Bispectral approach to analyze nonlinear cochlear active mechanisms in transient evoked otoacoustic emissions

A new approach to study nonlinearity in cochlear active mechanisms, as evaluated in transient evoked otoacoustic emissions (TEOAEs), is presented. TEOAEs are signals generated in the cochlea by a mix of linear and nonlinear mechanisms. This new approach was designed to complement the traditional TEOAE analysis performed by currently available systems used in objective hearing screening and assessment. Nonlinearity of TEOAEs was studied by means of the bispectrum, which is able to find out quadratic frequency couplings (QFCs) that occur when a frequency is not only generated by an independent cochlear source, but it is the result of the interaction among a number of cochlear sources. To fit with the technical constraints of currently available TEOAE systems, the bispectrum was estimated by the third-order scaled polyperiodogram. The proposed method was characterized with synthesized TEOAEs as a function of the main TEOAE parameters and then used to analyze TEOAEs recorded in normal hearing adults and full-term neonates. Results revealed the presence of QFCs in both adult and neonatal TEOAEs, with peculiar patterns and significantly different frequency content in the two groups: adults had QFCs mainly around 2 kHz and neonates had QFCs mainly in the range 3.5-4 kHz.