Lee Mendoza

Within‐ and across‐channel processes in modulation detection interference

Modulation detection interference (MDI) describes the effect of an amplitude-modulated sound on detection of amplitude modulation (AM) on a spectrally distant target sound. In MDI, AM detection is worsened by an AM interfering sound. Previous research has implicated both within-channel and across-channel processes as the basis for MDI. This study examined the detection of 10-Hz sinusoidal AM as a function of the spectral relationship, gating synchrony, and ear of presentation of the target and interfering sounds. Using a 993-Hz target sound, AM detection improved as the frequency of the interferer increased from 1250 to 2188 Hz, and as it decreased from 788 to 450 Hz. MDI was less for continuous interferers than for interferers gated synchronously with the target. In the continuous interferer conditions, however, MDI was still greatest for interferers most proximal to the target. The effects of frequency proximity and gating asynchrony were also evident using interferers presented to the ear contralateral to the target ear. While a 1250-Hz interferer led to more MDI than a 788-Hz interferer when the interferers were presented to the same ear as a 993-Hz target, no significant asymmetry was noted with dichotic stimulation. Taken together, the results of these experiments indicated that MDI is the result of across-channel, and to a limited extent, within-channel processes.

Modulation detection interference using random and sinusoidal amplitude modulation.

The detection of amplitude modulation (AM) of a target sound is made more difficult by the presence of a modulated sound some spectral distance from the target. This effect (modulation detection interference, or MDI) was examined for stimuli with random amplitude modulations (RAM) and for sounds with sinusoidal amplitude modulations (SAM) as a function of average modulation depth (m) of the interferer. In an experiment comparing comodulated and independent RAM targets and interferers, the amount of interference was not related to the modulation coherence of the target and interferer. Elevations in AM threshold increased as a function of m in a similar way for both conditions. The MDI for RAM and SAM targets and interferers was also compared. While no difference was found for AM detection of RAM and SAM, MDI was found to be greater for the RAM stimuli than for the SAM stimuli. A subsidiary experiment comparing RAM and SAM modulation depth discrimination indicated that RAM discrimination is more difficult than SAM discrimination. Taken together, these results are quantitatively consistent with a mechanism resembling AM discrimination as the underpinning of MDI in conditions where the target and interferer are synchronously gated.

Masker interaural phase and the MLD: effects of conductive hearing loss

Sensitivity to binaural signals that were interaurally antiphasic with respect to the masking noise was examined as a function of the interaural phase of the masking noise, for listeners with normal hearing, and listeners with conductive hearing losses. Some of the hearing-impaired listeners were tested both before and after middle ear surgery. In agreement with previous findings, the normal-hearing listeners showed the lowest thresholds when the masking noise had no interaural phase shift, with thresholds increasing monotonically as the interaural phase of the center frequency of the masker approached +/- 180 degrees. Although many of the masked threshold functions of the hearing-impaired listeners showed significant changes in thresholds as a function of masker interaural phase, most of the functions were abnormal in shape, and few showed peaks for the interaural masker phase of 0 degrees. Although functions often continued to be abnormal after middle ear surgery, a few subjects obtained postsurgery functions that were correlated with the average normal function. The results indicate that although normal-hearing listeners generally have the lowest antiphasic signal threshold for a masker with 0 degrees interaural phase, conductively-impaired listeners often do not show a clear minimum for antiphasic signal threshold at any particular masker interaural phase.

Perceptual organization in a comodulation masking release interference paradigm: exploring the role of amplitude modulation, frequency modulation, and harmonicity.

Two areas of interest within auditory scene analysis are the identification of cues which contribute to auditory grouping and/or segregation, and the methodology employed to measure such effects. The present study explored the roles of three purported grouping cues--correlated AM, correlated FM, and harmonicity--using a comodulation masking release interference paradigm. This method was reasoned to be free of alterations in the subjects mode of listening (analytic versus synthetic). The results suggested that correlated AM was a viable grouping cue. For some listeners, the presence of FM also appeared to provide a grouping cue, but, generally, no effect of coherence of FM was observed. No effect of harmonicity was observed in this paradigm.

Comodulation masking release: The effect of the characteristics of noise bands presented before and after a signal

CMR was measured as a function of masker duration in three conditions. In the first condition, a 400‐ms pure‐tone signal was presented in the temporal center of comodulated masking bands ranging in duration from 400 to 1200 ms. No effect of masker duration was observed: CMR was equivalent to that found in continuous maskers. In the second condition, continuous independent bands identical in spectra to the comodulated bands were gated off before, and on following, the comodulated bands. The presence of the independent bands reduced CMR considerably. Increasing the duration of the comodulated bands increased CMR somewhat; however, CMR was still substantially smaller than that observed without the independent bands. In the third condition, band limited noise spanning the same range as the comodulated bands was substituted for the independent bands. The bandlimited noise had little effect on CMR. In a second experiment, independent bands were gated on for various durations preceding spectrally identical 400‐m...

