Jesko L. Verhey

Within-channel cues in comodulation masking release (CMR): Experiments and model predictions using a modulation-filterbank model

Experiments and model calculations were performed to study the influence of within-channel cues versus across-channel cues in comodulation masking release (CMR). A class of CMR experiments is considered that are characterized by a single (unmodulated or modulated) bandpass noise masker with variable bandwidth centered at the signal frequency. A modulation-filterbank model suggested by Dau et al. [J. Acoust. Soc. Am. 102, 2892-2905 (1997)] was employed to quantitatively predict the experimental data. Effects of varying masker bandwidth, center frequency, modulator bandwidth, modulator type, and signal duration on CMR were examined. In addition, the effect of band limiting the noise before or after modulation was shown to influence the CMR in the same way as a systematic variation of the modulation depth. It is demonstrated that a single-channel analysis, which analyzes only the information from one peripheral channel, quantitatively accounts for the CMR in most cases, indicating that an across-channel process is generally not necessary for simulating results from this class of CMR experiments. True across-channel processes may be found in another class of CMR experiments.

The psychophysics and physiology of comodulation masking release

The ability to detect auditory signals from background noise may be enhanced by the addition of energy in frequency regions well removed from the frequency of the signal. However, it is important that this energy is amplitude-modulated in a coherent way across frequencies, i.e. comodulated. This enhancement of signal detectability is known as comodulation masking release (CMR), and in this review we show that CMR is largest if: (1) the total maskers bandwidth is large, (2) the modulation frequency is low, (3) the modulation depth is high, (4) the envelope is regular and, (5) the maskers spectrum level is high. Possible physiological correlates of CMR have been found at different levels of the auditory pathway. Current hypotheses for the underlying physiological mechanisms, including wide-band inhibition or the disruption of masker modulation envelope response, are discussed.

Intrinsic envelope fluctuations and modulation‐detection thresholds for narrow‐band noise carriers.

A model is presented which calculates the intrinsic envelope power of a bandpass noise carrier within the passband of a hypothetical modulation filter tuned to a specific modulation frequency. Model predictions are compared to experimentally obtained amplitude modulation (AM) detection thresholds. In experiment 1, thresholds for modulation rates of 5, 25, and 100 Hz imposed on a bandpass Gaussian noise carrier with a fixed upper cutoff frequency of 6 kHz and a bandwidth in the range from 1 to 6000 Hz were obtained. In experiment 2, three noises with different spectra of the intrinsic fluctuations served as the carrier: Gaussian noise, multiplied noise, and low-noise noise. In each case, the carrier was spectrally centered at 5 kHz and had a bandwidth of 50 Hz. The AM detection thresholds were obtained for modulation frequencies of 10, 20, 30, 50, 70, and 100 Hz. The intrinsic envelope power of the carrier at the output of the modulation filter tuned to the signal modulation frequency appears to provide a good estimate for AM detection threshold. The results are compared with predictions on the basis of the more complex auditory processing model by Dau et al.

Responses of Dorsal Cochlear Nucleus Neurons to Signals in the Presence of Modulated Maskers

The detection of a signal in noise is enhanced when the masking noise is coherently modulated over a wide range of frequencies. This phenomenon, known as comodulation masking release (CMR), has been attributed to across-channel processing; however, the relative contribution of different stages in the auditory system to such across-channel processing is unknown. It has been hypothesized that wideband or lateral inhibition may underlie a physiological correlate of CMR. To further test this hypothesis, we have measured the responses of single units from the dorsal cochlear nucleus in which wideband inhibition is particularly pronounced. Using a sinusoidally amplitude-modulated tone at the best frequency of each unit as a masker, a pure-tone signal was added in the dips of the masker modulation. Flanking bands (FBs, also amplitude-modulated pure tones) were positioned to fall within the inhibitory sidebands of the receptive field of the unit. The FBs were either in phase (comodulated) or out of phase (codeviant) with the on-frequency masker. For the majority of units, the addition of the comodulated FBs produced a strong reduction in the response to the masker modulation, making the signal more salient in the post stimulus time histograms. The change in spike rate in response to the signal between the masker and signal-plus-masker conditions was greatest for the comodulated condition for 29 of 45 units. These results are consistent with the hypothesis that wideband inhibition may play a role in across-channel processing at an early stage in the auditory pathway.

