We investigated phonetic confusion in patients with a sensorineural hearing loss (SNHL) to normal subjects using some modified Japanese monosyllables: /ki/, /sa/, /te/, /ha/ and /ri/. The percentage of correct answers were calculated in each group and each stimulus. In all groups, the percentage of correct answers, derived from the stimuli whose consonant and voice onset time (VOT) were amplified, showed a higher value than those of the original monosyllables. In contrast, the percentage of correct answers to the prolonged modification of stimuli showed lower value than those of the original monosyllables. Especially with /te/ and /ri/, the correct answer ratio deteriorated when the stimuli with prolonged process was used. We applied the factor analysis method to further analysis of the data. As for /ki/ and /sa/, some of the extracted factors were estimated as the factors in common between both groups; impaired and normal hearing. When the presented sound was classified by phonetic characters, there was no specific tendency. These results suggest that (1) the vowel following the consonant masks the consonant, (2) the perception of a monosyllable is possible only when enough information is taken in a certain duration, (3) the processing of sound based on the phonetic classification is not effective for improving the perception of patients with SNHL.
After recording evoked otoacoustic emissions (e-OAE) to click in guinea pigs by using ILO88 which was made by Kemp and Bray for easy recording and analysis of e-OAE, the changes after intravenous administration of furosemide (30mg/kg or 50mg/kg) were examined. The wave of e-OAE could be detected from 20 of 24 ears. After i.v. injection of furosemide (30mg/kg), e-OAE powers (total echo power and highest peak power in FFT pictures) were decreased quickly and showed minimum values after 5-10 minutes. Then they were increased rapidly and recovered normaly within 60 minutes after injection. On the other hand, e-OAE powers of the majority of the ears showed no response during 5-15 minutes after furosemide injection (50mg/kg). Then they recovered slowly as compared with the furosemide (30mg/kg) injection group. Above mentioned changes are similar to those of endocochlear potential (EP) after furosemide injection. From these results, we supposed that EP can contribute to the generation mechanism of e-OAE.
Sequential recording of auditory brain stem response (ABR), middle latency response (MLR), and slow vertex response (SVR) under logarithmic time scale was tried with several kinds of stimuli to elucidate the effects of rise time, duration time, and influence of stimulus envelope. The subjects were an adult man and 2 anesthetized adult cats. Sequential processing of the responses were carried out using a minicomputer Micro Vax II. The results were as follows; 1) With extending stimulus risetime, the early and middle component of auditory evoked potentials increased in latency. 2) With extending stimulus risetime, the early component decreased in amplitude. 3) With extending stimulus duration up to approximately 40ms, the wave P2 of SVR increased in amplitude.
The characteristics of ABR, MLR and SVR in two awake Japanese monkeys were investigated. Seven peaks were observed in ABR. The latency of five peaks, from the first to the fifth, which showed high reproducibility, was very similar to that of human ABR. In MLR two positive peaks were noted between about 17 and 19msec and between 30 and 35msec, respectively. In SVR, two positive peaks were manifested at 50msec and 100msec in both monkeys. The configurations of MLR and SVR were similar to those of humans. These data suggest that ABR, MLR and SVR in awake Japanese monkeys are similar to those of human, comparing to those of small animals like cats and guinea pigs, and the monkey is suitable as clinical models for humans.
A two-interval, two-alternative forced choise task was used to estimate psychophysical tuning curves (PTCs) in simultaneous masking. In each experiment, the masked PTCs for signal levels of 10, 20 and 30dBSL caused by the addition of a 500ms masker were determined as a function of masker frequency. The results were as follows: 1) The PTC for the highest signal level had the shallowest lower branch and steepest upper branch, and it had the narrowest tip. The PTC for the lowest signal level had the steepest lower branch and shallowest upper branch, and it the broadest tip. These properties of PTCs had been explained in terms of excitation-pattern model. 2) The bandwidths of PTCs were not dependent on the level and durations of signal, however, if we measured the bandwidths of tuning curves and filter shapes 10dB from the tip, we found that the widths of the auditory filters are 1.5-2.0 times the widths of the PTCs. This phenomenon was adequate to explain the basic effects of off-frequency listening on PTCs. 3) Temporal integration for signal level of 10 and 20dBSL were shown, however, for its level of 30dBSL, PTC tips were independent of signal duration. From these results, we found that the masked threshold of short duration and high level signal was independent of critical band. 4) The conventional PTC associated with combination tone and off-frequency gave a more accurate indication of frequency selectivity of the auditory system and showed that the bandwidth of PTC was to reflect properly the critical bandwidth.
The thalamocortical projections from the suprageniculate nucleus (SG) in the rat were studied with the anterograde and double fluorescent retrograde tracing techniques. Axon terminals labeled anterogradely with Phaseolus vulgaris-leucoagghutinin (PHA-L) were distributed not only in the temporal cortical region but also in the frontal cortical region, following PHA-L injection into SG. The laminar distribution of the terminals was found in the superficial and deep cortical layers. In the frontal cortex, SG axon terminals were distributed largely in the motor areas including the frontal eye field and those in the temporal cortex were mainly localized in the primary and associational auditory areas. The retrograde fluorescent tracers in a combination of Fluolo Gold/Nulclear Yellow or Fast Blue/Fluoro Gold were injected respectively in the frontal cortex and the temporal cortex. In all experiments, it was remakable that no double labeled cells were seen in SG and the medial geniculate body. The present results suggest that auditory inputs ascending through individually different SG neurons have influence upon the frontal region as well as the temporal region and especially, these inputs arriving at the frontal region may play an important role in the motor function linked with the auidtory stimuli.
The recent findings on auditory frequency selectivity and critical band for simultaneous masking were reviewed psychophysically and physiologically. The mechanisms of off-frequency listening, cubic distortion product, olivocochlear bundle, and lateral inhibition contributes significantly to sharp tuning curve and critical band formation. The origin of the tuning curves and the critical bands filters lies in the frequency resolution of the basilar membrane. Compared with frequency selectivity, which is measured behaviorally, three characteristics of the critical bands, i.e. (1) appropriate absolute filter handwidths, (2) sound intensity independence of filter bandwidths, (3) linear spectal integration within the filter, are not present in the discharge rates and average temporal patterns of single neurons in the auditory nerve and ventral cochlear nucleus. These all bandwidth-related properties of the internal auditory filter, however, are found in single neurons of the inferior colliculus. Frequency selectivity and critical band filtering in a behaviorally relevant way are therefore established at the auditory midbrain level. The information in a sound of signal is conveyed by more than 10 parallel brainstem projections to the inferior colliculus. Thus a inferior colliculus neuron may participate in parallel functional networks processing information about spectral, temporal, and spatial information at the same time.