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.
Effective parts of tone pips to evoke anditory brain stem response (ABR-Na) were studied using 6 cats. Amplitudes of the responses evoked by equal-slope-tone-pips with different rise-times were analysed at three frequencies (500, 1000 and 2000Hz). The results indicated that the effective parts measured from onset were more than 4msec at 500 and 1000Hz, and more than 2msec at 2000Hz. Each cycle of this range contributed to evoke the responses, at weak intensity level above response thresholds. Contribution of the initial part of the tone pips was larger as the slpoe was broadened. The results of this experiments provide useful informations to determine the appropriate stimuli for objective audiometry.
Power spectral analysis and digital filtration of the auditory brain stem responses to pure-tone stimuli at the frequencies from 0.5 to 4kHz were performed in three adult subjects with normal hearing. Three dominant peaks ranging in frequency 50-150Hz (peak I), 500-600Hz (peak II) and 1000-1100Hz (peak III) were observed in the spectrograms of the responses to 4kHz at 80dB SL. Powers of peaks II and III were markedly decreased or completely disappeared with decreased intensity as well as decreased frequency of stimulus, while peak I around 100Hz was consistently appeared. Main spectral components of the individual waves in the responses were revealed as follows: peaks II and III for waves I, III and V, peak III for waves II and IV, peak II for waves VI and VII, and peak I for the slow positive deflection in the responses.
Auditory Brain Stem Responses (ABR) were recorded in a 5-year-old girl with encephalitis of the brain stem. The patient was admitted to the hospital in semicomatous condition with irregular respiration. Hyperreflexia, abnormal reflex and occasional opistotonic posture were also seen. ABR in the right ear on admission showed wave-I only and prolonged latency. In the left ear, the wave from I to IV with normal latency were obserevd. The other waves in the both ears were not identified. Her consciousness and neurological abnormalities were gradually recovered during next 7 weeks. Improvement of the clinical features were almost parallel to that of ABR. The latency of wave V was still prolonged 20 months after the onset of the disease, but gradually improved. ABR can be usefull for diagnosis and follow-up of the brain stem disorders.
Band-limiting experiments were performed at five spectrum level differences (10, 20, 30, 40 and 50dB) between the 1st and 2nd bandpass noise maskers for simultaneous, forward and backward masking. Masked threshold for 2kHz sinusoidal signal (duration 15ms and rise-fall time 5ms) was studied as a function the two bandpass maskers (duration 600ms and rise-fall time 10ms or 5ms), and its center was fixed at 2kHz, and the 1st masker level was fixed at 30dB SPL. The results were as follows. 1) Masked threshold for a 2kHz sinusoidal signal was increased and reached its magnitude for the bandwidths greater than ten times of critical band at the delay time of 8ms in the simultaneous masking, and the critical bands estimated at the delay time 300ms in the simultaneous masking were described by a level-dependent critical band with upward spread. 2) The signal was always and immediately preceded (in the backward masking) or immediately followed (in the forward masking) the 1st masker. The 1st masker-plus-signal and the 2nd masker or the signal and 2nd masker were presented simultaneously. The suppression effect was increased at supracritical bands and the suppression effect at the masker band width 4kHz, level differences between the 1st and 2nd masker, from 10dB to 30dB, increased for the forward masking, but level differences between the 1st and 2nd masker, from 30dB to 50dB were increased rapidly for the backward masking. 3) These differences in the suppression effect observed in the forward and backward masking was consistent with an interpretation that the suppression in the forward masking was determind masking by a reduction of the effective level of the masker in the peripheral processes, and that the suppression in the backward masking was dominated by lateral inhibition of the central processes, possibly the dorsal cochlear nucleus.