The mechanisms underlying multiple lobes in SOAE suppression tuning curves in a transmission line model of the cochlea

Spontaneous otoacoustic emissions (SOAE) can be suppressed by presenting an acoustical stimulus. For stimuli with frequencies close to the SOAE frequency, the SOAE either show a beating pattern or are heavily suppressed while the spectral energy of the stimulus increases. This effect indicates that the self-sustained oscillations in the cochlea underlying SOAE are entrained by the oscillation evoked by the stimulus. The level required to entrain SOAE needs to be higher for a larger spectral distance between SOAE and stimulus compared to a smaller spectral distance, leading to a V-shaped tuning curve. When these tuning curves are measured over a broad frequency range spanning several octaves, additional lobes of suppression can be found with a spectral distance of about half an octave. It has been proposed that SOAEs are generated by a standing wave pattern in the cochlea that arises by interference of multiple reflections between the best place and the oval window [1]. It has further been hypothesized that the additional side lobes in the SOAE suppression tuning curves are the result of interaction of the stimulus with the nodes and antinodes of the standing wave pattern underlying the SOAE [4]. In the present study, a nonlinear and active transmission line model of the cochlea is used to investigate this hypothesis. The model is able to produce SOAEs with plausible characteristics and further shows the suggested standing wave pattern. This approach hence makes it possible to disentangle contributions of entrainment and compression of the forward-traveling wave to the SOAE suppression tuning curves.