Acoustic correlates of tonal tinnitus.

A sensitive microphone has been developed which can pick up tonal signals (spontaneous acoustic emissions) in the sealed ear-canal of certain subjects. Various properties of these frequency components suggest that they arise from an active, frequency-selective self-limiting feedback process within the cochlea and that they rely on internal reflection from the middle ear. An external tone can synchronize, frequency-lock, suppress of frequency-shift the acoustic component. These interactions are frequency-dependent in a way suggestive of cochlear tuning properties. Positive or negative middle-ear pressure can also influence the components by increasing their frequency and in some cases can enhance one component at the expense of a neighbouring one. Some subjects hear these components as tinnitus and can report on the measured changes. Other subjects do not hear the measured signals, which otherwise behave similarly. A third group of subjects have tinnitus but no objective sound can be detected. In this last group there are, nevertheless, sometimes notches or other discontinuities in the audiogram which correspond to their tinnitus pitch-matches. It appears likely that the recordable type of tinnitus is essentially non-pathological and represents hypersensitivity of the system, whereas the non-recordable type might be associated with local pathological changes at the end-organ or more centrally.