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

In this study, a preparation was developed which continuously monitored the f2−f1, 2 f1−f2, and 3f1−2 f2 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 2 f1−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.