Biomedical subjects

Ray Meddis

Publications and source records attributed to Ray Meddis.

12 recordsLinked to original sources

PubMed2006

Auditory-nerve first-spike latency and auditory absolute threshold: a computer model.

A computer model of the auditory periphery was used to address the question of what constitutes the physiological substrate of absolute auditory threshold. The model was first evaluated to show that it is consistent with experimental findings that auditory-nerve fiber spikes can be predicted to occur when the running integral of stimulus pressure reaches some critical value [P. Heil and H. Neubauer, J. Neurosci. 15, 7404-7415 (2001)]. It was then modified to examine two ways in which the accumulation and clearance of receptor presynaptic calcium might explain this effect. Both methods gave results that matched the animal data. It was also shown how the rate of clearance of presynaptic calcium could be used to explain the origin of differences between low and high spontaneous-rate fiber types. When spiking activity is aggregated across a number of similar high spontaneous-rate fibers and used as the input to a model of a cochlear nucleus coincidence neuron, its response can be used to judge whether or not a stimulus is present. A simulated psychophysical experiment then demonstrated that this simple decision procedure can reproduce measurements of absolute auditory threshold for tones in quiet where the threshold is a joint function of both time and level.

Acoustic Stimulation↗

PubMed2006

Reply to comment on "Auditory-nerve first-spike latency and auditory absolute threshold: a computer model".

Krisha [J. Acoust. Soc. Am., in press (2006)] has commented that an explanation based on presynaptic calcium accumulation at the inner hair cell is an incorrect explanation for the success of a model of the auditory periphery [Meddis, R., J. Acoustic. Soc. Am. 119, 406-417 (2006)] in explaining data on first-spike auditory nerve latency. This reply accepts the criticism and accepts the strength of an alternative explanation based on expected latencies in random sequences of low-probability events. This reply also goes on briefly to explore the application of this argument to other phenomena, including the dependence of absolute auditory threshold on the duration of the stimulus. This has wide-ranging implications for the concept of "temporal integration" in psychophysics.

Action Potentials↗

PubMed2006

Virtual pitch in a computational physiological model.

A computational model of nervous activity in the auditory nerve, cochlear nucleus, and inferior colliculus is presented and evaluated in terms of its ability to simulate psychophysically-measured pitch perception. The model has a similar architecture to previous autocorrelation models except that the mathematical operations of autocorrelation are replaced by the combined action of thousands of physiologically plausible neuronal components. The evaluation employs pitch stimuli including complex tones with a missing fundamental frequency, tones with alternating phase, inharmonic tones with equally spaced frequencies and iterated rippled noise. Particular attention is paid to differences in response to resolved and unresolved component harmonics. The results indicate that the model is able to simulate qualitatively the related pitch-perceptions. This physiological model is similar in many respects to autocorrelation models of pitch and the success of the evaluations suggests that autocorrelation models may, after all, be physiologically plausible.

Auditory Threshold↗

PubMed2005

Cochlear compression in listeners with moderate sensorineural hearing loss.

Psychophysical estimates of basilar membrane (BM) responses suggest that normal-hearing (NH) listeners exhibit constant compression for tones at the characteristic frequency (CF) across the CF range from 250 to 8000 Hz. The frequency region over which compression occurs is broadest for low CFs. This study investigates the extent that these results differ for three hearing-impaired (HI) listeners with sensorineural hearing loss. Temporal masking curves (TMCs) were measured over a wide range of probe (500-8000 Hz) and masker frequencies (0.5-1.2 times the probe frequency). From these, estimated BM response functions were derived and compared with corresponding functions for NH listeners. Compressive responses for tones both at and below CF occur for the three HI ears across the CF range tested. The maximum amount of compression was uncorrelated with absolute threshold. It was close to normal for two of the three HI ears, but was either slightly (at CFs < or =1000 Hz) or considerably (at CFs > or =4000 Hz) reduced for the third ear. Results are interpreted in terms of the relative damage to inner and outer hair cells affecting each of the HI ears. Alternative interpretations for the results are also discussed, some of which cast doubts on the assumptions of the TMC-based method and other behavioral methods for estimating human BM compression.

Acoustic Stimulation↗

PubMed2005

A computer model of the auditory-nerve response to forward-masking stimuli.

A computer model of the auditory periphery is used to study the involvement of auditory-nerve (AN) adaptation in forward-masking effects. An existing model is shown to simulate published AN recovery functions both qualitatively and quantitatively after appropriate parameter adjustments. It also simulates published data showing only small threshold shifts when a psychophysical forward-masking paradigm is applied to AN responses. The model is extended to simulate a simple but physiologically plausible mechanism for making threshold decisions based on coincidental firing of a number of AN fibers. When this is used, much larger threshold shifts are observed of a size consistent with published psychophysical observations. The problem of how stimulus-driven firing can be distinguished from spontaneous activity near threshold is also addressed by the same decision mechanism. Overall, the modeling results suggest that poststimulatory reductions in AN activity can make a substantial contribution to the raised thresholds observed in many psychophysical studies of forward masking.

