the frequency range of cochlear partitions higher than 2kHz, while those responsive for BSR extended to a octave lower range.It is considered that the latencies of responses are dependent to the most basal part of the cochlear partition, which is stimulated when the stimulus conditions are held constant. The amplitudes of AP are decided by the length of the stimulated cochlear partition, when the stimlus conditions areheld constant and the lower frequency bands of masking are changed.The following results were obtained from the second examination. The thresholds of responses were 5dB above subjective ones to the 4kHz and 81kHz tone pip stimuli.On the other hand AP and BSR thresholds to the 2kHz tone pips were 20dB and 15dB above subjective threshold respectively. The AP responses were not obtained and BSR were unstable to the the tone pips at 1kHz.The intensity-latency curves of AP and BSR were shifted in parallel with the frequency from high to low.It is concluded that the effective frequency of the tone pips were 4kHz or higher for AP, and 2kHz or higher for BSR in clinical application. The frequency selective masking of the band stop noise is necessary for us to obtain the responses from more restricted part of the cochlear partition.

BSR audiometry was administered to 20 normal and 44 hearing impaired subjects. The hearing impaired group consisted of 15 subjects with flat loss, 17 high tone loss and 12 low tone loss. One-and-a-half cycles of a 500Hz sinusoidal wave (500Hz click) and one cycle of a 3kHz sinusoidal wave (3kHz click) were used as stimuli. Intensities of these stimuli were calibrated by the averaged subjective threshold from 20 normal hearing ears. (dBHL)In normal subjects, the averaged threshold of the BSR evoked by a 500Hz click was 13.1dBHL and by a 3kHz click, 4.2dBHL.BSR frequency specificity was determined by measuring the BSR (to 500Hz and 3kHz clicks) in hearing impaired subjects. The 500Hz click evoked a BSR in subjects whose pure tone audiograms were impaired above 500Hz. Also, in cases of low tone hearing loss, the BSR was evoked at the best-hearing frequency above 500Hz. The 3kHz click evoked a BSR in subjects with hearing impairment above 1kHz.These results indicate that the BSR provide information regarding hearing ability as low frequency as 500Hz.

In standardizing acoustic stimuli of electrocochleography and brainstem response audiometry, a realistic method for measuring transient tone stimuli as tone pips or clicks should be established as often pointed out. In this study, the frequency-specificity of the acoustic stimuli was evaluated by means of a real-time digital frequency analyser with the KEMAR manikin constructed as an ear simulator for hearing aid and related acoustic research. In addition, the effect of the frequency-specific tone pips on auditory brainstem responses was studied. The application of the KEMAR manikin could reproduce and simulate conditons of realistic stimulation in electric response audiometry. In particular, tone pips with a rise and fall time of 3 cycles seemed favorable both for the frequency specificity of the acoustic stimuli and for the neurophysiological synchronization.

Auditory evoked brain stem responses elicited by air and bone-conducted sinusoidal one wave stimuli (0.5, 1, 2 and 4kHz) were studied in patients affecteed by conductive, neurosensory or mixed type hearing disorders. We set 0dBNSL as the mean sensation level of six normal hearing adults. As the maximum bone-conducted stimuli producible be our equipment is 45dBNSL, no responses may by elicited by bone-conducted stimuli when the patient's sensation level is more than 35dBNSL.The A-B gaps obtained by ABR were compared with the ones by pure-tone audiometry.The A-B gaps obtained by these two methods were approximately equal, but in some cases showed some differences varying from-20dB to+20dB.The bone-conducted ABR thresholds were compared with the puretone auditory thresholds for bone conduction. The ABR thresholds elicitied by low frequency stimuli were influenced by hearing level in high frequencies. But the thresholds by both methods were similar in all frequencies (0.5, 1, 2 and 4kHz). As regards frequency specificity, we also found the usefulness of the bone-conducted ABR thresholds in clinical cases.