The main focus of DPOAE measurements so far had been as a tool to evaluate the inner ear mechanics by using simple acoustical stimuli such as pure tones (f1 < f2). They arise at certain frequencies which are mathematically related to the chosen stimulation frequencies, and are measured in the outer ear channel with a sensitive microphone.
A new experimental procedure, the residual OAE protocol, allows the use of more complex sound stimuli, like communication calls, for investigation of inner ear nonlinearity. This leads to a comprehensive assessment of external signal changes that originate in the inner ear.
To measure residual OAEs, three recording time windows are relevant during the experiment: (1) An acoustical stimulus is applied through the first loudspeaker. (2) The same stimulus is applied with a higher level through the second loudspeaker. (3) The third step involves the stimuli from both loudspeakers at the same time. The OAE residual is calculated from the time signals of all three application windows as follows: R = signal 3 - (signal 1 + signal 2). If the inner ear would work in a linear way, the subtraction would be zero. But sensitive ears have highly non-linear mechanical properties and change incoming signals in a profound way, resulting in OAE residuals. An advantage lies in the use of complex signals, such as pre-recorded communication calls, which contain several frequencies. Hence, a broad range of hair cells is activated, in contrast to DPOAE measurements which only cover selective areas of the cochlea.
The measurements are going to be carried out in the short-tailed fruit bat Carollia perspicillata. These tropical bats produce communication and echolocation calls and have a wide hearing range of about 5 to 150 kHz.
Schlenther D., Voss C., Kössl M. (2011): Cubic and quadratic distortion-product otoacoustic emissions (DPOAE) in awake and anesthetized short-tailed fruit bats. 9th Göttingen Meeting of the German Neuroscience Society: T17-6A