Hendrikus Duifhuis

Consequences of peripheral frequency selectivity for nonsimultaneous masking

The frequency selectivity of the peripheral ear (e.g., at the VIIIth nerve level) is so acute that onset and offset transients in responses to short signals produce a nonnegligible extension of the signal duration. Thus, peripheral excitation patterns produced by signals which were separated in time can overlap and thereby mask each other. We refer to this type of masking as transient masking. Published data on nonsimultaneous masking and the results of two new experiments are compared with the masking that may be expected from filter transients. It is concluded that backward masking is mainly due to interactions at the level of the filter outputs, and that in forward masking, in addition to a short‐term component, a long‐term component is distinguishable. The latter has an exponential decay with a time constant of approximately 75 msec, and is probably related to physiological adaptation effects.

Measurement of pitch in speech: An implementation of Goldstein’s theory of pitch perception

Recent developments in hearing theory have resulted in the rather general acceptance of the idea that the perception of pitch of complex sounds is the result of the psychological pattern recognition process. The pitch is supposedly mediated by the fundamental of the harmonic spectrum which fits the spectrum of the complex sound optimally. The problem of finding the pitch is then equivalent to finding the best harmonic match. Goldstein [J. Acoust. Soc. Am. 54, 1496-1516 (1973)] has described an objective procedure for finding the best fit for stimuli containing relatively few spectral components. He uses maximum likelihood criterion. Application of this procedure to various data on the pitch of complex sounds yielded good results. This motivated our efforts to apply the pattern recognition theory of pitch to the problem of measuring pitch in speech. Although we were able to follow the main line of Goldsteins procedure, some essential changes had to be made. The most important is that in our implementation not all spectral components of the complex sound have to be classified as belonging to the harmonic pattern. We introduced a harmonics sieve to determine whether components are rejected or accepted at a candidate pitch. A simple criterion, based on the components accepted and rejected, led to the decision on which candidate pitch was to be finally selected. The performance and reliability of this psychoacoustically based pitch meter were tested in a LPC-vocoder system.

Cochlear nonlinearity and second filter: Possible mechanism and implications

We indicate that the directional sensitivity of the hair cell together with a directional distribution of frequency over the hair cells comprise a possible physiological basis for the second filter. Tuning disparity of the first and second filter denotes the difference in tuning frequency; at a given position x the tuning frequency of the first filter is αCF, of the second CF, with α≳1. This accounts for the asymmetry in location of two‐tone suppression areas. The compressive nonlinearity is described by a νth law device with ν<1. We analyze implications of this model for two‐tone suppression, sharpening, pure‐tone masking, and combination tone generation. Basic features of these phenomena are described adequately. For combination tones the propagation problem needs further study. On the basis of a comparison of literature data and theoretical predictions we estimate α=1.2 and ν=0.6. Regarding accurate shape of the first and second filter, the discussed data provide means for a qualitative evaluation only...

Level effects in psychophysical two-tone suppression.

Measurements of psychophysical two-tone suppression in a number of subjects are described. Levels of the stimulus components (suppressee, L1, and suppressor, L2) were the primary experimental variables. In all experiments the pulsation threshold was used with the probe frequency fr fixed at the suppressee frequency f1. In an initial experiment f1 was fixed at 1 kHz. The suppressor frequency f2 ranged from 0.2 to 1.4 kHz. At appropriate levels all subjects showed significant suppression. Suppression was found to decrease to zero as f2 approached f1. The amount of suppression depended on both L1 and L2 in a way not accounted for by any of the current theories of two-tone suppression. At higher overall levels suppression became increasingly prominent. The amount of two-tone suppression in a given stimulus condition depended strongly on the subject. The maximum amount of suppression measured was about 35 dB. In a second experiment it was verified that suppression follows the same pattern at other frequencies f1 (0.5, 2, and 4 kHz). Data for equal f2/f1 ratios were quite similar. The two-tone suppression effect decreased in a noisy environment. Within a 20-dB range of signal-to-noise ratios the effect of noise changed from negligible to the virtually complete elimination of two-tone suppression.

