Physiologically active cochlear micromechanics--one source of tinnitus

Spontaneous oscillatory vibrations within the cochlea exist in many normal ears and can become audible, providing one source of mild tonal tinnitus. These vibrations have been studied experimentally using an acoustic ear-canal recording technique. The spontaneous oscillations are directly related to stimulated acoustic emissions (cochlear echoes). They arise because of the development of strong cochlear resonances by a feedback process. With the aid of cochlear resonance theory the bandwidths of spontaneous cochlear mechanical activity can be used to determine intracochlear broad-band noise levels. Ways of modifying cochlear mechanical activity have been explored. Activity can be increased or decreased by changes in middle-ear mobility or by brief overstimulation. In the latter case a biphasic recovery cycle has been identified which also seems to be relevant to temporary noise-induced tinnitus and temporary threshold shift. Tinnitus due to spontaneous cochlear vibrations is here called cochlear mechanical tinnitus. Identification criteria are given for such tinnitus involving cochlear mechanical resonance. This form is mild and likely to be found in normally hearing people with tinnitus and in those with only middle-ear disorders. It is speculated that localized oscillation of mechanically isolated cochlear elements could induce much higher levels of tinnitus without producing any externally detectable vibration.