Bias-tone suppression of the auditory-nerve initial-peak (ANIP) response supports the hypothesis that ANIP is driven by cortilymph-organ-of-Corti-core longitudinal motion.

The auditory-nerve initial peak (ANIP) is the earliest response from moderate-to-high-level clicks in cat auditory-nerve type-1 fibers with characteristic frequencies (CFs) <3 kHz. ANIP is inhibited by medial-olivocochlear-efferent stimulation, and is suppressed when low-frequency (50 Hz) "bias" tones place the outer-hair-cell (OHC) mechano-electric-transduction (MET) function into low-slope, saturating edges. These properties show that ANIP emanates from OHC motility. Unexpectedly, the bias-tone phase that produces the most suppression (the major-suppression phase, MSP) is opposite for ANIP versus for low-level click or tone responses. We present data showing this difference, plus this hypothesis for its origin: Low-level responses take several cycles to build up and their gain is set by the local MET-function slope; their MSP is when the bias tone quasi-statically deflects the MET operating point to its nearest low-slope, saturation edge. In contrast, ANIP is the first half-cycle response from higher-level clicks, and requires OHC stereocilia deflection only in one direction. When a bias tone places the OHC-MET operating point at its nearest saturating edge, a rarefaction-click's unidirectional initial stereocilia deflection is away from this saturation, enabling a large traverse of the MET-function high-slope, high-gain region. The bias-tone level necessary to reach criterion suppression was higher for ANIP than for low-level clicks, which indicates that ANIP suppression occurred more basal where BM stiffness is higher. We hypothesize that ANIP is driven by apically-directed transverse motion of cortilymph and nearby organ-of-Corti-core tissue, i.e., is separate from the traveling-wave. Waxing-and-waning click responses also show that traveling waves are not simple, unitary waves.