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R Meddis

Publications and source records attributed to R Meddis.

At least 19 recordsLinked to original sources
PubMed2001

Physiological correlates of comodulation masking release in the mammalian ventral cochlear nucleus.

Comodulation masking release (CMR) enhances the detection of signals embedded in wideband, amplitude-modulated maskers. At least part of the CMR is attributable to across-frequency processing, however, the relative contribution of different stages in the auditory system to across-frequency processing is unknown. We have measured the responses of single units from one of the earliest stages in the ascending auditory pathway, the ventral cochlear nucleus, where across frequency processing may take place. A sinusoidally amplitude-modulated tone at the best frequency of each unit was used as a masker. A pure tone signal was added in the dips of the masker modulation (reference condition). Flanking components (FCs) were then added at frequencies remote from the unit best frequency. The FCs were pure tones amplitude modulated either in phase (comodulated) or out of phase (codeviant) with the on-frequency component. Psychophysically, this CMR paradigm reduces within-channel cues while producing an advantage of approximately 10 dB for the comodulated condition in comparison with the reference condition. Some of the recorded units showed responses consistent with perceptual CMR. The addition of the comodulated FCs produced a strong reduction in the response to the masker modulation, making the signal more salient in the poststimulus time histograms. A decision statistic based on d' showed that threshold was reached at lower signal levels for the comodulated condition than for reference or codeviant conditions. The neurons that exhibited such a behavior were mainly transient chopper or primary-like units. The results obtained from a subpopulation of transient chopper units are consistent with a possible circuit in the cochlear nucleus consisting of a wideband inhibitor contacting a narrowband cell. A computational model was used to confirm the feasibility of such a circuit.

Acoustic Stimulation↗
PubMed2001

A computational algorithm for computing nonlinear auditory frequency selectivity.

Computational algorithms that mimic the response of the basilar membrane must be capable of reproducing a range of complex features that are characteristic of the animal observations. These include complex input output functions that are nonlinear near the site's best frequency, but linear elsewhere. This nonlinearity is critical when using the output of the algorithm as the input to models of inner hair cell function and subsequent auditory-nerve models of low- and high-spontaneous rate fibers. We present an algorithm that uses two processing units operating in parallel: one linear and the other compressively nonlinear. The output from the algorithm is the sum of the outputs of the linear and nonlinear processing units. Input to the algorithm is stapes motion and output represents basilar membrane motion. The algorithm is evaluated against published chinchilla and guinea pig observations of basilar membrane and Reissner's membrane motion made using laser velocimetry. The algorithm simulates both quantitatively and qualitatively, differences in input/output functions among three different sites along the cochlear partition. It also simulates quantitatively and qualitatively a range of phenomena including isovelocity functions, phase response, two-tone suppression, impulse response, and distortion products. The algorithm is potentially suitable for development as a bank of filters, for use in more comprehensive models of the peripheral auditory system.

Algorithms↗
PubMed2001

A human nonlinear cochlear filterbank.

Some published cochlear filterbanks are nonlinear but are fitted to animal basilar membrane (BM) responses. Others, like the gammatone, are based on human psychophysical data, but are linear. In this article, a human nonlinear filterbank is constructed by adapting a computational model of animal BM physiology to simulate human BM nonlinearity as measured by psychophysical pulsation-threshold experiments. The approach is based on a dual-resonance nonlinear type of filter whose basic structure was modeled using animal observations. In modeling the pulsation threshold data, the main assumption is that pulsation threshold occurs when the signal and the masker produce comparable excitation, that is the same filter output, at the place of the BM best tuned to the signal frequency. The filter is fitted at a discrete number of best frequencies (BFs) for which psychophysical data are available for a single listener and for an average response of six listeners. The filterbank is then created by linear regression of the resulting parameters to intermediate BFs. The strengths and limitations of the resulting filterbank are discussed. Its suitability for simulating hearing-impaired cochlear responses is also discussed.

Basilar Membrane↗
PubMed1999

Analog very large-scale integrated (VLSI) implementation of a model of amplitude-modulation sensitivity in the auditory brainstem.

