Two-tone suppression of stimulus frequency otoacoustic emissions

Stimulus frequency otoacoustic emissions (SFOAEs) measured using a suppressor tone in human ears are analogous to two-tone suppression responses measured mechanically and neurally in mammalian cochleae. SFOAE suppression was measured in 24 normal-hearing adults at octave frequencies $(f_p=0.5–8.0\mathrm{kHz})$ over a $40\mathrm{dB}$ range of probe levels $\left(L_p\right)$ . Suppressor frequencies $\left(f_s\right)$ ranged from $-2.0$ to 0.7 octaves re: $f_p$ , and suppressor levels ranged from just detectable suppression to full suppression. The lowest suppression thresholds occurred for “best” $f_s$ slightly higher than $f_p.$ SFOAE growth of suppression (GOS) had slopes close to one at frequencies much lower than best $f_s$ , and shallow slopes near best $f_s$ , which indicated compressive growth close to $0.3\mathrm{dB}∕\mathrm{dB}$ . Suppression tuning curves constructed from GOS functions were well defined at 1, 2, and $4\mathrm{kHz}$ , but less so at 0.5 and $8.0\mathrm{kHz}$ . Tuning was sharper at lower $L_p$ with an equivalent rectangular bandwidth similar to that reported behaviorally for simultaneous masking. The tip-to-tail difference assessed cochlear gain, increasing with decreasing $L_p$ and increasing $f_p$ at the lowest $L_p$ from 32 to $45\mathrm{dB}$ for $f_p$ from 1 to $4\mathrm{kHz}$ . SFOAE suppression provides a noninvasive measure of the saturating nonlinearities associated with cochlear amplification on the basilar membrane.