Under bone conduction (BC) stimulation, the sound energy propagates from the stimulation site to both cochleae via five main BC pathways (Stenfelt and Goode, 2005a), however, under physiological conditions, the most prevalent (Stenfelt, 2016) are the ones involving the motion (i.e., inertial effect) and deformation (i.e., compressing and tension) of the otic capsule. Several of these BC pathways have been evaluated previously, both experimentally (Stenfelt et al., 2002; Stenfelt et al., 2003; Stenfelt, 2015; Stump et al. 2018; Dobrev et al. 2019; Dobrev et al. 2020a) and numerically (Stenfelt, 2016; Chang et al., 2016).

A key aspect in quantifying the contribution of each BC pathway and understanding the underlying physical phenomena, and specifically the role of the otic capsule, is the objective estimation of the cochlea activation as well as the corresponding hearing perception. An excellent method to assess cochlear activation is intracochlear sound pressure measurements in scala tympani and scala vestibuli. This methodology has been established by several groups for air conduction (AC) (Dancer and Franke, 1980; Nakajima et al., 2009; Frear et al., 2018; Peacock et al., 2018; Raufer et al.,2021) and BC (Mattingly et al., 2020; Stieger et al., 2018; Borgers et al., 2019), using off-the-shelf (Grossöhmichen et al.,2016) or custom-made pressure probes (Olson, 1998; Pfiffner et al., 2016; Liyanage et al., 2021). Applying this method under BC stimulation carries numerous challenges, primarily due to the potential for relative motion between the pressure probe and the cochlear walls (Stieger et al., 2018; Borgers et al., 2019). However, such a methodology has been successfully applied to the study of cochlear activation under both osseous and non-osseous BC stimulation conditions (Dobrev et al., 2022).

Current methodologies for direct intracochlear pressure measurements are not suitable for clinical purposes and are not widely available for research applications. However, measurements of bone vibration have been readily available, via accelerometers or laser Doppler vibrometers (Stenfelt and Goode, 2005b; Reinfeldt et al., 2006; Dobrev et al., 2022), and have contributed to the definition of standards for BC hearing (ISO 389-3). It has been shown that motion measurement close to the cochlea best represents cochlear activity (Egg-Olofsson et al., 2013; Beros et al., 2021). It has also been shown that the measurement of 3D cochlear motion is more representative of hearing sensation than 1D measurements (Dobrev and Sim, 2018).

Our group has studied the propagation of sound along the skull's surface and base previously (Dobrev et al., 2020; Farahmandi et al., 2022). The endpoint of our previous measurements has been the quantification of the vibration of the promontory or the skull's lateral surface. However, the 3D motion of the temporal bone is not well understood (Stenfelt et al., 2003; Raufer et al., 2019; Farahmandi et al., 2022), nor is the relation of promontory motion to intracochlear pressure changes (Steiger et al., 2018; Borgers et al., 2019; Dobrev et al., 2022). Intracochlear pressure may correlate to hearing more precisely than promontory motion in BC stimulation. While the promontory is in the immediate vicinity of the otic capsule, it is a single point, which is insufficient to describe the motion of the whole otic capsule (Lim et al., 2023). Thus, the 3D velocity at numerous points across the lateral and medial surface of the skull base and temporal bone (particularly the medal surface near the otic capsule) is hypothesized as being more representative of the activation of the cochlea. Therefore, the relation between the 3D motion of the promontory and intracochlear pressure change will be studied. This may be the basis for the development of an objective standard to compare different BCHAs. New BCHAs (Adamson et al., 2010; Dobrev et al., 2018), new coupling methods (Dobrev et al., 2021), or different stimulation sites (Dobrev et al., 2020b; Stenfelt and Goode, 2005a) could then be objectively compared in an experimental setting. These findings may also guide the development of new BCHAs and direct further measurements in subjects or patients (Stump et al., 2018).