An analog very large-scale integrated (VLSI) implementation of a model of signal processing in the auditory brainstem is presented and evaluated. The implementation is based on a model of amplitude-modulation sensitivity in the central nucleus of the inferior colliculus (CNIC) previously described by Hewitt and Meddis [J. Acoust. Soc. Am. 95, 2145-2159 (1994)]. A single chip is used to implement the three processing stages of the model; the inner-hair cell (IHC), cochlear nucleus sustained-chopper, and CNIC coincidence-detection stages. The chip incorporates two new circuits: an IHC circuit and a neuron circuit. The input to the chip is taken from a "silicon cochlea" consisting of a cascade of filters that simulate basilar membrane mechanical frequency selectivity. The chip which contains 142 neurons was evaluated using amplitude-modulated pure tones. Individual cells in the CNIC stage demonstrate bandpass rate-modulation responses using these stimuli. The frequency of modulation is represented spatially in an array of these cells as the location of the cell generating the highest rate of action potentials. The chip processes acoustic signals in real time and demonstrates the feasibility of using analog VLSI to build and test auditory models that use large numbers of component neurons.

Audiometry, Pure-Tone↗
PubMed1997

A unitary model of pitch perception.

A model of the mechanism of residue pitch perception is revisited. It is evaluated in the context of some new empirical results, and it is proposed that the model is able to reconcile a number of differing approaches in the history of theories of pitch perception. The model consists of four sequential processing stages: peripheral frequency selectivity, within-channel half-wave rectification and low-pass filtering, within-channel periodicity extraction, and cross-channel aggregation of the output. The pitch percept is represented by the aggregated periodicity function. Using autocorrelation as the periodicity extraction method and the summary autocorrelation function (SACF) as the method for representing pitch information, it is shown that the model can simulate new experimental results that show how the quality of the pitch percept is influenced by the resolvability of the harmonic components of the stimulus complex. These include: (i) the pitch of harmonic stimuli whose components alternate in phase; (ii) the increased frequency difference limen of tones consisting of higher harmonics; and (iii) the influence of a mistuned harmonic on the pitch of the complex as a function of its harmonic number. To accommodate these paradigms, it was necessary to compare stimuli along the length of the SACF rather than relying upon the highest peak alone. These new results demonstrate that the model responds differently to complexes consisting of low and high harmonics. As a consequence, it is not necessary to postulate two separate mechanisms to explain different pitch percepts associated with resolved and unresolved harmonics.

Hair Cells, Auditory, Inner↗
PubMed1997

Time decay of auditory stream biasing.

In an experiment designed to investigate the time decay of auditory stream biasing (ASB), subjects were required to listen to a 10-sec induction sequence of repeated tones (AAAA...) designed to bias the listener's percept toward hearing an A stream. The induction sequence was followed immediately by a silent interval (0-8 sec), and then a short ABAB ... test sequence. To measure the amount of ASB remaining at the end of the silent interval, subjects were asked to indicate whether the test sequence was temporally coherent or had segregated into separate A and B streams. A plot of the mean number of segregation responses against silent-interval duration indicated that the overall time decay of ASB can be ascribed by an exponential decay function with a time constant of tau = 3.84 sec, with musicians having a longer time constant (tau = 7.84 sec) than nonmusicians (tau = 1.42 sec). The length of the time constants for musicians and nonmusicians suggests that the mechanism responsible for ASB is associated with long auditory storage and that future experiments investigating auditory streaming phenomena should use interstimulus intervals of at least 8 sec.

Adult↗
PubMed1996

Computer simulation of auditory stream segregation in alternating-tone sequences.

A computer model is described that takes a novel approach to the problem of accounting for perceptual coherence in alternating pure-tone sequences by using simple physiological principles that operate at a low level. Using the same set of parameter values, the model is able to reproduce a number of phenomena associated with auditory stream segregation. These are (1) the buildup of stream segregation over time, (2) the temporal coherence and fission boundaries obtained from human listeners, and (3) the trill threshold. Whereas these phenomena are generally accounted for in terms of an auditory scene-analysis process that works on the basis of Gestalt perceptual principles, the operation of the model suggests that some Gestalt auditory grouping may be the product of low-level processes.

Acoustic Stimulation↗
PubMed1996

A physical model of sound diffraction and reflections in the human concha.

An approximated physical model of the frequency transfer function of the human concha is developed in this paper. This formulation includes diffraction, reflection, and interference phenomena in the concha cavity. The performance of the proposed diffraction/ reflection model is compared with that of the single-delay-and-add approximation by checking their predictions against the experimental transfer function of a metal spiral-shaped diffracting/reflecting system. Results show that the diffraction/reflection model performs considerably better at predicting both the absolute center frequency of spectral minima and the relative frequency spacing between them. The diffraction/reflection model is then applied to a realistic concha shape and its predictions are compared with experimental head-related transfer functions for azimuth- and elevation-varying sound sources. In this case, the model predicts the elevation-dependent spectral features related to the transverse dimensions of the concha. Additionally, the diffraction/reflection model predicts that, because of sound diffraction, similar spectral features must be generated in the concha for sources at all azimuths within the frontal part of the ipsilateral hemisphere. Experimental and theoretical evidence supporting this prediction is presented.

