On the electrical responses of the cochlea and the auditory tract of the cat to a phase reversal produced in a continuous musical tone

Abstract

The audible effects of changing by π (180°), the phase of a pure musical tone, have been described in a number of earlier publications (Hartridge 1921, 1936). Under certain conditions of frequency of the tone, and temporal spacing of the phase changes, the resulting sensation was found to be comparable to what Myers (1928) has described as the “rattling” stage of beats produced by two musical tones slightly out of unison. For this reason it has been considered justifiable to apply to the phenomenon the term “phase-change beat”. The possibility that this beat might be attributable to some transient disturbance in amplitude or frequency of the wave series other than the change of phase itself has been negatived by a more recent investigation (Hallpike, Hartridge and Rawdon-Smith 1936). In this the wave-form of the phase change in the response from a loudspeaker was assessed by means of a piezo-crystal microphone, amplifier, and cathode-ray oscillograph. By these means it was shown to be possible to elicit from the loud-speaker a tone whose wave form was approximately sinusoidal; and which showed, moreover, at each change of phase, fair reproduction of the wave form of the impressed voltage. Even under such conditions the audibility of the “phase-change beat” was found to be undiminished, thus justifying the view that the beat was attributable to the change of phase and to that alone. The work to be described in this paper concerns the investigation of the electrical events which take place in the mammalian (cat) internal ear and auditory tracts, in response to such a phase change; events which may be considered likely to correspond to the subjective sensation now described as the phase-change beat. By such an investigation it has been hoped firstly to throw light upon the physical nature of the vibrating elements of the internal ear responsible for the initiation of the nervous impulses the action potentials of which can be recorded in the manner first described by Davies and Saul (1932) in the auditory tracts of the mid-brain; and, further, by a comparison of the response to a phase change of these tract potentials with the corresponding response of the cochlear potentials described by Wever and Bray (1930), to elucidate the still obscure problem of the latter’s origin.