Abstract
In a sensitive cochlea, the basilar membrane response to transient excitation of any kind–normal acoustic or artificial intracochlear excitation–consists of not only a primary impulse but also a coda of delayed secondary responses with varying amplitudes but similar spectral content around the characteristic frequency of the measurement location. The coda, sometimes referred to as echoes or ringing, has been described as a form of local, short term memory which may influence the ability of the auditory system to detect gaps in an acoustic stimulus such as speech. Depending on the individual cochlea, the temporal gap between the primary impulse and the following coda ranges from once to thrice the group delay of the primary impulse (the group delay of the primary impulse is on the order of a few hundred microseconds). The coda is physiologically vulnerable, disappearing when the cochlea is compromised even slightly. The multicomponent sensitive response is not yet completely understood. We use a physiologically-based, mathematical model to investigate (i) the generation of the primary impulse response and the dependence of the group delay on the various stimulation methods, (ii) the effect of spatial perturbations in the properties of mechanically sensitive ion channels on the generation and separation of delayed secondary responses. The model suggests that the presence of the secondary responses depends on the wavenumber content of a perturbation and the activity level of the cochlea. In addition, the model shows that the varying temporal gaps between adjacent coda seen in experiments depend on the individual profiles of perturbations. Implications for non-invasive cochlear diagnosis are also discussed.
Highlights
IntroductionThe response of the basilar membrane (BM) to transient external acoustic excitation consists of a primary impulse response and of a coda of delayed secondary responses (sometimes called echoes or ringing) with varying amplitudes but similar spectral content centered near the best frequency of the measurement location [1,2,3] (see Fig 1A and 1B for an illustration)
In a sensitive cochlea, the response of the basilar membrane (BM) to transient external acoustic excitation consists of a primary impulse response and of a coda of delayed secondary responses with varying amplitudes but similar spectral content centered near the best frequency of the measurement location [1,2,3]
The fluid-structure-electrical interaction in the cochlea enable the basilar membrane, one of the most important structures in the cochlear partition, to display different dynamic patterns depending on the frequency content of the incoming sound
Summary
The response of the basilar membrane (BM) to transient external acoustic excitation consists of a primary impulse response and of a coda of delayed secondary responses (sometimes called echoes or ringing) with varying amplitudes but similar spectral content centered near the best frequency of the measurement location [1,2,3] (see Fig 1A and 1B for an illustration). In click-evoked BM responses from the base of the chinchilla cochlea [1], the temporal gaps were less than twice the primary group delay. They observed secondary coda with long or extended tails when the sound pressure levels were low. Similar temporal gaps between adjacent wave packets were found by Shera and Cooper [3]; in addition, the inter-packet delays were not always constant, sometimes displaying a wax and wane. This intricate behavior is not completely understood
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