Dynamics of the auditory evoked potential and the auditory evoked neuromagnetic field obtained with the interstimulus interval of 300msec, in response to various kinds of natural sounds and artificial sounds, were investigated and the following results were obtained; 1) Remarkable left-right auditory evoked potential differences depending upon the physical characteristics of sounds were found, and this mechanism depends not on any higher function, but on a more basic function. 2) The results obtained by the above method were in complete agreement with those obtained by Tsunoda's key-tapping test. 3) In the noraml Japanese, the right hemisphere was dominant for the sounds with the following physical characteristics; (1) single formant (or formant-like structure) or simple attenuation in higher frequency ranges. (2) two or more formants (or formant-like structures) in harmonic relations. (3) small pitch fluctuations and rich harmonic structures, particularly, when sounds consist only of harmonic tones. 4) The most active area of N1-components of the AEPs was in general the midtemporal in either the left or the right hemisphere in accordance with the relation between the physical characteristics of sounds and the left-right AEP differences. The most active area of P1-components was more sensitive to the physical structures of sounds. 5) The gravity center of the equivalent current dipole corresponding to N1-componint was estimated to be in or near the primary auditory cortex, while that to P1-component was estimated to be in the acoustic radiation or its neighborhood. Left-right differences in the quantities of the tangential dipole moments corresponding to both P1-and N1-components were in agreement with those of the auditory evoked potentials.
The purpose of this paper is to discuss the results of hearing tests on 14 children suspected of nonorganic hearing loss and to advocate some diagnostic procedures to avoid incorrect nonorganic hearing loss. The effective test methods were: (1) play audiometry in 6 cases; (2) re-test of standard pure tone audiometry in 4 cases and (3) random presentation of test sounds in the pure tone audiometry in 1. In 11 out of the 14 cases, nonorganic hearing loss was finally denied by utilizing these tests. It was considered that these 11 children merely could not understand the instructions given them during the previous test. Therefore, prior to ABR test, other hearing tests such as play audiometry, etc., should be given to those children suspected of nonorganic hearing loss to confirm their actual hearing acuity. We stress that the diagnosis of nonorganic hearing loss should be made only after the hearing tests mentioned above were carefully used.
The effect of band noise masking on persistent tinnitus was investigated in 191 patients. The effect of tinnitus masker was evaluated by the patient's complaints after two hours' masking. The causes of tinnitus in eluded pesbycusis, nose dnduced deafness, endlymphatic hydrops, sudden deafness sensorineural deafness of unknown etiology. The band noise masker was most effective on tinnitus due to presbycusis. Other influential factors in the effect of the tinnitus masker were age and hearing level at the tinnitus frequency. The suppression of tinnitus was more prominent in older patients and the patients whose hearing levels of the tinnitus frequencies were less than 30dBHL.
I obtained 118 extracellular single-unit recordings from the medial geniculate body (MG) in unanesthetized guinea pigs, and studied responses of the MG neurons to pure tone stimuli and to species-specific vocalized sounds. About 20% of the MG neurons were more sharply tuned to the characteristic frequency (CF) than primary neurons. These sharply tuned neurons were mainly located in the ventral division. Of 79 neurons with a CF within the frequency range of the vocalized sound, only 23 neurons (29%) responded to the vocalized sound in the response pattern that was predicted from the response properties to pure tone stimuli. Five neurons (6%) were inhibited by the vocalized sound, and 24 neurons (30%) had no response to the vocalized sound. The neurons responding in each way seemed to be distributed equally in the different subdivisions. One MG neuron had no response to any pure tone but a strong excitatory response to one vocalized sound. The responsiveness and predictability of MG neurons to the vocalized sound were low. These results suggest the presence of the neurons described as the feature detectors in the sub-cortical MG level.
Functional hearling loss has resently been increased. However, the etiological mechanism has not yet been fully clarified. The authors studied on recognition at the cortical level using CNV objective audiometory. This study disclosed that the recognition of sounds is seen in pseudohypoacusis and in patients with psychogenic hearing loss. This finding seems to disprove the notion that psychogenic hearing loss is an impairment of the auditory pathway at the subconscious level. Based on this result, we may conclude that the place where psychogenic hearing loss occurs does not lie in the subconscious level, but lies in higher-level of mental activity (i.e., the place which is related to the question of “how to grasp the sound inside oneself”). A fundamental difference between psychogenic hearing loss and pseudohypoacusis lies in that the former is based on the mentally immature self and a mental conflict within themselves, while the latter in based on the patient's will to malinger. Therefore, in cases of psychogenic hearing loss, elevated auditory thresholds should be observed as an indicator of psychological imbalance.
Basic chracteristics of hearing elicited by applying the amplitude modulated electrical stimulation to the external ear were investigated by means of psychophysical technique. The site in which sound was generated was discussed based on sound lateralization characteristics produced by applying electrical and sound stimulations to both ears. As the results, perceived sound by electrical stimulation was found to be pure tone and its threshold depended on applied voltage. The minimum auditory threshold was obtained at 800Hz signal frequency, at 100% modulation rate and at 50% duty ratio in the case of 50kHz carrier frequency. In the experiment of sound lateralization, perceived sound was proved to be caused by electrophonic effect and the site in which sound was generated was the skin surface of the external ear under the electrode.