Investigating sound-induced motion in the human cochlear hook region

Understanding the mechanical behaviour of the human cochlea remains a major challenge in auditory science. While most cochlear models focus on the basilar membrane (BM) and Organ of Corti, recent studies suggest that neighbouring structures, such as the osseous spiral lamina (OSL), cochlear partition bridge (CPB), and secondary spiral lamina (SSL), may also contribute to intracochlear motion. Here, we used spectral-domain optical coherence tomography vibrometry to characterize sound-induced motion across multiple cochlear structures in eight fresh (<24 h post-mortem) human temporal bones. Measurements were performed through the round window membrane, allowing minimally invasive access to the cochlear hook region. Frequency-dependent motion responses were quantified for acoustic stimulation between 0.5 and 5 kHz, a range relevant for speech perception. Linear mixed models (LMM) were used to assess velocity and phase across cochlear partition structures. The results show that at low frequencies (<1.5 kHz), OSL and SSL motion amplitudes matched or exceeded those of the BM, while phase differences between structures along the cochlear partition were more apparent. As frequency increased, motion became more localized to the CPB-BM junction and phase gradients along the cochlear partition flattened, indicating more uniform radial motion. However, at mid-frequency, the LMM showed the largest phase difference was apparent at the OSL. These findings suggest structure-specific mechanical functions for each structure, with the OSL and SSL potentially contributing to low-frequency filtering or damping. These results emphasize the need for investigating the cochlear partition mechanics beyond the basilar membrane.