Bone conduction hearing in the Guinea Pig: Sensitivity,directionality and vibration patterns

According to our comparable BC threshold, we can do some further research. The vibration was measured in three orthogonal directions where the dominating vibration directions was in line with the stimulation direction, here the ventral direction. The BC thresholds lay between -10 and 3 dB re 1 μm/s. The slopes of CAP growth function were similar for AC and BC at low and high frequencies, but slightly lower for BC than AC at frequencies between 8 and 16 kHz. This was attributed to differences in the stimulus levels used for the slope estimation and not a real difference in CAP slopes between the stimulation modalities.   

At the same time, the effect of a middle ear lesion, here modelled by severing the ossicles (ossicular discontinuity) and gluing the ossicles to the bone (otosclerosis), is investigated for both AC and BC. Two kinds of middle ear lesions, ossicular discontinuity and stapes glued to the surrounding bone, gave threshold shifts of between 23 and 53 dB for AC while it was below 16 dB when the stimulation was by BC. Statistically different threshold shifts between the two types of lesions were found where the AC threshold shifts for a glued stapes at 2 and 4 kHz were 9 to 18 dB greater than for a severed ossicular chain, and the BC threshold shifts for a glued stapes at 4 and 12 kHz were 8 to 9 dB greater than for a severed ossicular chain. 

Moreover, the direction of the vibration influences BC hearing also is investigated in our study. This direction sensitivity was investigated guinea pigs by providing BC stimulation in five different directions at the vertex of the guinea pig skull. The hearing thresholds for BC stimulation was obtained in the frequency range of 2 to 20 kHz by measurements of  the guinea pigs’ compound action potential. During the stimulation by  BC, the vibration of the cochlear promontory was measured with a three-dimensional laser Doppler vibrometer resulting in a set of unique three-dimensional vibration combinations for each threshold estimation. The sets of three-dimensional vibration at threshold were used to investigate six different predictors of BC hearing based on cochlear promontory vibration, three single direction (x, y and z directions in isolation), one linear combination of the three-dimension vibrations, one square-rooted sum of the squared vibration magnitudes, and one sum of the weighted three-dimensional vibrations based on a restricted minimum mean square error (MMSE) estimation. The MMSE gave the best predictions of the hearing threshold based on the cochlear promontory vibration while using only a single direction gave the worst predictions of the hearing thresholds overall. According to the MMSE estimation, at frequencies up to 8 kHz the vibration direction between the right and left side gave the greatest contribution to BC hearing in the guinea pig while at the highest frequencies measured, 16 and 20 kHz, the anteroposterior direction of the guinea pig head gave the greatest contribution.  

We do the further research to compare the vibrational patterns of human and guinea pig cochleae accurately, we developed and validated a novel finite element model of the guinea pig, leveraging it to analyze vibrational patterns in the cochlea. This approach is mirrored in our examination of the human cochlear model, providing granular insights into the nuances of human bone conduction hearing. The comparative analysis reveals that the guinea pig cochlea mirrors human cochlear vibrational patterns, thus serving as an efficient proxy for exploring human cochlear function. The convenient and comparable sites for bone conduction stimulation are identified as the human mastoid and the upper region of the guinea pig's skull. The cochlear vibration pattern encompasses a mix of rigid, rotational, and compressive motion.