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Enrique A Lopez-Poveda

Publications and source records attributed to Enrique A Lopez-Poveda.

12 recordsLinked to original sources
PubMed2006

A biophysical model of the inner hair cell: the contribution of potassium currents to peripheral auditory compression.

The term peripheral auditory compression refers to the fact that the whole range of audible sound pressure levels is mapped into a narrower range of auditory nerve responses. Peripheral compression is the by-product of independent compressive processes occurring at the level of the basilar membrane, the inner hair cell (IHC), and the auditory nerve synapse. Here, an electrical-circuit equivalent of an IHC is used to look into the compression contributed by the IHC. The model includes a mechanically driven transducer potassium (K(+)) conductance and two time- and voltage-dependent basolateral K(+) conductances: one with fast and one with slow kinetics. Special attention is paid to faithfully implement the activation kinetics of these basolateral conductances. Optimum model parameters are provided to account for previously reported in vitro observations that demonstrate the compression associated with the gating of the transducer and of the basolateral channels. Without having to readjust its parameters, the model also accounts for the in vivo nonlinear IHC transfer characteristics. Model simulations are then used to investigate the relative contribution of the transducer and basolateral K(+) currents to the nonlinear IHC input/output functions in vivo. The simulations suggest that the voltage-dependent activation of the basolateral currents compresses the DC potential for stereocilia displacements above approximately 5 nm. The degree of compression exceeds 2-to-1 and is similar for all stimulation frequencies. The AC potential is compressed in a similar way, but only for frequencies below 800 Hz. The simulations further suggest that the nonlinear gating of the transducer current is responsible for the expansive growth of the DC potential with increasing sound level (slope of 2 dB/dB) at low sound pressure levels.

Animals↗
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

Two new directions in speech processor design for cochlear implants.

Two new approaches to the design of speech processors for cochlear implants are described. The first aims to represent "fine structure" or "fine frequency" information in a way that it can be perceived and used by patients, and the second aims to provide a closer mimicking than was previously possible of the signal processing that occurs in the normal cochlea.

Acoustic Stimulation↗
PubMed2005

Detection of high-frequency spectral notches as a function of level.

High-frequency spectral notches are important cues for sound localization. Our ability to detect them must depend on their representation as auditory nerve (AN) rate profiles. Because of the low threshold and the narrow dynamic range of most AN fibers, these rate profiles deteriorate at high levels. The system may compensate by using onset rate profiles whose dynamic range is wider, or by using low-spontaneous-rate fibers, whose threshold is higher. To test these hypotheses, the threshold notch depth necessary to discriminate between a flat spectrum broadband noise and a similar noise with a spectral notch centered at 8 kHz was measured at levels from 32 to 100 dB SPL. The importance of the onset rate-profile representation of the notch was estimated by varying the stimulus duration and its rise time. For a large proportion of listeners, threshold notch depth varied nonmonotonically with level, increasing for levels up to 70-80 dB SPL and decreasing thereafter. The nonmonotonic aspect of the function was independent of notch bandwidth and stimulus duration. Thresholds were independent of stimulus rise time but increased for the shorter noise bursts. Results are discussed in terms of the ability of the AN to convey spectral notch information at different levels.

Acoustic Stimulation↗
PubMed2004

Inferred basilar-membrane response functions for listeners with mild to moderate sensorineural hearing loss.

Psychophysical estimates of cochlear function suggest that normal-hearing listeners exhibit a compressive basilar-membrane (BM) response. Listeners with moderate to severe sensorineural hearing loss may exhibit a linearized BM response along with reduced gain, suggesting the loss of an active cochlear mechanism. This study investigated how the BM response changes with increasing hearing loss by comparing psychophysical measures of BM compression and gain for normal-hearing listeners with those for listeners who have mild to moderate sensorineural hearing loss. Data were collected from 16 normal-hearing listeners and 12 ears from 9 hearing-impaired listeners. The forward masker level required to mask a fixed low-level, 4000-Hz signal was measured as a function of the masker-signal interval using a masker frequency of either 2200 or 4000 Hz. These plots are known as temporal masking curves (TMCs). BM response functions derived from the TMCs showed a systematic reduction in gain with degree of hearing loss. Contrary to current thinking, however, no clear relationship was found between maximum compression and absolute threshold.

Adult↗
PubMed2003

The commissure of the inferior colliculus shapes frequency response areas in rat: an in vivo study using reversible blockade with microinjection of kynurenic acid.

The commissure of the inferior colliculus (CoIC) interconnects corresponding frequency-band laminae in the two inferior colliculi (ICs). Although the CoIC has been studied neurophysiologically in vitro, the effect of the CoIC on the responses of IC neurons to physiological stimuli has not been addressed. In this study, we injected the glutamate receptor blocker kynurenic acid into one IC while recording the frequency response areas (FRAs) of neurons in the other, to test the hypothesis that frequency response properties of IC neurons are influenced by commissural inputs from the contralateral IC. Following blockade of the commissure, 10 of 12 neurons tested exhibited an increase or a decrease in their FRAs. In most neurons (9/12) the response area changed in the same direction, irrespective of whether the neuron was stimulated monaurally (at the ear contralateral to the recorded IC) or binaurally. In one neuron, blockade of the CoIC resulted in an expansion of the response area under binaural stimulation and a contraction under monaural stimulation. In the remaining two units, no effect was observed. Changes in response areas that exceeded the criterion ranged between 17 and 80% of control values with monaural stimulation, and 35 and 77% with binaural stimulation. Area changes could also be accompanied by changes in spike rate and monotonicity. From our observation that FRAs contract following commissure block, we infer that the commissure contains excitatory fibres. The expansion of response areas in other cases, however, suggests that the commissure also contains inhibitory fibres, or that its effects are mediated by disynaptic as well as monosynaptic circuits. The small sample size precludes a definitive conclusion as to which effect predominates. We conclude that inputs from the contralateral IC projecting via the CoIC influence the spectral selectivity and response gain of neurons in the IC.

Acoustic Stimulation↗
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

An approximate transfer function for the dual-resonance nonlinear filter model of auditory frequency selectivity.

The dual-resonance nonlinear filter [Meddis et al., J. Acoust. Soc. Am. 109, 2852-2861 (2001)] was presented as a digital time-domain algorithm to model nonlinear auditory frequency selectivity. This report extends previous work by presenting an approximate analytic transfer function that allows calculating and analyzing its level-dependent frequency-domain response. The transfer function is derived on the assumption that the filter behaves linearly for any given input amplitude. It matches accurately the response (gain and phase) of the digital filter for tones. Practical uses for the transfer function are suggested.

Algorithms↗
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↗