Gregory P. Schooneveldt

Temporal modulation transfer functions for band-limited noise in subjects with cochlear hearing loss

The modulation depth required for the detection of sinusoidal amplitude modulation was measured as a function of modulation rate, giving temporal modulation transfer functions (TMTFs). The carrier was a one-octave wide noise centred at 2 kHz, and it was presented in an unmodulated background noise lowpass filtered at 5 kHz. Three subjects with unilateral cochlear hearing loss were tested. For each subject, the normal ear was tested both at the same sound pressure level (SPL) and at the same sensation level (SL) as the impaired ear. The TMTFs were essentially the same for the normal and impaired ears, both at equal SPL and at equal SL. The better ears of three subjects with bilateral cochlear losses were also tested. Again, TMTFs were essentially the same as obtained for normal ears. These results suggest that temporal resolution is not necessarily adversely affected by cochlear hearing loss, at least as measured by this task.

Across-channel masking and comodulation masking release.

These experiments on across-channel masking (ACM) and comodulation masking release (CMR) were designed to extend the work of Grose and Hall [J. Acoust. Soc. Am. 85, 1276-1284 (1989)] on CMR. They investigated the effect of the temporal position of a brief 700-Hz signal relative to the modulation cycle of a 700-Hz masker 100% sinusoidally amplitude modulated (SAM) at a 10-Hz rate, which was either presented alone (reference masker) or formed part of a masker consisting of the 3rd to 11th harmonics of a 100-Hz fundamental. In the harmonic maskers, each harmonic was either SAM with the same 10-Hz modulator phase (comodulated masker) or with a shift in modulator phase of 90 degrees for each successive harmonic (phase-incoherent masker). When the signal was presented at the dips of the envelope of the 700-Hz component, the comodulated masker gave lower thresholds than the reference masker, while the phase-incoherent masker gave higher thresholds, i.e., a CMR was observed. No CMR was found when the signal was presented at the peaks of the envelope. In experiment 1, we replicated the experiment of Grose and Hall, but with an additional condition in which the 600- and 800-Hz components were removed from the masker, in order to investigate the role of within-channel masking effects. The results were similar to those of Grose and Hall. In experiment 2, the signal was added at the peaks of the envelope of the 700-Hz component, but in antiphase to the carrier of that component and at a level chosen to transform the peaks into dips. No CMR was found. Rather, performance was worse for both the comodulated and phase-incoherent maskers than for the reference masker. This was true even when the flanking components in the maskers were all remote in frequency from 700 Hz. In experiment 3, the masker components were all 50% SAM and the signal was added in antiphase at a dip of the envelope of the 700-Hz component, thus making the dip deeper. Performance was worse for the phase-incoherent than for the reference masker and was worse still for the comodulated masker. The results of all three experiments indicate strong ACM effects. CMR was found only when the signal was placed in the dips of the masker envelope and when it produced an increase in level relative to that in adjacent bands.

Comodulation masking release for various monaural and binaural combinations of the signal, on‐frequency, and flanking bands

The threshold for a signal masked by a narrow band of noise centered at the signal frequency (the on-frequency band) may be reduced by adding to the masker a second band of noise (the flanking band) whose envelope is correlated with that of the first band. This effect is called comodulation masking release (CMR). These experiments examine two questions. (1) How does the CMR vary with the number and ear of presentation of the flanking band(s)? (2) Is it possible to obtain a CMR when a binaural masking level difference (BMLD) is already present, and vice versa? Thresholds were measured for a 400-ms signal in a continuous 25-Hz-wide noise centered at signal frequencies (fs) of 250, 1000, and 4000 Hz. This masker was presented either alone or with one or more continuous flanking bands whose envelopes were either correlated or uncorrelated with that of the on-frequency band; their frequencies ranged from 0.5fs to 1.5fs. CMRs were measured for six conditions in which the signal, the on-frequency band, and the flanking band(s) were presented in various monaural and binaural combinations. When a single flanking band was used, the CMR was typically around 2-3 dB. The CMR increased to 5-6 dB if an additional flanking band was added. The effect of the additional band was similar whether it was in the same ear as the original band or in the opposite ear. At the lowest signal frequency, a large CMR was observed in addition to a BMLD and vice versa. At the highest signal frequency, the extra release from masking was small. The results are interpreted in terms of the cues producing the CMR and the BMLD.

