Sound emission from cochlear filters and foveae--does the auditory sense organ make sense?

Sense organs filter relevant information from a broad background of physical interactions and discard possible perceptual input that has not proven useful during the course of biological evolution. Sense organs not only limit the access to physical reality, under certain conditions they have a life of their own and produce responses even in the absence of physical stimulation. As a perfect example, the inner ear, the cochlea, in addition to detecting incoming sound waves, it also is capable of producing sound energy. Such "active" processes, however, seem to be necessary to push detection thresholds close to physical limits. The price that has to be paid are "cochlear artifacts" like otoacoustic emissions. In the following, measurement of sound that is emitted by the ear will be introduced as a noninvasive means to assess cochlear function and to help to unravel the mechanical interaction between sensory cells and supporting structures that ultimately leads to sensitive and sharply tuned auditory perception. One focus will be on the cochlea of echo-locating bats that use audition as the main window of perception to their environment and therefore have highest demands on cochlear performance.