Abstract
PURPOSE: Biasing of the cochlear partition by tones below 100 Hz is commonly used to investigate cochlear non-linearity. Their long periodicity allows resolution of the suppression of cochlear responses as the partition is displaced periodically away from its resting position. The purpose of this study was to quantify by how much the biasing tone (BT) level needed to be increased to keep an equal suppression depth in OAE and an equal criterion suppression of loudness while the levels of the response-evoking stimuli were increased. METHOD: Suppression-period patterns were obtained for the distortion-product and stimulus-frequency otoacoustic emissions (DPOAE: N = 8 ears, SFOAE: N = 6 ears) using a 55-Hz BT, conditions in which the primary frequencies were at least 3 octaves above the BT frequency. A prior search for primary parameters that allowed the highest recording SNR was conducted, including an optimization of the lower primary tone level (L1) for the DPOAE. The BT level was adjusted so that the maximum suppression depth was kept constant at 9 dB for both OAEs, for various primary levels. Also, the BT level required to reach an equal criterion suppression in the loudness of 1- and 2-kHz tone-pip probes was measured psychoacoustically (N = 4) at various probe levels using the same BT. RESULTS: The increase in suppressor level per increase in probe level was similar (~ 0.4 dB/dB) for equal-loudness suppression of the tone pips, 9-dB suppression of the SFOAE, and the 2F1-F2 DPOAE, when the increase in L1 was considered. Average BT levels increased linearly over the whole range of probe levels tested. CONCLUSION: As the observed iso-suppression rate is broadly consistent with the growth of basilar membrane vibration with increasing stimulus level in the active region of the traveling wave, we conclude that equal suppression of cochlear responses during partition biasing occurs when bias displacement grows in proportion to the probe's traveling wave amplitude in its active region.