Electro-Cochlear Potentials Elicited by Sinusoidally Modulated Signals

Abstract

Responses of the guinea pig cochlea to amplitude-modulated stimuli were measured with the aid of a gross electrode. The dynamic characteristics of this part of the auditory system was studied by varying several parameters of the applied signal. The signals used as carriers in our experiments were either white noise or pure tones of 1 and 4 kHz. The modulation frequency, dynamic and intensity characteristics were determined by varying the modulating frequency, the modulation depth and the intensity of the applied signal. To get an idea about possible non-linear aspects of the system under investigation, we always computed the Fourier transform of the response data and plotted the amplitude of the various harmonics and the phase of the fundamental separately as functions of the signal parameter in question. The greatest response was always found at a modulation frequency of about 200 Hz, with a relatively gradual rise up to this frequency and a sharper drop above 200 Hz. The phase of the fundamental changes very rapidly at frequencies above 200 Hz. The distortion is mainly second-harmonic and has a maximum about 1 octave lower than the fundamental. The carrier frequency and the intensity of the stimulus were not found to have a great influence on the frequency characteristic. For small modulation depths, the system is nearly linear; at higher intensities and modulation depths saturation occurs, coinciding with a relative increase in the intensity of the second harmonic with respect to the fundamental.

Key Words:

• Electro-cochlear potentials
• Amplitude-modulated signals
• Dynamic behaviour
• Frequency characteristic
• Dynamic characteristic