Comodulation masking release as a function of type of signal, gated or continuous masking, monaural or dichotic presentation of flanking bands, and center frequency

Thresholds were measured for detecting a signal centered in a narrow-band noise (NBN) masker (on-frequency band, OFB), for the OFB alone, and with two flanking bands (FBs) added to the OFB, one centered above and one below the OFB. The FBs were either correlated with the OFB or were independent and were presented either to the same ear as the signal plus OFB (monaural condition) or to the opposite ear (dichotic condition). The OFB and FBs were either gated with the signal, or were presented continuously. Three signal types were used: a pure tone; an NBN uncorrelated with the OFB; and an NBN correlated with the OFB. The signal was centered at 0.5, 2, or 6 kHz. Comodulation masking release was estimated either as the difference between threshold with the OFB alone and with the OFB plus correlated FBs [CMR(R-C)], or as the difference between thresholds using correlated and uncorrelated FBs [CMR(U-C)]. Although there were marked individual differences, positive CMR(R-C) values were found in all conditions for all three signal types. CMR(U-C) values were often larger than those for CMR(R-C), reflecting the fact that the uncorrelated FBs tended to produce interference effects, especially for the gated maskers, and at 6 kHz. Values of CMR were larger and more consistent across subjects for continuous than for gated maskers. For continuous maskers, the values of CMR tended to be smallest for the correlated-NBN signal. Results are discussed in terms of available cues and in terms of perceptual grouping mechanisms.

Comodulation masking release in subjects with unilateral and bilateral hearing impairment

Three subjects with unilateral cochlear hearing loss and three subjects with bilateral cochlear hearing loss were tested in three experiments. In the first, their auditory filter shapes were measured for center frequencies of 700 and 2000 Hz, using the notched-noise method. The auditory filters were generally broader for the impaired than for the normal ears. In experiment 2, the threshold for detecting a 2000-Hz signal centered in a band of noise was measured as a function of the noise bandwidth for a Gaussian noise, and for that same noise multiplied (modulated) by a second noise low-pass filtered at 12.5 Hz. For the Gaussian noise, thresholds increased up to a certain noise bandwidth and then flattened off. This bandwidth was usually greater for the impaired than for the normal ears, consistent with the broader auditory filters of the impaired ears. For the modulated noise, thresholds tended to decrease when the noise bandwidth was increased beyond a certain value, indicating comodulation masking release (CMR). The decrease occurred at wider bandwidths for the impaired than for the normal ears. For the unilaterally impaired subjects, the amount of decrease was smaller for the impaired than for the normal ears when tested at equal SPL, but not when tested at equal SL. In experiment 3, the threshold for detecting a 700-Hz signal centered in a 20-Hz-wide band of noise (the on-frequency band, ONB) was measured in the presence of eight flanking bands (FBs) whose envelopes were either identical with that of the ONB (correlated condition) or were uncorrelated. CMR was defined as the difference in threshold between the correlated and uncorrelated conditions. The ONB and the FBs were presented either to the same ear (monaural condition) or to opposite ears (dichotic condition). CMRs tended to be greatest at high levels of the ONB and the FBs. CMRs in the monaural condition were smaller for hearing-impaired than for normal ears. However, at high levels, CMRs in the dichotic condition were similar for normal, bilaterally impaired, and unilaterally impaired subjects. In the latter case, CMRs were similar when the ONB was presented to the normal ear and to the impaired ear of each subject.(ABSTRACT TRUNCATED AT 400 WORDS)

Comodulation masking release as a function of bandwidth and time delay between on‐frequency and flanking‐band maskers

The threshold for a signal masked by a narrow band of noise centered at the signal frequency (the on-frequency band) may be reduced by adding to the masker a second band of noise (the flanking band) whose envelope is correlated with that of the first band, an effect called comodulation masking release (CMR). This paper examines CMR as a function of masker bandwidth and time delay between the envelopes of the on-frequency and flanking bands. The 1.0-kHz sinusoidal signal had a duration of 400 ms. The on-frequency band was presented alone (reference condition) or with the flanking band. The flanking-band envelope was either correlated or uncorrelated with that of the on-frequency band. Flanking-band center frequencies ranged from 0.25-2.0 kHz. The flanking band was presented either in the same ear as the on-frequency band (monaural condition) or in the opposite ear (dichotic condition). The noise bands had bandwidths of 6.25, 25, or 100 Hz. In the correlated conditions, the flanking-band envelope was delayed with respect to that of the on-frequency band by 0, 5, 10, or 20 ms. For the 100-Hz bandwidth, CMRs were small (typically less than 1 dB) in both monaural and dichotic conditions at all delay times. For the 25-Hz bandwidth, CMRs were about 3.5 dB for the 0-ms delay, and decreased to about 1.5 dB for the 20-ms delay. For the 6.25-Hz bandwidth, CMRs averaged about 5 dB and were almost independent of delay time. The results suggest that the absolute delay time is not the critical variable determining CMR. The magnitude of CMR appears to depend on the correlation between the envelopes of the on-frequency and flanking bands. However, the results do not support a model of CMR that assumes that signal threshold corresponds to a constant change in across-band envelope correlation when the correlation is transformed to Fishers z.