Acoustic Stimulation↗

PubMed2004

The representation of periodic sounds in simulated sustained chopper units of the ventral cochlear nucleus.

The nature of the neural processing underlying the extraction of pitch information from harmonic complex sounds is still unclear. Electrophysiological studies in the auditory nerve and many psychophysical and modeling studies suggest that pitch might be extracted successfully by applying a mechanism like autocorrelation to the temporal discharge patterns of auditory-nerve fibers. The current modeling study investigates the possible role of populations of sustained chopper (Chop-S) units located in the mammalian ventral cochlear nucleus (VCN) in this process. First, it is shown that computer simulations can predict responses to periodic and quasiperiodic sounds of individual Chop-S units recorded in the guinea-pig VCN. Second, it is shown that the fundamental period of a periodic or quasiperiodic sound is represented in the first-order, interspike interval statistics of a population of simulated Chop-S units. This is true across a wide range of characteristic frequencies when the chopping rate is equal to the f0 of the sound. The model was able to simulate the results of psychophysical studies involving the pitch height and pitch strength of iterated ripple noise, the dominance region of pitch, the effect of phase on pitch height and pitch strength, pitch of inharmonic stimuli, and of sinusoidally amplitude modulated noise. Simulation results indicate that changes in the interspike interval statistics of populations of Chop-S units compare well with changes in the pitch perceived by humans. It is proposed that Chop-S units in the ventral cochlear nucleus may play an important role in pitch extraction: They can convert a purely temporal pitch code as observed in the auditory nerve into a temporal place code of pitch in populations of cochlear-nucleus, Chop-S with different characteristic frequencies, and chopping rates. Thus, populations of cochlear-nucleus Chop-S units, together with their target units presumably located in the inferior colliculus, may serve to establish a stable rate-place code of pitch at the level of the auditory cortex.

Action Potentials↗

PubMed2004

The temporal representation of speech in a nonlinear model of the guinea pig cochlea.

The temporal representation of speechlike stimuli in the auditory-nerve output of a guinea pig cochlea model is described. The model consists of a bank of dual resonance nonlinear filters that simulate the vibratory response of the basilar membrane followed by a model of the inner hair cell/auditory nerve complex. The model is evaluated by comparing its output with published physiological auditory nerve data in response to single and double vowels. The evaluation includes analyses of individual fibers, as well as ensemble responses over a wide range of best frequencies. In all cases the model response closely follows the patterns in the physiological data, particularly the tendency for the temporal firing pattern of each fiber to represent the frequency of a nearby formant of the speech sound. In the model this behavior is largely a consequence of filter shapes; nonlinear filtering has only a small contribution at low frequencies. The guinea pig cochlear model produces a useful simulation of the measured physiological response to simple speech sounds and is therefore suitable for use in more advanced applications including attempts to generalize these principles to the response of human auditory system, both normal and impaired.

Animals↗

PubMed2003

Adaptation in a revised inner-hair cell model.

A revised computational model of the inner-hair cell (IHC) and auditory-nerve (AN) complex was recently presented [Sumner et al., J. Acoust. Soc. Am. 111, 2178-2188 (2002)]. One key improvement is that the model reproduces the rate-intensity functions of low- (LSR), medium- (MSR), and high-spontaneous rate (HSR) fibers in the guinea-pig. Here we describe the adaptation characteristics of the model, and how they vary with model fiber type. Adaptation of the revised model for a HSR fiber is in line with an earlier version of the model [Meddis and Hewitt, J. Acoust. Soc. Am. 90, 904-917 (1991)]. In guinea-pig, poststimulus time histograms (PSTH) have been found to show less adaptation in LSR fibers. Evidence from chinchilla suggests that this is due to chronic adaptation resulting from short interstimulus intervals, and that fully recovered LSR fibers actually show more adaptation. However, the model is able to account for both variations of PSTH shape when fully recovered from adaptation. Interstimulus interval can also affect recovery in the model. The model is further tested against data previously used to evaluate models of AN adaptation. The tests are (i) recovery from adaptation of spontaneous rate and (ii) the recovery of response to acoustic stimuli ("forward masking"), (iii) the response to stimulus increments and (iv) decrements, and (v) the conservation of transient components. A HSR model fiber performs similarly to the earlier version of the model. However, there is considerable variation in response to increments and decrements between different model fibers.

Acoustic Stimulation↗

PubMed2003

Cochlear nonlinearity between 500 and 8000 Hz in listeners with normal hearing.

