Evaluation of distortion products produced by the human auditory system in response to two-tone signals
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A preponderance of experimental evidence indicates that signal distortion is a by-product of normal auditory function. During the simultaneous presentation of two tones, intermodulation distortion products can be measured acoustically in the ear canal and electrically as auditory evoked potentials detected by electrodes placed on the scalp. The purpose of this investigation was to elucidate the sources of nonlinearity within the human auditory system responsible for generating the quadratic difference tone (QDT) and the cubic difference tone (CDT). Three experiments were conducted during the investigation. During the first experiment, measurements of distortion-product otoacoustic emissions and auditory evoked potentials were obtained from 24 normal-hearing adults (12 male, 12 female) in conditions with and without presentation of a contralateral noise. The effects of two-tone signal duration and mode of presentation (monotic vs. dichotic) on measurements of auditory evoked potential distortion products were examined in the second and third experiments. The results of the first experiment indicated that overall, both acoustic and electric distortion products were suppressed during presentation of a contralateral noise. However, suppression of acoustic distortion products was dependent on the sex of the subject, with males exhibiting greater suppression than females. Suppression of electric distortion products was dependent on the frequency of the two-tone signals and on the type of distortionproduct, with greater suppression for low-frequency signals and at the CDT compared to the QDT. The second experiment revealed that increases in the duration of the two-tone signals caused increases in the amplitudes of both CDT and QDT distortion products. However, differences in the rate of growth between CDT and QDT distortion products were observed. The results of the third experiment demonstrated that the proportion of individuals exhibiting both CDT and QDT distortion products was greater in the monotic condition compared to the dichotic condition. The findings from the first experiment of the investigation supported the conjecture that a cochlear nonlinearity produced CDT acoustic and electric distortion products. Evidence from the second and third experiments concerning the origin of the QDT distortion-product was inconclusive, and contributions from both cochlear and neural nonlinear sources could not be ruled out.