Acoustics↗
PubMed1995

A computer model of dorsal cochlear nucleus pyramidal cells: intrinsic membrane properties.

Manis [P. B. Manis, J. Neurosci. 10, 2338-2351 (1990)] studied "simple spiking," pyramidal cells of the dorsal cochlear nucleus (DCN) maintained in vitro. Response profiles to hyperpolarizing and depolarizing current pulses were generated. Hyperpolarization of the cell membrane followed by depolarization produced markedly different response profiles from those generated when no prehyperpolarization was imposed. By manipulating the magnitude of the hyperpolarizing and depolarizing pulses, "chopper," "pauser" and "build-up" response patterns, similar to those in vivo, could be generated by individual cells. Manis concluded that the different response profiles resulted from the modulation of intrinsic membrane conductances by the prehyperpolarizing pulses. Here a computer model is used to show that (a) steady-state hyperpolarization can influence cell responding to subsequent depolarization in a manner consistent with the data reported by Manis; and (b) the effects reported can be generated by the addition of a modeled transient potassium conductance to the standard Hodgkin-Huxley model of spike generation [A. L. Hodgkin and A. F. Huxley, J. Physiol. 117, 500-544 (1952)]. The model will be of use to those who wish to consider the role of various excitatory and inhibitory inputs to pyramidal cells and to establish their functional role within the DCN.

Cochlear Nucleus↗
PubMed1994

A computer model of amplitude-modulation sensitivity of single units in the inferior colliculus.

A computer model is presented of a neural circuit that replicates amplitude-modulation (AM) sensitivity of cells in the central nucleus of the inferior colliculus (ICC). The ICC cell is modeled as a point neuron whose input consists of spike trains from a number of simulated ventral cochlear nucleus (VCN) chopper cells. Input to the VCN chopper cells is provided by simulated spike trains from a model of the auditory periphery [Hewitt et al., J. Acoust. Soc. Am. 91, 2096-2109 (1992)]. The performance of the model at the output of the auditory nerve, the cochlear nucleus and ICC simulations in response to amplitude-modulated stimuli is described. The results are presented in terms of both temporal and rate modulation transfer functions (MTFs) and compared with data from physiological studies in the literature. Qualitative matches were obtained to the following main empirical findings: (a) Auditory nerve temporal-MTFs are low pass, (b) VCN chopper temporal-MTFs are low pass at low signal levels and bandpass at moderate and high signal levels, (c) ICC unit temporal-MTFs are low pass at low signal levels and broadly tuned bandpass at moderate and high signal levels, and (d) ICC unit rate-MTFs are sharply tuned bandpass at low and moderate signal levels and flat at high levels. VCN and ICC units preferentially sensitive to different rates of modulation are presented. The model supports the hypothesis that cells in the ICC decode temporal information into a rate code [Langner and Schreiner, J. Neurophysiol. 60, 1799-1822 (1988)], and provides a candidate wiring diagram of how this may be achieved.

Animals↗
PubMed1993

The response of guinea pig auditory-nerve fibers with high spontaneous discharge rates to increments in intensity.

We have re-examined the response of auditory-nerve fibres with high spontaneous discharge rates to increments in intensity as a function of the delay of the increment. In agreement with previous studies, the response measured over a relatively long time window (10 ms), emphasising the properties of short-term adaptation, did not decrease with the delay of the increment. However, the response to an increment in intensity, measured over a short time window (0.64 ms), was significantly larger when the increment was coincident with the stimulus onset than when it was delayed by either 5, 10 or 15 ms.

Acoustic Stimulation↗
PubMed1993

Regularity of cochlear nucleus stellate cells: a computational modeling study.

This article reports on a computational modeling study designed to investigate the generation of the transient chopper response of cochlear nucleus stellate cells. The model is based on a simulation of the auditory periphery which feeds a generic stellate-cell model. Physiological recordings of transient chopper units in response to short, best frequency, tone bursts show a brief initial period (typically < 10 ms) of rapid rate adaptation as evidenced by a rapid rise in mean interspike interval. Associated with this rate adaptation is a significant increase in firing irregularity. The changes in rate and irregularity have recently been attributed to the activation of noisy inhibitory inputs on the cell [e.g., Banks and Sachs, J. Neurophysiol. 65, 606-629 (1991)]. However, the results show that the transient chopper response pattern can be generated without the need for inhibitory inputs. The transience of the initial chopping pattern is sensitive to the following model parameters: (a) the firing threshold of the cell, (b) the number of excitatory inputs that converge on the cell, and (c) the magnitude of the current delivered to the cell for each active input. The response was also found to be relatively insensitive to changes in the degree of dendritic filtering imposed on the auditory-nerve input. The results of each simulation can be explained by considering the pattern of depolarization the cell receives during the course of a tone burst.