Comodulation masking release as a function of masking noise‐band temporal envelope similarity in normal hearing and cochlear impaired listeners.

Thresholds for a pure‐tone signal (1000 Hz) were obtained from both normal hearing and hearing‐impaired listeners in a variety of masker conditions. Masking stimuli consisted of five Gaussian noise bands, each 20 Hz wide, and centered on 500, 750, 1000, 1250, and 1500 Hz. Two such base stimuli were created. In the first stimulus set, all noise bands had the same temporal envelope (comodulated). In the second set, each noise band was generated independently of the other bands, and thus each had independent temporal envelopes. Additional masking stimuli were generated by combining the comodulated and independent bands at specific comodulated/independent intensity ratios (CIR = 25, 20, 15, 10, and 5 dB), with overall level of the combined noise bands held constant. The result of increasing CIR was a progressive increase in the similarity of temporal envelopes of the noise bands masking the signal. Compared to threshold for the pure‐tone signal in independent bands of noise, threshold steadily decreased as th...

Modulation detection interference (MDI) for randomly fluctuating envelopes

Modulation detection interference (MDI) was measured in normal‐hearing listeners for stimuli with randomly fluctuating envelopes. The stimuli were generated by modulating pure‐tone carriers by dc‐shifted low‐pass noises (cutoff=10 Hz). The target carrier was 1000 Hz and the interferer carrier 2250 Hz. The target could be presented either alone (baseline condition) or in conjunction with the interferer that was either gated synchronously with the target or was presented continuously. The interferer could be unmodulated, comodulated with the target, or modulated independently from the target. Results indicated substantial MDI in the presence of the interferer both for comodulated and independently modulated envelopes. The differential gating effects will be compared to those for sinusoidal amplitude modulations. [Work supported by the NIDCD R01‐DC00418.]

Comodulation masking release (CMR) for multiple temporal envelope maskers

Thresholds for a pure‐tone signal were measured in masking noise composed of multiple noise bands. In some conditions, one group of flanking bands (consisting of half of the flanking bands) was comodulated, and independent of the other group of comodulated flanking bands in terms of temporal envelope. The center frequencies of the flanking bands were arranged such that noise bands from the two groups were spectrally interleaved. In these conditions, the band of noise centered on the signal was a combination of two noise bands, which were each comodulated with one group of flanking bands. CMR was obtained but was not of the magnitude observed when all noise bands shared a common temporal envelope. Potential cues in the multiple envelope maskers included the information in the summed representation of the flanking bands, and the degree of temporal envelope correlation between the on‐signal band and the flankers. Additional stimulus conditions examined the viability of these cues. The results of these experi...

Temporal asynchrony effects in listeners with cochlear hearing loss

Temporal asynchrony appears to play a dominant role in the perceptual organization of multi‐source sounds. Two paradigms in which temporal asynchrony results in perceptual segregation are comodulation masking release (CMR) and modulation detection interference (MDI). Both of these paradigms involve amplitude‐modulated stimuli. Because listeners with hearing loss of cochlear origin often exhibit reduced temporal resolution for modulated stimuli, this study examined whether these listeners are as susceptible to asynchrony effects as are listeners with normal hearing. The CMR task was similar to one described previously [J. H. Grose and J. W. Hall, J. Acoust. Soc. Am. 93, 2896–2902 (1993)]. The modulation interference task was novel. The baseline condition was a measure of sequention envelope discrimination for a 20‐Hz wide band of noise centered at 1125 Hz. Modulation interference was measured as a reduction in performance brought about by the synchronous presentation of a second independent band of noise c...

Stream segregation in normal‐hearing and cochlear‐impaired listeners

Cochlear‐impaired and normal‐hearing listeners were compared on two tasks intended to assess auditory stream segregation. One task measured gap discrimination between two tones that were presented either in isolation or were embedded into a sequential stream of tones. For tone pairs distally spaced in frequency, performance was poor when the tones were presented in isolation but improved when they were drawn into separate auditory streams, presumably because temporal judgments could now be made within a stream. Both groups of listeners gave similar patterns of results although some differences were apparent. The second task was a melody recognition procedure [J. A. P. M. de Laat and R. Plomp, J. Acoust. Soc. Am. 78, 1574–1577 (1985)] and involved identifying a 4‐note target melody embedded between two competing melodies as a function of the proximity and temporal synchrony of the competing melodies. In both groups, marked individual differences were observed. Results from both tasks will be discussed in t...