Volume 106, Issue 4, October 1999
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
106(1999); http://dx.doi.org/10.1121/1.427935View Description Hide Description
This paper presents a phenomenological model of the cochlea. It consists of a bank of nonlinear time-varying parallel filters and an active distributed feedback. Realistic filter shapes are obtained with the all-pole gamma-tone filter (APGF), which provides both a good approximation of the far more complex wave propagation or cochlear mechanicsmodels and a very simple implementation. Special care has been taken in modeling nonlinear properties in order to mimic the responses of the cochlea to complex stimuli. As a result, the model reproduces several observed phenomena including compression, two-tone suppression, and suppression of tones by noise. The distributed feedback, based on physiological evidence from outer hair cell (OHC) functioning, controls the damping parameter of the APGF and provides good modeling of both low-side and high-side suppression. Responses to more complex stimuli as well as a study of the model’s parameters are also presented. Areas of application of this type of model include understanding of signal coding in the cochlea and auditory nerve, development of hearing aids,speech analysis, as well as input to neural models of higher auditory centers.
106(1999); http://dx.doi.org/10.1121/1.427936View Description Hide Description
The limited dynamic range of the majority of auditory-nerve fibers represents a difficulty in accounting for normal hearing capabilities over the known psychoacoustic intensity range. The presence of noise is an additional complication because it will tend to saturate these fibers, thereby considerably reducing their dynamic range, i.e., the range of mean firing rates. In this study, simulations involving a model of auditory nerve and cochlear nucleus neurons were conducted using pure-tone stimuli in the presence of noise. The main focus is on the role of inhibition in regulating the activity of cells, improving their capability to represent signals in background noise. This concerns in particular those inhibitory neurons that receive input from a wide range of auditory-nerve fibers and respond with an onset chopper pattern. A detailed model of stellate cells is used. It allows several parameters such as the number, location, and strength of inputs to be manipulated. The first part of this paper presents the model and its responses to pure-tone and noise stimuli presented separately. The model’s capacity to generalize to tone/noise combinations is then tested. Responses to these stimuli are found to be qualitatively similar to neurophysiological findings. Model neurons exhibit appropriate shifts in their rate-level functions and their responses are inhibited or suppressed by tones outside their characteristic frequency. The model stellate cell is also found to display many of the temporal patterns reported in electrophysiological studies as a result of appropriate settings of certain parameters. Therefore, the model is sufficient to account for a large number of findings and should serve as a basis for predicting responses to novel stimuli, or as a building block for modeling larger networks.
106(1999); http://dx.doi.org/10.1121/1.427937View Description Hide Description
The extraordinary fine-tuning characteristic of normal mammalian hearing is attributed to physiological mechanisms collectively known as the cochlear amplifier (CA), which amplifies and sharpens the basilar membrane (BM) vibration response to incoming acoustic pressure oscillations. Electromechanical properties of outer hair cells (OHCs) are believed to be the critical component of the CA, but its “circuitry” as yet remains unknown. Here, the required frequency-space response characteristics of the CA are computationally determined when typical in vivo tuning data are introduced as input to a linear hydroelastic cochlear model whose cross-sectional dynamics are represented by two coupled vibrational degrees of freedom. It is assumed that the CA senses motion at the tectorial membrane (TM) reticular lamina (RL) and applies proportional, equal, and opposite forces to the BM and the RL. The results show the CA to be tonotopically tuned, meaning it conforms to a space-frequency similarity principle like other cochlear dynamical responses. This requires that the active mechanism use information distributed along the cochlear partition. The physiological mechanism responsible for such behavior remains unknown, but here the computed CA characteristics can be qualitatively reproduced by a circuit spanning the length of the cochlea. This does not preclude other mechanisms, but is intended to motivate closer experimental investigation of extracellular and intercellular ionic flow pathways.
106(1999); http://dx.doi.org/10.1121/1.427938View Description Hide Description
The phenomenology of spontaneous otoacoustic emissions (OAEs) is compared to theoretical predictions given by models in which they are produced by active nonlinear oscillators. Along with the well-known Van der Pol oscillator, a new active oscillator model is proposed and analyzed here. Numerical simulations and multi-scale analytical computation results are compared to experimental data of neonatal spontaneous and evoked OAEs. A simple analysis technique is proposed, in which the time evolution after a click stimulus of the amplitude of each spectral line corresponding to a spontaneous OAE is studied. Apart from a few stationary lines, an approximately exponential decay law, with characteristic damping coefficients in the 20–200 Hz range, was found to fit the data. These results are not compatible with a Van der Pol oscillator model, and show that some important aspects of the OAE phenomenology can be better explained by the proposed oscillator. Other interesting features of the spontaneous end evoked OAE phenomenology, such as spontaneous OAE suppression by external tones and the following recovery, as well as stimulus/response curves in the linear and nonlinear mode of acquisition, are also well reproduced by the proposed model.