A upward-sweeping FM sound and a downward-sweeping sound were used to estimate psychophysical tuning curves (PTCs) in simultaneous masking. In each experiment, masked PTCs for signal level of 15, 20, 30 and 35dB SL caused by the addition of a 800ms masker (FM sound) were determined as a function of masker frequency. The results were as follows: 1) When a upward-sweeping FM sound was used to estimate masked PTCs, the PTC for the higher signal level had the shallower low-frequency slope and steeper high frequency slope, and 10dB bandwidth for PTC coincided in restricted PTC. 2) When a downward-sweeping FM sound was used to estimate masked PTCs, the PTC for the higher signal level had the steeper low-frequency slope and shallower high-frequency slope, and 10dB bandwidth for PTC coincided in unrestricted PTC. 3) The PTCs as determined by masking in the presense of FM sounds apparently restricted subjects to listen off-frequency, and it was shown that the bandwidth of PTC properly refects the critical band. 4) The properties of these PTCs can be easily explained by a neural network model of lateral inhibition, therefore, we hypothesized that hierarchical processing is incorporated within inferior colliculus for parallel signal processing. 5) We concluded that frequency resolution and critical band filtering are therefore realized in a psychophysically relevant way in the autitory midbrain level.
40-Hz auditory steady-state response (SSR), ABR and MLR were recorded in the waking and sleeping states in 9 adults with normal hearing. The responses were elicited with 500Hz tone pips at 55dBnHL given with intervals of 25ms and 125ms for recording SSR and ABR-MLR, respectively. Each averaged response was the composite of 2048 stimulus presentations. Since the stage 2 sleep was most frequently observed in the sleeping state, amplitude measurement of the averaged responses was exclusively made in the responses recorded during the stage 2 sleep. Synthesized SSRs (sSSR) were made from the recorded ABR-MLR waveforms and their amplitudes were compared with those of the really recorded SSRs (rSSR). In the waking state, the ratio of mean amplitudes of rSSR to sSSR reached 0.845:1, whereas the ratio decreased to 0.557:1 during sleep. Statistically significant difference was found in the values of ratios between awake and asleep states. The results supported the assumption that auditory SSR in the waking state was obtained from the linear super-imposition of ABR and MLR. However, such linear relationship between SSR and ABR-MLR could not be considered to exist in the sleeping state.
Electorical dipole moments of auditory brainstem responses (ABR) were investigated by dipole localization methods in humans. For wave I and wave V, dipole magnitude variability between subjects at a specific age was considered too large to accept as a useful measure. A spherical head shape and homogeneous conductivity were assumed in the present study. Variability of the estimated dipole magnitudes might be caused by the shape of the head, inhomogeneous conductivity, and errors of the assumed dipole location and the electrodes location. Wave I dipole magnitude reduced greatly with a decrease of stimulus intensity from 90 to 60dBnHL, and decreased gradually with a further decrease of intensity. Wave V dipole magnitude showed a smaller change with stimulus intensity than wave I dipole magnitude.
The suitable MPO levels of hearing aids were compared with the UCLs in 279 cases with mild and moderate sensorineural hearing loss. The suitable MPO levels were fixed by repeated adjustment and audition of hearing aids, and UCLs were measured beforehand. In most cases, MPO levels were fixed lower than UCLs, and the difference between the two was large in a few cases. In few cases, MPO levels were fixed higher than UCLs, but the difference between the two was small. The difference between MPO levels and UCLs varied in each cases, and definite relation between MPO levels and UCLs was not found. Clinically, UCL is not useful as a guideline for adjustment of MPO Level.
The effects of transient local anoxia of one hour duration upon the EP and the cochlear blood flow measured with Laser Doppler method were examined under hypothermic condition (32.5°C, n=5 and 28°C, n=5) using albino guinea pigs, and the results were compared with those of the controls (37°C, n=8). The pre-anoxic EP values ranged from 79mV to 100mV, and there was no significant difference between the control and the hypothermic animals. On termination of local anoxia, the EP rapidly increased, reached the maximum value (EP peak) and then attained a certain atatic levels after one hour (Static EP). The results were summarized as follow; 1) No difference was found in the lowest levels of the EP during anoxia between the controls and the hypothermic animals.; 2) The levels of the EP peak and the Static EP in animals with the temperature of 28°C were significantly higher than those of the controls (P<0.01).; 3) Cochlear blood flow during and after one hour anoxia exhibited the almost same values in the controls and the hypothermic animals. The present result indicates that the reduced metabolism due to hypothermia decreases the damage of the strial cells, resulting in the higher recovery level of the EP after transient local anoxia.
The effect of transient local anoxia of one hour duration ear upon the EP and the cochlear blood flow in the albino guinea pigs treated with phenytoin (PHT) and mannitol was observed, and the results were compared with those of controls. Animals were treated before inducing local anoxia with following drugs, i.e.; 1) PHT 10mg/kg i.v. (n=3); 2) PHT 20mg/kg i.v. (n=3); 3) PHT 30mg/kg i.v. (n=5); 4) mannitol 10ml/kg i.v. (n=4); 5) PHT 30mg/kg and mannitol 10ml/kg i.v. (n=5). The results obtained in the present study were as follow; 1) The post-anoxic levels of the EP were significantly higher in the animals treated with PHT 30mg/kg and mannitol 10ml/kg than those of the control (p<0.01).; 2) The cochlear blood flow after anoxia was generally higher in the animals treated with PHT 20, 30mg/kg and PHT 30mg/kg and mannitol 10ml/kg than in those of controls. Mannitol has an action of free radical scavenger and PHT is considered to have several protecting mechanisms under anoxic condition. The higher recovery levels of the EP in the present study indicate that these actions of PHT and mannitol protect the strial cells from the damage due to anoxia.