Search PubMed⌕ Search

Biomedical subjects

Stephan D Ewert

Publications and source records attributed to Stephan D Ewert.

6 recordsLinked to original sources
PubMed2005

Modulation masking produced by second-order modulators.

Recent studies suggest that an auditory nonlinearity converts second-order sinusoidal amplitude modulation (SAM) (i.e., modulation of SAM depth) into a first-order SAM component, which contributes to the perception of second-order SAM. However, conversion may also occur in other ways such as cochlear filtering. The present experiments explored the source of the first-order SAM component by investigating the ability to detect a 5-Hz, first-order SAM probe in the presence of a second-order SAM masker beating at the probe frequency. Detection performance was measured as a function of masker-carrier modulation frequency, phase relationship between the probe and masker modulator, and probe modulation depth. In experiment 1, the carrier was a 5-kHz sinusoid presented either alone or within a notched-noise masker in order to restrict off-frequency listening. In experiment 2, the carrier was a white noise. The data obtained in both carrier conditions are consistent with the existence of a modulation distortion component. However, the phase yielding poorest detection performance varied across experimental conditions between 0 degrees and 180 degrees, confirming that, in addition to nonlinear mechanisms, cochlear filtering and off-frequency listening play a role in second-order SAM perception. The estimated magnitude of the modulation distortion component ranges from 5%-12%.

Adolescent↗
PubMed2005

Perceptual interaction between carrier periodicity and amplitude modulation in broadband stimuli: a comparison of the autocorrelation and modulation-filterbank model.

Recent temporal models of pitch and amplitude modulation perception converge on a relatively realistic implementation of cochlear processing followed by a temporal analysis of periodicity. However, for modulation perception, a modulation filterbank is applied whereas for pitch perception, autocorrelation is applied. Considering the large overlap in pitch and modulation perception, this is not parsimonious. Two experiments are presented to investigate the interaction between carrier periodicity, which produces strong pitch sensations, and envelope periodicity using broadband stimuli. Results show that in the presence of carrier periodicity, detection of amplitude modulation is impaired throughout the tested range (8-1000 Hz). On the contrary, detection of carrier periodicity in the presence of an additional amplitude modulation is impaired only for very low frequencies below the pitch range (<33 Hz). Predictions of a generic implementation of a modulation-filterbank model and an autocorrelation model are compared to the data. Both models were too insensitive to high-frequency envelope or carrier periodicity and to infra-pitch carrier periodicity. Additionally, both models simulated modulation detection quite well but underestimated the detrimental effect of carrier periodicity on modulation detection. It is suggested that a hybrid model consisting of bandpass envelope filters with a ripple in their passband may provide a functionally successful and physiologically plausible basis for a unified model of auditory periodicity extraction.

Adult↗
PubMed2004

External and internal limitations in amplitude-modulation processing.

Three experiments are presented to explore the relative role of "external" signal variability and "internal" resolution limitations of the auditory system in the detection and discrimination of amplitude modulations (AM). In the first experiment, AM-depth discrimination performance was determined using sinusoidally modulated broadband-noise and pure-tone carriers. The AM index, m, of the standard ranged from -28 to -3 dB (expressed as 20 log m). AM-depth discrimination thresholds were found to be a fraction of the AM depth of the standard for standards down to -18 dB, in the case of the pure-tone carrier, and down to -8 dB, in the case of the broadband-noise carrier. For smaller standards, AM-depth discrimination required a fixed increase in AM depth, independent of the AM depth of the standard. In the second experiment, AM-detection thresholds were obtained for signal-modulation frequencies of 4, 16, 64, and 256 Hz, applied to either a band-limited random-noise carrier or a deterministic ("frozen") noise carrier, as a function of carrier bandwidth (8 to 2048 Hz). In general, detection thresholds were higher for the random- than for the frozen-noise carriers. For both carrier types, thresholds followed the pattern expected from frequency-selective processing of the stimulus envelope. The third experiment investigated AM masking at 4, 16, and 64 Hz in the presence of a narrow-band masker modulation. The variability of the masker was changed from entirely frozen to entirely random, while the long-term average envelope power spectrum was held constant. The experiment examined the validity of a long-term average quantity as the decision variable, and the role of memory in experiments with frozen-noise maskers. The empirical results were compared to predictions obtained with two modulation-filterbank models. The predictions revealed that AM-depth discrimination and AM detection are limited by a combination of the external signal variability and an internal "Weber-fraction" noise process.