Cochlear nonlinearity was estimated over a wide range of center frequencies and levels in listeners with normal hearing, using a forward-masking method. For a fixed low-level probe, the masker level required to mask the probe was measured as a function of the masker-probe interval, to produce a temporal masking curve (TMC). TMCs were measured for probe frequencies of 500, 1000, 2000, 4000, and 8000 Hz, and for masker frequencies 0.5, 0.7, 0.9, 1.0 (on frequency), 1.1, and 1.6 times the probe frequency. Across the range of probe frequencies, the TMCs for on-frequency maskers showed two or three segments with clearly distinct slopes. If it is assumed that the rate of decay of the internal effect of the masker is constant across level and frequency, the variations in the slopes of the TMCs can be attributed to variations in cochlear compression. Compression-ratio estimates for on-frequency maskers were between 3:1 and 5:1 across the range of probe frequencies. Compression did not decrease at low frequencies. The slopes of the TMCs for the lowest frequency probe (500 Hz) did not change with masker frequency. This suggests that compression extends over a wide range of stimulus frequencies relative to characteristic frequency in the apical region of the cochlea.

Adult↗

PubMed2003

A nonlinear filter-bank model of the guinea-pig cochlear nerve: rate responses.

The aim of this study is to produce a functional model of the auditory nerve (AN) response of the guinea-pig that reproduces a wide range of important responses to auditory stimulation. The model is intended for use as an input to larger scale models of auditory processing in the brain-stem. A dual-resonance nonlinear filter architecture is used to reproduce the mechanical tuning of the cochlea. Transduction to the activity on the AN is accomplished with a recently proposed model of the inner-hair-cell. Together, these models have been shown to be able to reproduce the response of high-, medium-, and low-spontaneous rate fibers from the guinea-pig AN at high best frequencies (BFs). In this study we generate parameters that allow us to fit the AN model to data from a wide range of BFs. By varying the characteristics of the mechanical filtering as a function of the BF it was possible to reproduce the BF dependence of frequency-threshold tuning curves, AN rate-intensity functions at and away from BF, compression of the basilar membrane at BF as inferred from AN responses, and AN iso-intensity functions. The model is a convenient computational tool for the simulation of the range of nonlinear tuning and rate-responses found across the length of the guinea-pig cochlear nerve.

Animals↗

PubMed2003

Louder sounds can produce less forward masking: effects of component phase in complex tones.

The influence of the degree of envelope modulation and periodicity on the loudness and effectiveness of sounds as forward maskers was investigated. In the first experiment, listeners matched the loudness of complex tones and noise. The tones had a fundamental frequency (F0) of 62.5 or 250 Hz and were filtered into a frequency range from the 10th harmonic to 5000 Hz. The Gaussian noise was filtered in the same way. The components of the complex tones were added either in cosine phase (CPH), giving a large crest factor, or in random phase (RPH), giving a smaller crest factor. For each F0, subjects matched the loudness between all possible stimulus pairs. Six different levels of the fixed stimulus were used, ranging from about 30 dB SPL to about 80 dB SPL in 10-dB steps. Results showed that, at a given overall level, the CPH and the RPH tones were louder than the noise, and that the CPH tone was louder than the RPH tone. The difference in loudness was larger at medium than at low levels and was only slightly reduced by the addition of a noise intended to mask combination tones. The differences in loudness were slightly smaller for the higher than for the lower F0. In the second experiment, the stimuli with the lower F0s were used as forward maskers of a 20-ms sinusoid, presented at various frequencies within the spectral range of the maskers. Results showed that the CPH tone was the least effective forward masker, even though it was the loudest. The differences in effectiveness as forward maskers depended on masker level and signal frequency; in order to produce equal masking, the level of the CPH tone had to be up to 35 dB above that of the RPH tone and the noise. The implications of these results for models of loudness are discussed and a model is presented based on neural activity patterns in the auditory nerve; this predicts the general pattern of loudness matches. It is suggested that the effects observed in the experiments may have been influenced by two factors: cochlear compression and suppression.

Adult↗

PubMed2002

A revised model of the inner-hair cell and auditory-nerve complex.

A revised computational model of the inner-hair cell (IHC) and auditory-nerve (AN) complex is presented and evaluated. Building on previous models, the algorithm is intended as a component for use in more comprehensive models of the auditory periphery. It combines smaller components that aim to be faithful to physiology in so far as is practicable and known. Transduction between cochlear mechanical motion and IHC receptor potential (RP) is simulated using a modification of an existing biophysical IHC model. Changes in RP control the opening of calcium ion channels near the synapse, and local calcium levels determine the probability of the release of neurotransmitter. AN adaptation results from transmitter depletion. The exact timing of AN action potentials is determined by the quantal and stochastic release of neurotransmitter into the cleft. The model reproduces a wide range of animal RP and AN observations. When the input to the model is taken from a suitably nonlinear simulation of the motion of the cochlear partition, the new algorithm is able to simulate the rate-intensity functions of low-, medium-, and high-spontaneous rate AN fibers in response to stimulation both at best frequency and at other frequencies. The variation in fiber type arises in large part from the manipulation of a single parameter in the model: maximum calcium conductance. The model also reproduces quantitatively phase-locking characteristics, relative refractory effects, mean-to-variance ratio, and first- and second-order discharge history effects.

Algorithms↗