Numerical methods for solving one‐dimensional cochlear models in the time domain

In this article, a robust numerical solution method for one-dimensional (1-D) cochlear models in the time domain is presented. The method has been designed particularly for models with a cochlear partition having nonlinear and active mechanical properties. The model equations are discretized with respect to the spatial variable by means of the principle of Galerkin to yield a system of ordinary differential equations in the time variable. To solve this system, several numerical integration methods concerning stability and computational performance are compared. The selected algorithm is based on a variable step size fourth-order Runge-Kutta scheme; it is shown to be both more stable and much more efficient than previously published numerical solution techniques.

Audibility of high harmonics in a periodic pulse

A periodic pulse consisting of sufficiently narrow pulses has a frequency spectrum which contains all harmonics with equal amplitude. Owing to the limited resolving power of the hearing organ, only the low harmonics can be perceived separately. The high harmonics are heard together as one complex signal. We have found that harmonics above a certain number are audible when spectrally absent (the other harmonics being present at normal amplitude). The solution of this seeming paradox is to be found in an analysis of the time structure of the signal. This time analysis of the stimulus is (in a limited way) possible at the basilar membrane because of its limitation in frequency‐resolving power.

Automated correction of spin-history related motion artefacts in fMRI: Simulated and phantom data

This paper concerns the problem of correcting spin-history artefacts in fMRI data. We focus on the influence of through-plane motion on the history of magnetization. A change in object position will disrupt the tissues steady-state magnetization. The disruption will propagate to the next few acquired volumes until a new steady state is reached. In this paper we present a simulation of spin-history effects, experimental data, and an automatic two-step algorithm for detecting and correcting spin-history artefacts. The algorithm determines the steady-state distribution of all voxels in a given slice and indicates which voxels need a spin-history correction. The spin-history correction is meant to be applied before standard realignment procedures. To obtain experimental data a special phantom and an MRI compatible motion system were designed. The effect of motion on spin-history is presented for data obtained using this phantom inside a 1.5-T MRI scanner. We show that the presented algorithm is capable of detecting the occurrence of a displacement, and it determines which voxels need a spin-history correction. The results of the phantom study show good agreement with the simulations.

Modelling the Cochlear Partition with Coupled van der Pol Oscillators

Within the context of interest in analyzing ‘active’ and nonlinear processes in the cochlea we have been studying a model cochlea in which the local membrane impedance is described by a Van der Pol-oscillator. The behaviour of the undriven and sinusoidally driven discretized model is examined numerically. The undriven model describes the behaviour of a discrete number of coupled oscillators, which, if uncoupled, would have limit cycles gradually differing in frequency. In the coupled case the limit cycle behaviour is less predictable: it appears to exhibit quasi stochastic properties. In the driven model a sufficiently strong stimulus causes entrainment to the stimulus, and odd order harmonics appear. In the range where the driven response is small compared with the average limit cycle, the response is almost linear. The strict Van der Pol-damping function, which is parabolic in velocity, produces strong saturation. A generalized Van der Pol-damping term, which causes small-amplitude instability and large-amplitude stability, produces much the same general behaviour, but the intensity response can be modelled more realistically.

Spatial periodicity in the cochlea : The result of interaction of spontaneous emissions?

In this study the results of simulations with a nonlinear macromechanical model of the human cochlea are presented. In this model it is possible to investigate the interactions between large numbers of spontaneous emissions. The focus of this study is on the effect of a local maximum in emission strength on its direct surroundings. The suppression that such an emitter causes depends on the distance between suppressor and suppressee in a strongly nonmonotonic manner. This nonmonotonicity leads to natural minimal distances between emission frequencies, without additional assumptions about the mechanics of the cochlea. The origin of this nonmonotonicity in the suppression lies in reflection of the generated energy at the stapes. The fact that these minimum distances correspond to distances observed between measured SOAE frequencies in adults, but not in newborns, suggests that the nonmonotonicity may play a role in the development of the cochlea in the early years of childhood.

Distortion product otoacoustic emissions: a time domain analysis.

Distortion product otoacoustic emissions (DPOAEs) provide relatively easily accessible external information about internal properties of the inner ear. A primary result is that the detection of normal DPOAE components is a strong indicator of normal functioning of the cochlear mechanics. The details of amplitude and phase behavior of the DPOAEs, and the development over time (DPOAE delay) are not fully understood and subject of some debate. In this paper, I would like to emphasize the view that precise time domain experiments and time domain model analysis provide a superb tool, the power of which is generally underestimated. Results are presented from a model analysis in which the primary-tone phase variation technique introduced by Whitehead et al. was employed successfully.