Experimental Exploration of the Soft Tissue Conduction Pathway from Skin Stimulation Site to Inner Ear

Abstract

Objectives: Auditory sensation can be elicited by air conduction (AC) and by bone conduction (BC). It is also possible to elicit such responses by applying the standard clinical bone vibrator to the skin over soft tissue sites on the head, neck, or thorax of humans and animals. This mode of auditory stimulation has been called soft tissue conduction (STC). This study was designed to investigate the pathway between soft tissue sites and the ear. Methods: The air in the middle ear was replaced with saline solution in an animal with unique anatomy — The fat sand rat, in which about 70% of a thin-walled inner ear bulges into the middle ear bulla cavity — While we recorded the auditory brain stem responses (ABRs) to AC, BC, and STC stimulation. Results: This replacement of air with saline solution led to a significant improvement in STC threshold. With AC stimulation, the ABR threshold was elevated and the latency of the first ABR wave was prolonged. Consistent changes were not seen with BC stimulation. Conclusions: When the air (which has a very low acoustic impedance) that normally surrounds most of the inner ear is replaced with saline solution (which has an acoustic impedance similar to that of soft tissues), the STC threshold is improved. This improvement may be due to improved transmission of acoustic energy from the soft tissues to the inner ear.