Failure to obtain comodulation masking release with frequency‐modulated maskers

These experiments were intended to determine whether comodulation masking release (CMR) occurs for maskers that are modulated in frequency rather than in amplitude. In experiment I, thresholds for a sinusoidal signal were measured in the presence of two continuous sinusoidal maskers: one was centered at the signal frequency (1.0 kHz), and the other was positioned at flanking frequencies ranging from 0.5 to 2.0 kHz. The two maskers were frequency modulated (FM) by the same low-pass-noise modulator (correlated condition) or by independent noise modulators (uncorrelated condition). Thresholds were the same for the correlated and uncorrelated maskers, i.e., no CMR occurred. This was also true when the flanking band was presented in the ear opposite to that containing the signal and the on-frequency masking band. In experiment II, 25-Hz-wide noise maskers were used. The on-frequency band was sinusoidally frequency modulated, while the off-frequency band either had the same FM or no FM. Thresholds were similar for the two conditions, again indicating that no CMR occurred. The results suggest that, unlike amplitude modulation, correlated FM of the masker in different frequency bands does not give rise to a release from masking.

The continuum between comodulation masking release (CMR) and profile analysis (PA): Modulation depth

The existence of a continuum between CMR and PA was investigated by varying the modulation depth of five sinusoidal components. In the unmodulated condition (PA), a 2.0‐kHz tone was presented either along (reference), or with four flanking tones presented to the same or opposite ear. In the modulated conditions (CMR), each tone was sinusoidally amplitude modulated (SAM) from 20% to 100%. The signal was an increment to the 2.0‐kHz tone. The conditions were run with or without a random level variation of +10 dB about a median value of 50 dB per SAM tone. Randomizing level impaired performance in the reference condition at low modulation depths, and a masking release occurred with both monaural and dichotic presentation of the flanking SAM tones. At high modulation depths (60%–100%), randomizing level did not impair performance, probably because the signal produced a change in modulation depth that could be used as a cue. In this case, there was no masking release.

A comparison of comodulation masking release and profile analysis

Large reductions in signal thresholds may be observed under stimulus conditions in which there is common information in a number of auditory channels or critical bands. In comodulation masking release (CMR) tasks, a reduction in the threshold of a signal in a narrow‐band noise occurs when a flanking or cue band of noise is also present, provided that the flanking band has an amplitude envelope which is correlated with that of the masker band. This reduction in threshold occurs in dichotic conditions, where the flanking band is presented in the opposite ear to the signal‐plus‐masker. In profile analysis (PA), the overall level is randomized from one stimulus to the next. The threshold for detecting an increment in level of a single component is reduced when flanking components equal in amplitude to the pedestal are added. In general, this effect has not been found when the flanking components are presented dichotically. The present work examines the extent of the similarity, and divergence, between CMR and...

Comodulation masking release (CMR) as a function of signal frequency, flanking‐band frequency, masker bandwidth, and flanking‐band level

Thresholds for 400‐ms signals were measured in the presence of a continuous narrow‐band noise centered at signal frequencies (fs) ranging from 250–4000 Hz in 1‐oct steps. The masker was presented either alone or together with a second band of noise (the flanking band) whose envelope was either correlated with that of the on‐frequency band or was uncorrelated; its frequency ranged from 0.5 to 1.5 fs. CMR was defined as the difference between thresholds for the correlated and uncorrelated conditions. The CMR showed two components: a broadly tuned component occurring at all signal frequencies and all flanking‐band frequencies, and a component restricted to flanking‐band frequencies close to fs, which increased in magnitude with increasing fs. The second component was probably not a true CMR, but resulted from “beating” between the carrier frequencies of the two masker hands. Additional experiments, in which the bandwidth of the masker and the level of the flanking band were varied, support this interpretatio...