Upon acoustic stimulation of the mammalian cochlea, a travelling wave will move in a base-to-apex direction, peaking close to the resonant place of the basilar membrane. A gradient of different resonances along the basilar membrane gives rise to frequency-place coding, or tonotopicity, throughout the majority of the cochlea. Per our previous findings, the organ of Corti in the guinea pig cochlear apex does not display tonotopicity. The behavior of the basilar membrane has not been explored throughout the cochlear apex, and so hypothetically tonotopicity is conserved at the level of this structure. Outer hair cells, which influence organ of Corti and basilar membrane mechanics, provide amplification throughout the cochlea. The mechanism for this process in the region close to the helicotrema, which is important for speech and music perception, is not fully described. Here, we describe the acoustically evoked vibrations measured using optical coherence tomography (OCT) at three locations within the guinea pig cochlear apex. We note a decline in the ratio of organ of Corti and basilar membrane gain with proximity to the helicotrema and show that distinct nonlinear behaviors are associated with each location. Phase differences between the cellular portion of the organ of Corti and basilar membrane were present and remained after furosemide treatment. We report that the frequency tuning of the apical basilar membrane is inconsistent with standard models. The analysis of nonlinearity indicates that the mechanisms governing sensory transduction, while still not elucidated, change rapidly within the apical turn—findings that are important for understanding how communication-relevant sounds are encoded by the cochlea.
Burwood et al. (2026) studied this question.