Evidence for a cochlear origin for acoustic re-emissions, threshold fine-structure and tonal tinnitus

Delayed acoustic waves have been measured in the sealed ear canal in response to click and tone-burst stimuli. The response shows compressive nonlinearity, evidence for 'active' processing, and frequency tuning. At low sound levels group and phase delays correspond, and represent about 15 waves delay for the mid-frequency region. Frequency positions of pressure peaks and dips for continous tone stimulation can be predicted from the phase delays and persist well below threshold. Fine-structure in the audiogram is also related to these irregularities. Hydrostatic pressure, induced by body tilt, can eliminate audiogram fine-structure and reve rse the positions of peaks and dips for inverted body positions. This is interpreted as due to changes of internal reflection from the middle ear caused by acoustic impedance changes. Tinnitus can be induced by this inverted body posture, can be measured objectively in certain cases, and demonstrates properties such as synchronisation and beating with an external signal. These findings support the view of Kemp that the phenomena of acoustic re-emission, threshold fine-structure, and tinnitus are interrelated and originate within the cochlea.