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Abstract
In order to understand fully the diagnostic significance of electorcochleography (E. Co. G.) in clinical audiology, the present study was designed to cover 3 problems concerning (1) objective threshold audiometry by E. Co. G., (2) objective bone-conduction audiometry by E. Co. G., and (3) objective differential audiometry of sensory-neural hearing loss by E. Co. G.
The compound action potential (AP) of the cochlear nerve was used as an indicator for this purpose. It was recorded simultaneously from both ears of the same subject with a promontory electrode or a meatal skin electrode by means of an average response computer.
Acoustic signals such as clicks or 3 different tone-pips with center frequencies of 2 000, 4 000, and 8 000 Hz, were given to the subject by both air-conduction and bone-conduction.
Five different electric indices were determined from the AP measurements as measures of neurophysiological activity of the cochlea, i. e., (1) the threshold of the AP, (2) the input-output function of N1, (3) the increments of the N1 amplitude, (4) the intensity-latency relation of N1, and (5) the wave form of the AP. These results lead to the following conclusions
(1) The promontory-recorded AP thresholds were in an excellent agreement statistically not only with the subjective thresholds for the same stimuli but also with the clinical hearing thresholds shown on the audiogram.
(2) The promontory-recorded AP provided significantly more precise and reliable data on E. Co. G. as compared with the meatal skin surface-recorded AP. The difference between the AP thresholds and the subjective thresholds, on the average, was + 0.2 dB, subjective more sensitive, for the promontory recording, and + 17.8 dB, subjective more sensitive, for the meatal skin surface recording. But the choice of electrodes must be according to what is required of the E. Co. G.
(3) The bone-conduction AP measurements could be carried out with the promontory electrode. There was a relatively good agreement between the bone-conduction AP thresholds and the bone-conduction subjective thresholds, but the agreement was worse than that for the air-conduction AP measurements.
(4) An air-bone gap was determined objectively from the input-output and intensity-latency relations of the bone-conduction AP compared to those of the air-conduction AP. The promontory-recorded AP measurements by bone-conduction should be very helpful in estimating objectively the degree of an air-bone gap.
(5) There were some interesting correlations between the changes in the electric indices of the AP measurements and the patterns of subtractive loss. Some patterns of subtractive loss seemed to be specified by various combinations of the threshold elevation of the AP, the 'H curve' type of input-output relation, the reduction in the maximum amplitude of N1, and the distortion of the AP wave form.
(6) A distinctive pattern of the distorted AP wave form appeared in Ménière's disease. The depression and recovery of the AP was closely correlated with the course of audiological and vestibular symptoms of this disease.
(7) On the basis of observations on the AP measurements for sensorineural hearing loss, 3 conceptual populations of sensory units are hypothesized, i.e., (a) the more sensitive population of sensory units capable of discharging well-synchronized impulses with lower thresholds, (b) the less sensitive population of sensory units capable of discharging well-synchronized impulses with higher thresholds, and (c) the more sensitive population of sensory units capable of discharging less well-synchronized or dispersed impulses with lower thresholds.
Finally, some serious problems confronting the routing clinical use of E. Co. G. are discussed, and the current status of E. Co. G. is described. It seems reasonable to speculate that E. Co. G., combined with electric response audiometry, will serve as the most practical method of objective differential audiometry as well as of objective threshold audiometry.