Efferently mediated changes in the quadratic distortion product (f2-f1)

In this study, a preparation was developed which continuously monitored the f2-f1, 2f1-f2, and 2f1-2f2 distortion products in guinea pigs, in response to various contralateral and ipsilateral tones and broadband noise. A consistent decline over time in the quadratic distortion product, f2-f1, with continuous ipsilateral stimulation was observed, confirming previous findings by Kirk and Johnstone [Hearing Res. 67, 20-34 (1993)]. Broadband noise applied to the contralateral ear simultaneously increased both the magnitude and the rate of decline. This effect was shown for primaries across all frequencies tested (f2 from 2-16 kHz, f2/f1 ratio of 1.26), though a maximal effect was found for f2 of 4 kHz. Experiments using contralateral tones demonstrated a half-octave shift in the frequency specificity of this effect. A more ''immediate'' suppression effect on 2f1-f2 (0.6 dB) was found in animals awakening from anesthesia, though this was even larger for f2-f1 (5.1 dB). Extended ipsilateral stimulation with contralateral noise results in completely diminishing f2-f1 to the noise floor. These findings suggest an adaptive process within the cochlea that may be modulated by efferent stimulation. A model relating the various even-and odd-order distortion products to outer hair cell receptor potentials is presented. (C) 1997 Acoustical Society of America.