Spectral loudness summation as a function of duration

Loudness was measured as a function of signal bandwidth for 10-, 100-, and 1000-ms-long signals. The test and reference signals were bandpass-filtered noise spectrally centered at 2 kHz. The bandwidth of the test signal was varied from 200 to 6400 Hz. The reference signal had a bandwidth of 3200 Hz. The reference levels were 45, 55, and 65 dB SPL. The level to produce equal loudness was measured with an adaptive, two-interval, two-alternative forced-choice procedure. A loudness matching procedure was used, where the tracks for all signal pairs to be compared were interleaved. Mean results for nine normal-hearing subjects showed that the magnitude of spectral loudness summation depends on signal duration. For all reference levels, a 6- to 8-dB larger level difference between equally loud signals with the smallest (delta f = 200 Hz) and largest (delta f = 6400 Hz) bandwidth is found for 10-ms-long signals than for the 1000-ms-long signals. The duration effect slightly decreases with increasing reference loudness. As a consequence, loudness models should include a duration-dependent compression stage. Alternatively, if a fixed loudness ratio between signals of different duration is assumed, this loudness ratio should depend on the signal spectrum.

Spatial dissociation of changes of level and signal-to-noise ratio in auditory cortex for tones in noise.

Functional magnetic resonance imaging has been used to investigate the signal representation in human auditory cortex for a sinusoidal signal in the presence of a noise masker. This paradigm is widely used in auditory research to study auditory processing. Five-note tonal melodies were presented in a masking noise for signal-to-noise ratios (S/N) from -18 dB to+24 dB in 6 dB-steps. For small S/N (-18 dB, -12 dB, -6 dB) the overall level of the sound is nearly constant, but the audibility of the tone varies with S/N. For S/N of 0 dB and above, the tone is always clearly audible, and the perceived change is mainly the increase in overall level. This interaction between S/N, overall level and perception is reflected by a spatial dissociation of the respective activation in auditory cortex. Brain regions mainly sensitive to level changes were found in various parts of the superior temporal lobes, including primary auditory cortex and Planum temporale, while those regions mainly sensitive to S/N changes were located at or close to lateral Heschls gyrus. The overlap between these two regions is small. The results are interpreted as indicating that the coding of overall level and, thus, loudness is different from the coding of audibility of a periodic signal. The S/N-sensitive region largely overlaps with the pitch-sensitive regions in lateral Heschls gyrus found in previous studies. The results from the present study further suggest that the audibility of a tone in noise is related to the overall pitch strength.

Comparison of loudness models for time-varying sounds

The loudness of a sound depends, among other parameters, on its temporal shape. Different loudness models were proposed to account for temporal aspects in loudness perception. To investigate different dynamic concepts for modeling loudness, predictions were made with the two current loudness models of Glasberg and Moore [J. Acoust. Soc. Am. 50, 331–341 (2002)] and Chalupper and Fastl [Acta Acustica united with Acustica 88, 378–386 (2002)] for a set of time-varying sounds. The predicted effects of duration, repetition rate, amplitude-modulation, temporal asymmetry, frequency modulation and the systematic variation of spectro-temporal structure on loudness were compared to data from the literature. Both models predicted the general trends of the data for single, repeated and asymmetric sound bursts and amplitude-modulated sounds. The model of Chalupper and Fastl seems to agree slightly better with loudness data for sounds with strong spectral variations over time, since it includes a dynamic stage which allows spectral loudness summation also for non-synchronous frequency components.

40-Hz multiple auditory steady-state responses to narrow-band chirps in sedated and anaesthetized infants.

