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As is well known, raising of the threshold of hearing is caused by exposure to a continuous tone. The magnitude of this effect is a function of the frequency, duration and intensity of the tone. In general it is small, with intensities which do not exceed 90–100 db. above normal threshold in the middle frequency range.

THE sensation resulting in the human subject from a change of phase of (180°) occurring in the course of a continuous musical tone has been the subject of a number of earlier publications1. Under certain conditions, the sensation has been found to resemble the beat produced by two pure tones slightly out of unison, and has been described as a ” phase-change beat”2. As stated by Hartridge1, it is demanded by the Helmholtz resonance theory that the physiological event which corresponds to such a phase-change beat must be a transient arrest of the resonant elements of the internal ear brought about by the opposition of the impressed forces following the change of phase to the after-swings enforced by resonance.

IT has been shown by Witmaack and more recently confirmed by Kaida1, that following division of the VIII nerve in the cat central to the peripheral cochlear and vestibular ganglia, the ramus cochlearis fails to conform with the well-known Wallerian law in that the nerve elements including the spiral ganglion distal to the point of section undergo degeneration. In the following experiment, use has been made of this fact to adduce evidence bearing upon the problem of origin of the potential changes generated within the intact mammalian cochlea, in response and of a frequency corresponding to physiologically applied sound waves (Wever and Bray phenomenon).

IT has been shown by Jellinek and Scheiber1, and since confirmed2,3,4, that stimulation of the human cochlea by audio-frequency alternating currents results in a sensation of tone corresponding in pitch to the frequency of the stimulus. The possibility that the mechanism of excitation is by direct stimulation of the cochlear nerve fibrils has an important bearing upon existing views on audition, in so far as it would provide clear evidence in support of the telephone theory. It has been suggested as an alternative possibility that the electrical currents excite the nerve elements indirectly by setting in motion, in a manner not understood, those cochlear elements which are normally set in motion by physiologically applied sound waves. Such a mechanism of excitation would imply localization of the response according to frequency by resonance, and so would make possible an explanation of the observed phenomena upon the basis of the Helmholtz resonance hypothesis. This movement theory of excitation is strongly supported by experimental evidence which is now described. As stated by Hart ridge5, sudden reversal in the phase of a continuous musical tone results in the human subject in a sensation described as a phase change beat. This, in accordance with the resonance hypothesis, has been correlated with a transient arrest of the basilar fibres due to the opposition of the applied force following the phase reversal to the after swings due to resonance. Further, objective evidence of the occurrence of such an arrest has been provided by a recent study of the auditory tract potentials in the cat6.

DIRECT evidence in favour of the view that the vibrating elements of the cochlea are differentially tuned for frequency has been adduced in the following way: The cochlea of the cat is exposed under Nembutal anaesthesia. Viewing the preparation through a dissecting microscope, and using a dental burr, small excavations are now made in the bone, one proximal to the round window, the other towards the apex of the cochlea; the floor of these concavities may be made so thin that the cochlear fluids seep through, a state of affairs which may be easily seen through the microscope. At this juncture, small beads of mercury are placed in the depressions, using a micro-pipette; in this way, further loss of fluid is prevented, and a good electrical contact with the fluids of the inner ear established, by the insertion of platinum electrodes into the mercury droplets.