Perception of the envelope-beat frequency of inharmonic complex temporal envelopes

Modulation masking and glimpsing of natural and vocoded speech during single-talker modulated noise: Effect of the modulation spectrum

Compared to notionally steady-state noise, modulated maskers provide a perceptual benefit for speech recognition, in part due to preserved speech information during the amplitude dips of the masker. However, overlap in the modulation spectrum between the target speech and the competing modulated masker may potentially result in modulation masking, and thereby offset the release from energetic masking. The current study investigated masking release provided by single-talker modulated noise. The overlap in the modulation spectra of the target speech and the modulated noise masker was varied through time compression or expansion of the competing masker. Younger normal hearing adults listened to sentences that were unprocessed or noise vocoded to primarily limit speech recognition to the preserved temporal envelope cues. For unprocessed speech, results demonstrated improved performance with masker modulation spectrum shifted up or down compared to the target modulation spectrum, except for the most extreme time expansion. For vocoded speech, significant masking release was observed with the slowest masker rate. Perceptual results combined with acoustic analyses of the preserved glimpses of the target speech suggest contributions of modulation masking and cognitive-linguistic processing as factors contributing to performance.

Modulation masking and glimpsing of natural and vocoded speech during single-talker modulated noise: Effect of the modulation spectrum

Compared to notionally steady-state noise, modulated maskers provide a perceptual benefit for speech recognition, in part due to preserved speech information during the amplitude dips of the masker. However, overlap in the modulation spectrum between the target speech and the competing modulated masker may potentially result in modulation masking, and thereby offset the release from energetic masking. The current study investigated masking release provided by single-talker modulated noise. The overlap in the modulation spectra of the target speech and the modulated noise masker was varied through time compression or expansion of the competing masker. Younger normal hearing adults listened to sentences that were unprocessed or noise vocoded to primarily limit speech recognition to the preserved temporal envelope cues. For unprocessed speech, results demonstrated improved performance with masker modulation spectrum shifted up or down compared to the target modulation spectrum, except for the most extreme time expansion. For vocoded speech, significant masking release was observed with the slowest masker rate. Perceptual results combined with acoustic analyses of the preserved glimpses of the target speech suggest contributions of modulation masking and cognitive-linguistic processing as factors contributing to performance.

The origin of binaural interaction in the modulation domain

The purpose of these experiments was to assess whether the detection of diotic 5 Hz "probe" modulation of a 4000 Hz sinusoidal carrier was influenced by binaural interaction of "masker" modulators presented separately to each ear and applied to the same carrier. A 50 Hz masker modulator was applied to one ear and the masker modulator applied to the other ear had a frequency of 55 or 27.5 Hz. The starting phase of the masker modulators was fixed, and the starting phase of the probe modulator was varied. For both pairs of masker modulators, the threshold for detecting the probe modulation varied slightly but significantly with probe starting phase. Further experiments measuring probe detectability as a function of probe modulation depth did not provide clear evidence to support the idea that the internal representations of the masker modulators interacted binaurally to produce a weak distortion component in the internal representation of the modulation at a 5 Hz frequency. Also, the obtained phase effects were not correctly predicted using a model based on short-term loudness fluctuations.

Neural representations of complex temporal modulations in the human auditory cortex

Natural sounds such as speech contain multiple levels and multiple types of temporal modulations. Because of nonlinearities of the auditory system, however, the neural response to multiple, simultaneous temporal modulations cannot be predicted from the neural responses to single modulations. Here we show the cortical neural representation of an auditory stimulus simultaneously frequency modulated (FM) at a high rate, f(FM) approximately 40 Hz, and amplitude modulation (AM) at a slow rate, f(AM) <15 Hz. Magnetoencephalography recordings show fast FM and slow AM stimulus features evoke two separate but not independent auditory steady-state responses (aSSR) at f(FM) and f(AM), respectively. The power, rather than phase locking, of the aSSR of both decreases with increasing stimulus f(AM). The aSSR at f(FM) is itself simultaneously amplitude modulated and phase modulated with fundamental frequency f(AM), showing that the slow stimulus AM is not only encoded in the neural response at f(AM) but also encoded in the instantaneous amplitude and phase of the neural response at f(FM). Both the amplitude modulation and phase modulation of the aSSR at f(FM) are most salient for low stimulus f(AM) but remain observable at the highest tested f(AM) (13.8 Hz). The instantaneous amplitude of the aSSR at f(FM) is successfully predicted by a model containing temporal integration on two time scales, approximately 25 and approximately 200 ms, followed by a static compression nonlinearity.

Comparison of level discrimination, increment detection, and comodulation masking release in the audio- and envelope-frequency domains

In general, the temporal structure of stimuli must be considered to account for certain observations made in detection and masking experiments in the audio-frequency domain. Two such phenomena are (1) a heightened sensitivity to amplitude increments with a temporal fringe compared to gated level discrimination performance and (2) lower tone-in-noise detection thresholds using a modulated masker compared to those using an unmodulated masker. In the current study, translations of these two experiments were carried out to test the hypothesis that analogous cues might be used in the envelope-frequency domain. Pure-tone carrier amplitude-modulation (AM) depth-discrimination thresholds were found to be similar using both traditional gated stimuli and using a temporally modulated fringe for a fixed standard depth (ms = 0.25) and a range of AM frequencies (4-64 Hz). In a second experiment, masked sinusoidal AM detection thresholds were compared in conditions with and without slow and regular fluctuations imposed on the instantaneous masker AM depth. Release from masking was obtained only for very slow masker fluctuations (less than 2 Hz). A physiologically motivated model that effectively acts as a first-order envelope change detector accounted for several, but not all, of the key aspects of the data.

Effect of modulation maskers on the detection of second-order amplitude modulation with and without notched noise

The mechanisms underlying the detection of second-order amplitude modulation (AM) were explored. The detectability of second-order AM (fixed depth for each subject) was measured for first- and second-order modulation rates of 16 and 2 Hz, respectively (slow-rate pair), and 50 and 10 Hz, respectively (fast-rate pair), with no masker, a low-band modulation masker (centered at 2 or 10 Hz), and a high-band modulation masker (centered at 16 or 50 Hz). This was done in the absence and presence of an audio-frequency notched noise centered at the carrier frequency of 4000 Hz. Both modulation maskers were "low-noise" noises, to prevent overmodulation. In the absence of notched noise, both modulation maskers impaired performance for the slow-rate pair, but only the low-band masker impaired performance for the fast-rate pair. When notched noise was present, the low-band masker had no significant effect for either rate pair and the high-band masker had an effect only for the slow-rate pair. These results suggest that second-order AM detection is mediated both by an envelope distortion component at the second-order rate and by slow fluctuations in the output of a modulation filter tuned to the first-order rate. When notched noise is present, the distortion component plays little role.