Acoustic Stimulation↗
PubMed1992

Modeling the identification of concurrent vowels with different fundamental frequencies.

Human listeners are better able to identify two simultaneous vowels if the fundamental frequencies of the vowels are different. A computational model is presented which, for the first time, is able to simulate this phenomenon at least qualitatively. The first stage of the model is based upon a bank of bandpass filters and inner hair-cell simulators that simulate approximately the most relevant characteristics of the human auditory periphery. The output of each filter/hair-cell channel is then autocorrelated to extract pitch and timbre information. The pooled autocorrelation function (ACF) based on all channels is used to derive a pitch estimate for one of the component vowels from a signal composed of two vowels. Individual channel ACFs showing a pitch peak at this value are combined and used to identify the first vowel using a template matching procedure. The ACFs in the remaining channels are then combined and used to identify the second vowel. Model recognition performance shows a rapid improvement in correct vowel identification as the difference between the fundamental frequencies of two simultaneous vowels increases from zero to one semitone in a manner closely resembling human performance. As this difference increases up to four semitones, performance improves further only slowly, if at all.

Acoustics↗
PubMed1992

A computer model of a cochlear-nucleus stellate cell: responses to amplitude-modulated and pure-tone stimuli.

A computer model of a ventral-cochlear-nucleus (VCN) stellate cell with chop-S type response properties is presented and evaluated. The model is based on a simplified model of spike generation preceded by a stage that simulates dendritic low-pass filtering. Input to the model is in the form of simulated auditory-nerve spikes produced by a model of the auditory periphery [Meddis and Hewitt, J. Acoust. Soc. Am. 89, 2866-2882 (1991)]. Outputs from the stellate-cell model are shown to qualitatively replicate a wide range of typical in vivo responses. These include: (a) realistic onset and steady-state rate-level functions, (b) "chopper"-type post-stimulus time histogram responses; (c) typical "chop-S"-type neuron responses characterized by a low coefficient of variance (CV less than 0.3) of interspike intervals as a function of time; (d) level-dependent amplitude-modulation transfer functions; (e) intrinsic oscillations in responses to pure-tone stimuli; (f) amplitude-modulation encoding over a wide dynamic range; and (g) frequency-limited phase locking to pure tones. It is shown that these responses can be explained primarily by the membrane properties of the cells. More specifically, how the model encodes signal amplitude modulation was studied and an explanation was suggested for the generation of the bandpass modulation transfer functions. Such functions are observed neurally in response to amplitude-modulated stimuli presented at moderate to high signal levels.

Animals↗
PubMed1991

Lateralization of very-short-duration tone pulses of low and high frequencies.

The position and image-width of the simultaneous images produced by very short tone pulses were measured as a function of interaural time difference (ITD) at both low- (250 and 800 Hz) and high- (2500 and 8000 Hz) frequencies using a direct-estimation technique. Primary images are lateralized towards the ear receiving the leading stimulus. At low frequencies image position is proportional to interaural phase-difference (IPD) below 90 degrees and remains at the lead-ear for larger values. At high frequencies images are reported is proportional to ITD up to 500-1000 microseconds. Secondary images are reported on the opposite side of the head for IPDs greater than 180 degrees at low frequencies, and at ITDs greater than 500 microseconds at high frequencies. Image width is approximately constant for all ITDs and both images at a given frequency, but becomes more compact as frequency increases. The data are discussed in terms of onset cues and stimulus fine-structure cues. The best explanation is in terms of an onset mechanism, but one that is calibrated in terms of IPD at low frequencies. The existence of double images is explained in terms of a breakdown in the mechanism determining fusion.

Adult↗
PubMed1991

A computer model of auditory stream segregation.

A computer model is described which simulates some aspects of auditory stream segregation. The model emphasizes the explanatory power of simple physiological principles operating at a peripheral rather than a central level. The model consists of a multi-channel bandpass-filter bank with a "noisy" output and an attentional mechanism that responds selectively to the channel with the greatest activity. A "leaky integration" principle allows channel excitation to accumulate and dissipate over time. The model produces similar results to two experimental demonstrations of streaming phenomena, which are presented in detail. These results are discussed in terms of the "emergent properties" of a system governed by simple physiological principles. As such the model is contrasted with higher-level Gestalt explanations of the same phenomena while accepting that they may constitute complementary kinds of explanation.

Attention↗