Low-frequency and high-frequency distortion product otoacoustic emission suppression in humans.

Distortion product otoacoustic emission suppression (quantified as decrements) was measured for f(2)=500 and 4000 Hz, for a range of primary levels (L(2)), suppressor frequencies (f(3)), and suppressor levels (L(3)) in 19 normal-hearing subjects. Slopes of decrement-versus-L(3) functions were similar at both f(2) frequencies, and decreased as f(3) increased. Suppression tuning curves, constructed from decrement functions, were used to estimate (1) suppression for on- and low-frequency suppressors, (2) tip-to-tail differences, (3) Q(ERB), and (4) best frequency. Compression, estimated from the slope of functions relating suppression "threshold" to L(2) for off-frequency suppressors, was similar for 500 and 4000 Hz. Tip-to-tail differences, Q(ERB), and best frequency decreased as L(2) increased for both frequencies. However, tip-to-tail difference (an estimate of cochlear-amplifier gain) was 20 dB greater at 4000 Hz, compared to 500 Hz. Q(ERB) decreased to a greater extent with L(2) when f(2)=4000 Hz, but, on an octave scale, best frequency shifted more with level when f(2)=500 Hz. These data indicate that, at both frequencies, cochlear processing is nonlinear. Response growth and compression are similar at the two frequencies, but gain is greater at 4000 Hz and spread of excitation is greater at 500 Hz.