Filtering of distortion-product otoacoustic emissions in the inner ear of birds and lizards

When the output magnitude of more than one order of distortion-product otoacoustic emission (DPOAE) is measured, they reach their maximum at the same DPOAE frequency. This fact led several authors to the assumption that, subsequent to their generation in the cochlea, the DPOAE are band-pass filtered. It was suggested that the tectorial membrane is the structure responsible for this filtering. In this report, we show that the same kind of 'DPOAE tuning' is shown by animals which have hearing organs with tectorial structures of very different morphology, or even with no tectorial membrane at all. We therefore conclude that it is unlikely that the filter is the tectorial membrane.