Since Dix, Hallpike, and Hood (1948), it has been widely believed that the recruitment phenomenon is not found in cases with retrocochlear hearing impairment. In fact, a number of investigators have reported that many cases with retrocochlear lesion do not show the recruitment phenomenon. But actually, a recruitment test, the author believes, has not been a conclusive cue in differential diagnosis in deafness due to cochlear and retrocochlear lesions. Though it is not usual, case with probable retrocochlear lesion sometimes shows the recruitment phenomenon and cochlear lesion sometimes does not.

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For the differential diagnosis of retrocochlear deafness, some binaural hearing tests, that is, binaural summation at the threshold of interrupted tone, binaural supplementation of SISI score, binaural fusion test using band-pass filtered speech sounds, and interaural discrimination or binaural separation test using dichotically presented speech sounds were investigated.

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I. Directional Hearing Test
The directional hearing test was performed, measuring interaural time difference between two ears to discriminate the tone image moving in the head or in its close proximity.

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Amplitude of fixed-frequency Békésy tracing in normal and abnormal ears was discussed. The paper covers following items: (1) components of amplitude of Békésy tracing, (2) normal amplitude for continuous and interrupted tones, (3) abnormal narrow amplitude in the ears with sensorineural deafness, (4) relation between narrow amplitude and recruitment phenomenon, (5) narrow amplitude and frequency of test tones, (6) narrow amplitude for continuous and interrupted test tones, (7) decrease in amplitude in normal ears after exposure to loud tones, (8) relation between narrow amplitude and abnormal auditory adaptation.

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The author attempted to develop a new testing method to diagnose affected parts in perceptive deafness easily, quickly, and accurately in the course of daily clinic.
The author found, as a result, that the diagnosis affected parts is possible by combining three test methods, namely the conventional audiometry (air and bone-conduction threshold test), a threshold test using an interrupted tone and a continuaus tone which are self-recorded, and a threshold difference test based on the masking level difference phenomenon of pure tones.
This report describes the principles of diagnosis the scope of application of each test method.
1. The conventional audiometry; This method should be considered to be threshold test designed to screen perceptive deafness out of various hearing disturbances. It should be noted that this routine audiometry, when employed for screening purposes, in subject to the following three restrictions: 1) the limit of the out-put level of bone conduction, 2) cases where the measurement of bone conduction threshold is impossible, and 3) cases where some types of perceptiv deafness fail to be screened.
This problem can be solved by devising a test method which would enable screening of perceptive deafness without measuring bone conduction thresholds. As a method to suit this purpose, I have introduced - combination of an air conduction threshold test and an intensity difference limen using "superimposed pip" test developed by the author.
2. Diagnosis by self-recording method; This is a new testing method developed by the author and his collaborators with a view to classify perceptive deafness into inner ear disturbances and retro-cochlear disturbances.
In diagnosis, the author used the threshold difference between the threshold obtained by the interrupted tone (duration 200msec; rise and fall time 25msec ; repetition time 400 msec) ascending method and the threshold obtained through the continuous tone descending method (after exposing the ear to the level of 85dB on the hearing loss dial for thirty seconds). The case may be regarded as inner ear deafness, if this threshold difference is within 20dB, and as retro-cochlear deafness if the value is over 20dB.
3. Diagnosis by Masking Level Difference Test (MLD-Test, EBIHARA'S) ; This method is employed to classify retrocochlear disturbances into those located in peripheral region and those located in central region of the tertiary neuron in auditory passway.
This principle employs the difference in pure tone threshold caused when the sound images of a pure tone and of a noise are fused or separated artificially in binaural hearing. With the 1000cps, tone used for the MLD-Test, there is no distur. bance if this threshold difference is above 5dB, and if the value is under 4dB, the central region of the tertiary neurons in central auditory passway will be considered affected

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The severe acute CO-poisoning series among the hundreds sufferers of the last great industrial accident at the "M" Coal Mine have been treated in the Kyushu University Hospital for over 2 years.
We had an opportunity, rarely to be experienced, to perform the binaural articulation testing method wit filtered speech sounds against these 17 patients of CO-poisoning anamnesis.
They showed various findings in neurological and variegated cerebropathological signs, the latter being amnesia of fixation, sensory aphasia, agnosia, agraphia, dementia of various degrees, emotional deterioration, and unsteadiness of feeling. They showed also abnormal EEGs. Three cases had tinnitus soon after the accident and one case after 6 months. Except these four cases, no hearing disorder complaints were made. There were 12 ears with cs dip in 8 cases. With the exception of certain cases which showed a slight hearing loss in high frequency range, the average hearing loss of the main speech frequency range in the majority was measured within normal limits.
The normal control value of the binaural lowand high-pass articulation at the speech intensity level of 60 dB was 60%.
The filtered speech sounds of our equipment consists in a steep elimination of the frequency range between 850- and 3400-cps. Nine cases of the CO-poisoning series, the majority of the total observed cases, however, showed values below 50 %. Four cases were 60-80%. Furthermore, inspite of excellent hearing ability for high tones, the articulation score of the binaural test with filtered speech sounds was remarkably poor, 30% in 2 other cases.
The reason why they showed such remarkably diminished values of articulation test by filtered speech sounds in spite of their results of the binaural articulation test with overall passed speech sounds, should be found in the disorder of the central auditory pass way above mesencephalon, especially at the involvement of the connecting fibers of corpus callosum caused by CO-intoxication-, which participate in role of central fusion to complete the projected sound images from both ears.

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