OBJECTIVE The general consensus to date has been that a stimulus repetition rate of 40Hz is not appropriate for the recording of auditory steady-state responses (ASSR) in sedated or anaesthetized infants. The aim of this study was to investigate whether reliable 40-Hz ASSR thresholds can be obtained in sedated infants using narrow-band chirp stimuli in the clinical routine. METHODS 40-Hz auditory brainstem responses (ABR) and 40-Hz ASSR were recorded in 34 infants below the age of 48 months under sedation or under general anaesthesia. ABR were evoked by broad-band chirp stimuli with a flat amplitude spectrum. ASSR were recorded simultaneously from both ears with an adaptive multiple stimulus paradigm using four narrow-band chirps centred at 500, 1000, 2000 and 4000Hz. ABR and ASSR thresholds were evaluated to determine differences between the estimates from the two methods. RESULTS Despite of sedation and anaesthesia, large wave V amplitudes of the chirp evoked 40-Hz ABR were found at levels as low as 10dB above the individual ABR threshold. ASSR thresholds for stimulus repetition rates of 40Hz could be consistently estimated in all 34 infants. Thresholds estimated from the ASSR for the four frequencies of the narrow-band chirps and the threshold derived from the broad-band chirp ABR differ, on average by 3.7dB for the frequency range of 1000-2000Hz and 7.1dB for the frequency range from 2000 to 4000Hz. CONCLUSION In contrast to the general assumption that 40-Hz ASSR are not appropriate for threshold estimation in infants our results demonstrate that multiple ASSR with a stimulus repetition rate of 40Hz can be recorded in sedated and anaesthetized infants using narrow-band chirps. Threshold estimates obtained with 40-Hz ASSR are, on average, slightly higher than those obtained with chirp evoked ABR but allow for a frequency specific characterisation of the hearing ability.

Fast Hearing-Threshold Estimation Using Multiple Auditory Steady-State Responses with Narrow-Band Chirps and Adaptive Stimulus Patterns

This paper describes the estimation of hearing thresholds in normal-hearing and hearing-impaired subjects on the basis of multiple-frequency auditory steady-state responses (ASSRs). The ASSR was measured using two new techniques: (i) adaptive stimulus patterns and (ii) narrow-band chirp stimuli. ASSR thresholds in 16 normal-hearing and 16 hearing-impaired adults were obtained simultaneously at both ears at 500, 1000, 2000, and 4000 Hz, using a multiple-frequency stimulus built up of four one-octave-wide narrow-band chirps with a repetition rate of 40 Hz. A statistical test in the frequency domain was used to detect the response. The recording of the steady-state responses was controlled in eight independent recording channels with an adaptive, semiautomatic algorithm. The average differences between the behavioural hearing thresholds and the ASSR threshold estimate were 10, 8, 13, and 15 dB for test frequencies of 500, 1000, 2000, and 4000 Hz, respectively. The average overall test duration of 18.6 minutes for the threshold estimations at the four frequencies and both ears demonstrates the benefit of an adaptive recording algorithm and the efficiency of optimised narrow-band chirp stimuli.

Role of suppression and retro-cochlear processes in comodulation masking release

Recent physiological studies suggest that comodulation masking release (CMR) could be a consequence of wideband inhibition at the level of the cochlear nucleus. The present study investigates whether the existence region of psychophysical CMR is comparable to the inhibitory areas of units showing a physiological correlate of CMR. Since the inhibitory areas are similar to suppressive regions at the level of the basilar membrane, the amount of CMR that can be accounted for by suppression was determined by predicting the data with a model incorporating a peripheral nonlinearity. A CMR of up to 6 dB could still be experimentally observed for a flanking band (FB) four octaves below the on-frequency masker (OFM). For FB frequencies below the OFM, the suggested model predicts CMR equal to the measured CMR for high levels of the FB. The model underestimates the magnitude of CMR for midlevels of the FB, indicating that suppression alone cannot account for CMR. The data are consistent with the hypothesis that wideband inhibition plays a role in CMR.