Acoustic Stimulation↗
PubMed2003

Modulation masking produced by complex tone modulators.

Thresholds were measured for detecting sinusoidal amplitude modulation in the presence of a complex-tone masker modulation. Both modulations were applied to the same sinusoidal carrier. Two different masker modulations were used: (i) a pair of components beating at the difference frequency and (ii) a three-tone complex producing a sinusoidal amplitude modulation of the modulation depth at the difference frequency between adjacent components. Both maskers show a periodicity in the waveform that is not contained in the envelope spectrum itself but can be observed when the envelope of the envelope, referred to as the "venelope" [Ewert et al., J. Acoust. Soc. Am. 112, 2921-2931 (2002)], is calculated. For a signal frequency equal to the masker-venelope periodicity, modulation depth at threshold was measured as a function of the signal phase relative to the phase of the masker-venelope component. Signal frequencies of 5, 30, and 90 Hz were used. It was found that masking was phase dependent for all three signal frequencies. Thresholds were lower for the in-phase condition, where maxima in the signal waveform coincided with maxima in the masker-venelope waveform, than for the antiphase condition. The maximum threshold difference was 15 dB. The results are in contrast to recent data [Moore et al., J. Acoust. Soc. Am. 106, 908-918 (1999)], where lowest thresholds were reported for the antiphase condition in a similar experiment. The present data are in line with the idea that a nonlinearity prior to a modulation filterbank extracts the venelope of the masker modulator. However, a compressive nonlinearity such as that associated with the processing on the basilar membrane cannot account for the empirical findings, since it predicts the opposite phase effect.

Adult↗
PubMed2002

Spectro-temporal processing in the envelope-frequency domain.

The frequency selectivity for amplitude modulation applied to tonal carriers and the role of beats between modulators in modulation masking were studied. Beats between the masker and signal modulation as well as intrinsic envelope fluctuations of narrow-band-noise modulators are characterized by fluctuations in the "second-order" envelope (referred to as the "venelope" in the following). In experiment 1, masked threshold patterns (MTPs), representing signal modulation threshold as a function of masker-modulation frequency, were obtained for signal-modulation frequencies of 4, 16, and 64 Hz in the presence of a narrow-band-noise masker modulation, both applied to the same sinusoidal carrier. Carrier frequencies of 1.4, 2.8, and 5.5 kHz were used. The shape and relative bandwidth of the MTPs were found to be independent of the signal-modulation frequency and the carrier frequency. Experiment 2 investigated the extent to which the detection of beats between signal and masker modulation is involved in tone-in-noise (TN), noise-in-tone (NT), and tone-in-tone (TT) modulation masking, whereby the TN condition was similar to the one used in the first experiment. A signal-modulation frequency of 64 Hz, applied to a 2.8-kHz carrier, was tested. Thresholds in the NT condition were always lower than in the TN condition, analogous to the masking effects known from corresponding experiments in the audio-frequency domain. TT masking conditions generally produced the lowest thresholds and were strongly influenced by the detection of beats between the signal and the masker modulation. In experiment 3, TT masked-threshold patterns were obtained in the presence of an additional sinusoidal masker at the beat frequency. Signal-modulation frequencies of 32, 64, and 128 Hz, applied to a 2.8-kHz carrier, were used. It was found that the presence of an additional modulation at the beat frequency hampered the subject's ability to detect the envelope beats and raised thresholds up to a level comparable to that found in the TN condition. The results of the current study suggest that (i) venelope fluctuations play a similar role in modulation masking as envelope fluctuations do in spectral masking, and (ii) envelope and venelope fluctuations are processed by a common mechanism. To interpret the empirical findings, a general model structure for the processing of envelope and venelope fluctuations is proposed.

Adult↗