Rate and synchrony at the level of the auditory nerve are not sufficient to account for behaviorally estimated cochlear compression - a modeling study

Methods based on psychoacoustical forward masking have been proposed to estimate the local compressive growth of the basilar membrane (BM). This results from normal outer hair cells function, which leads to level-dependent amplification of BM vibration. Psychoacoustical methods assume that cochlear processing can be isolated from the response of the overall system, that sensitivity is dominated by the tonotopic location of the probe and that the effect of forward masking is different for on- and off-characteristic frequency (CF) maskers. In the present study, a computational model of the auditory nerve (AN) in combination with signal detection theory was used to test these assumptions. The underlying idea was that, for the BM compression to be estimated using psychoacoustics, enough information should be preserved at the level of the AN, because this forms an information bottleneck in the ascending auditory pathway. The simulated AN responses were quantified in terms of rate and synchrony for different types of AN fibers and CFs. The results show that, when using a low-intensity probe, local activity at the tonotopic location of the probe frequency is the dominant contributor to sensitivity in the healthy auditory system. However, on- and off-CF maskers produced similar forward masking onto the probe, which was mainly encoded by high- and to little extent by medium-spontaneous rate fibers. The simulation results suggested that the estimate of compression based on the behavioral experiments cannot be derived from sensitivity at the level of the AN but may require additional contributions, supporting previous physiological studies.

Genetic hearing impairment affects cochlear processing and, consequently, speech recognition in noise

The relation between speech recognition and hereditary hearing loss is not straightforward. Impaired cochlear processing of sound might be determined by underlying genetic defects. Data obtained in nine groups of patients with a specific type of genetic hearing loss were evaluated. For each group, the affected cochlear structure, or site-of-lesion, was determined based on previously published animal studies. Retrospectively obtained speech recognition scores in noise were related to several aspects of supra-threshold cochlear processing, as assessed by psychophysical measurements. The differences in speech perception in noise between these patient groups could be explained by these factors, and partially by the hypothesized affected structure of the cochlea, suggesting that speech recognition in noise was associated with genetics-related malfunctioning of the cochlea.

Robust Cochlear-Model-Based Speech Recognition

Accurate speech recognition can provide a natural interface for human–computer interaction. Recognition rates of the modern speech recognition systems are highly dependent on background noise levels and a choice of acoustic feature extraction method can have a significant impact on system performance. This paper presents a robust speech recognition system based on a front-end motivated by human cochlear processing of audio signals. In the proposed front-end, cochlear behavior is first emulated by the filtering operations of the gammatone filterbank and subsequently by the Inner Hair cell (IHC) processing stage. Experimental results using a continuous density Hidden Markov Model (HMM) recognizer with the proposed Gammatone Hair Cell (GHC) coefficients are lower for clean speech conditions, but demonstrate significant improvement in performance in noisy conditions compared to standard Mel-Frequency Cepstral Coefficients (MFCC) baseline.

Rhythm judgments reveal a frequency asymmetry in the perception and neural coding of sound synchrony

In modern Western music, melody is commonly conveyed by pitch changes in the highest-register voice, whereas meter or rhythm is often carried by instruments with lower pitches. An intriguing and recently suggested possibility is that the custom of assigning rhythmic functions to lower-pitch instruments may have emerged because of fundamental properties of the auditory system that result in superior time encoding for low pitches. Here we compare rhythm and synchrony perception between low- and high-frequency tones, using both behavioral and EEG techniques. Both methods were consistent in showing no superiority in time encoding for low over high frequencies. However, listeners were consistently more sensitive to timing differences between two nearly synchronous tones when the high-frequency tone followed the low-frequency tone than vice versa. The results demonstrate no superiority of low frequencies in timing judgments but reveal a robust asymmetry in the perception and neural coding of synchrony that reflects greater tolerance for delays of low- relative to high-frequency sounds than vice versa. We propose that this asymmetry exists to compensate for inherent and variable time delays in cochlear processing, as well as the acoustical properties of sound sources in the natural environment, thereby providing veridical perceptual experiences of simultaneity.

Correlation between speech-evoked auditory brainstem responses and transient evoked otoacoustic emissions

Objective:To investigate the correlation between cochlear processing and brainstem processing.Method:Transient evoked otoacoustic emissions and speech-evoked auditory brainstem responses were recorded in 40 ears of normal-hearing individuals aged 18 to 23 years. Correlation analyses compared transient evoked otoacoustic emission parameters with speech-evoked auditory brainstem response parameters.Results:There was a significant correlation between speech-evoked auditory brainstem response wave V latency and transient evoked otoacoustic emission global emission strength; there were no other significant correlations between the two tests.Conclusion:Tests for transient evoked otoacoustic emissions and speech-evoked auditory brainstem responses provide unique and functionally independent information about the integrity and sensitivity of the auditory system. Therefore, combining both tests will provide a more sensitive clinical battery with which to identify the location of different disorders (e.g. language-based learning impairments and hearing impairments).