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The great sensitivity and frequency selectivity of mammalian hearing originates in the mechanical properties of the cochlea. Cochlear motions in response to sound are amplified using metabolic energy. The motor element of this cochlear amplification is presumed to be the outer hair cell (OHC), that expands and contracts lengthwise in response to a change in receptor potential at audio frequencies. How thousands of OHCs, acting through the local cytoarchitecture, work to achieve cochlear amplification is not fully understood. The properties of individual OHCs measured in micro-chamber environments are low-pass with a corner frequency of, at most, a few kHz. OHC corner frequencies within the cochlea have been estimated to be ∼3 kHz. Since in-vivo cochlear motion measurements show amplified frequency responses that extend up to many tens of kHz, but OHCs have corner frequencies of a few kHz, it has been suggested that OHCs are not active at the highest frequencies. How to reconcile these measurements has been a challenge. To address this, we made transverse cochlear motion measurements using a high-resolution optical-coherence-tomography (OCT) system at approximately the two ends of the OHC axial length: at the OHC bottom near the OHC-Deiter-Cell junction and at the OHC top near the reticular lamina. From these in-vivo measurements near the 42 kHz best frequency (BF) region of the gerbil cochlea, we determined the transverse differential response of OHCs. To remove phase from the traveling wave, we calculated the ratio of the RL motion to the motion at the OHC-Deiter-Cell junction. The OHC top-to-bottom phase difference was almost exactly ½-cycle at frequencies up to 35-40 kHz. As frequency increased above 40 kHz, the phase difference decreased until near BF there was very little phase difference. The OHC top-to-bottom amplitude ratio transitioned from the OHC top moving less than the bottom at low frequencies, to the OHC top moving more than the bottom, at about 30 kHz, which is approximately the frequency above which there is traveling-wave amplification. Our data show that OHCs had length changes at frequencies up to 50 kHz, more than a decade above their low-pass corner frequency in vitro. [ABSTRACT FROM AUTHOR]