Учебники / Hearing - From Sensory Processing to Perception Kollmeier 2007

Fig. 1 Schematic representation of the effect of steady-state factor on masker waveforms. The bottom two waveforms show the possible envelope-phase configurations for the probe

masker were shaped with 20-ms cos2 r/f ramps. Probe and masker levels were 67 and 57 dB SPL, respectively, with overall levels held constant regardless of AM depth or waveshape.

2.2Results and Discussion

Probe modulation rate affected the pattern of results (Fig. 2). For 4-Hz probe SAM, there was an interaction between factors r/f and ssf with no MDI, relative unmodulated-masker thresholds, in three of the six conditions in which the ssf was either 0.0 or 1.0. Shailer and Moore (1993) suggested that, for complex modulators with steep envelope slopes, masker salience can affect MDI. To evaluate the relationship between envelope waveshape and perceived salience, listeners judged by triadic comparison the perceptual prominence of various masker waveforms. Multidimensional scaling of judgments of salience indicated correlation between salience and fundamental amplitude in the modulation spectrum for modulators with higher values of rms amplitude (see Fig. 3 for spectra). For lower-level modulators, gross envelope waveshape accounted for salience judgments.

With 10-Hz probe SAM, thresholds varied by only slightly more than 3 dB across the complex masker-modulation conditions. The relatively constant masking effect contrasts with either change in masker salience with modulator waveshape or energetic analysis in the modulation domain. Across the 10-Hz-masker conditions, masker power at the probe SAM rate varies by over 35 dB and masker-modulator rms amplitude varies by more than 15 dB. As with 10-Hz modulation, the 4-Hz thresholds do not follow trends indicated by energetic analysis.

Fig. 4 Mean thresholds averaged across six listeners for detecting 4-Hz (left panel) or 10-Hz (right panel) probe SAM as a function of masker ssf with parameter probe-modulator phase. The masker-modulator r/f time was 5 ms. Error bars are one s.e. of the mean threshold

At both probe modulation rates, probe modulator phase had no effect (Fig. 4). This result is not consistent with MDI based on cross-spectral envelope summation coupled with detection determined by a max/min rule.

3Experiment II

3.1Method

Probe and masker modulators were either continuous 8-Hz SAM, or complex waveforms termed “dropped-cycle” modulators in which only the evenor odd-numbered fluctuation cycles were present with the modulator remaining at dc during the time of the omitted cycles (Fig. 5). These modulator types are labeled as “all”, “odd”, or “even.” The task was to detect probe AM with each of the three modulator types. The 1.8-kHz probe was either presented alone or in the presence of a two-carrier (0.75and 4.5-kHz) masker complex with the masker AM index either 0.0 or 1.0. Across conditions, each of the three types of masker AM was paired with each pattern of probe modulation. The overall level of each carrier of the probe and masker was 60 dB SPL regardless of AM depth or pattern. In the initial condition set, the 500-ms probe and masker were synchronously gated, while in the second set, masker duration was increased to 1000-ms with probe onset delayed 500-ms from the masker onset. All probe and masker waveforms were shaped with 5-ms cos2 r/f ramps.

Fig. 5 On the left, schematic representation of a “dropped-cycle” condition in which the probe carrier is modulated by only the even cycles of an 8-Hz sinusoidal function while the masker is modulated by the odd cycles. In both cases, the modulator remains at dc during the time of the omitted fluctuation cycles. The right panel shows the amplitude spectrum of a “dropped-cycle” modulator with m equal to 1.0

3.2Results and Discussion

Results are shown in Table 1. With synchronous gating of the probe and masker carriers, masker modulation elevated thresholds in all conditions. In conditions with continuous probe modulation (probe-condition “all”), eliminating half the masker-modulation cycles (masker-condition “odd” or “even”) did not significantly reduce MDI relative to the conventional MDI stimulus configuration (masker-condition “all”). This absence of a release from interference when omitting masker-modulator cycles was obtained despite the roughly 7-dB drop in the masker-modulator amplitude spectrum in the vicinity of the 8-Hz probe SAM rate (i.e., when integrating levels of the 4-, 8-, and 12-Hz masker-modulator spectral components; see Fig. 5).

With synchronous gating and “dropped-cycle” probe modulation (probeconditions “odd” and “even”), results again indicated an exception to energetic masking in the modulation domain. When “dropped-cycle” modulators were used for both the probe and masker modulators, it did not matter whether the probe and masker fluctuations were concurrent or sequential. Working with frequency modulation (FM), Gockel et al. (2002) reported significant MDI with probe modulation only during a single temporal gap in masker FM. In the present work, across the six modulated-masker conditions in which the probe modulation was not continuous, mean thresholds ranged from −15.9 to −17.7 dB. In all synchronous-masker conditions, the same data trends were obtained when the rate of sinusoidal envelope fluctuation was increased from 8 to 16 Hz.

Table 1 Mean AM detection thresholds (dB) averaged across four subjects with both continuous and “dropped-cycle” 8-Hz AM. In the synchronous-masker conditions, the probe and masker carriers were gated on and off together; for the asynchronous masker, the masker onset preceded the probe onset by 500 ms with the probe and masker offsets coterminous. Each column is for a specific probe-modulation pattern, and each row a given masker characteristic; s.e.s of the mean thresholds are shown in parentheses. Separate subject groups ran in the synchronousand asynchronous-masker conditions

Results from additional conditions indicated that the absence of an effect of cross-spectral modulator concurrency cannot be accounted for by temporal or nonsimultaneous masking of AM detection (e.g., Wojtczak and Viemeister 2005). In these additional conditions, the carrier(s) were gated off rather than remaining at dc during the time of omitted modulation cycles. AM adaptation should persist through the silent intervals of pulsed carriers. MDI, however, was generally not obtained with modulation of the pulsed carriers.

A second consideration concerns potential “ringing” of the AM filters of a modulation filterbank (Sek and Moore 2002). Figure 6 illustrates the effect of filtering on modulator waveforms. Though filter output shows oscillation during the times the modulator is at dc, this low-level effect is insufficient to account for the absence of effect of modulation concurrency in the MDI conditions.

In the asynchronous-masker conditions, there was a 500-ms delay of the probe-carrier onset. Results are shown in the bottom half of Table 1. Though the extent of MDI was greatly reduced by the gating asynchrony between the probe and masker carriers, data trends regarding the effects of variation in masker-modulator spectrum and cross-spectral concurrency of modulation are the same as obtained in the synchronous-masker conditions.

Fig. 6 Illustrations of the effect of filtering on ac-coupled modulators with the left column showing the input waveforms and the right column the filter outputs. Modulators were passed through an FIR filter centered at 8 Hz with a Q of 1. Filtering raises the correlation between “even” and “odd” modulators from 0.0 to 0.27. However, filter “ringing” introduces less than a 0.5-dB power increment during the “dropped” cycles in auditory simulations

4Discussion and Summary

Results from experiment I showed a significant effect of modulator waveshape only at 4 Hz, and not at 10 Hz. The 4-Hz effect in part relates to variation in masker salience, with salience presumably having some relationship to auditory attention. In experiment II, there was no effect of envelope-fluctuation concurrency between the probe and masker. It is difficult to imagine a scheme for MDI based on grouping by common modulation in which concurrency would not come into play. In both experiments, significant departures from energetic masking in the modulation domain were obtained. As an alternative to basing MDI on auditory grouping of the probe and masker, the nonenergetic results suggest involvement of informational masking. Stimulus uncertainty is often associated with informational masking. For the MDI task, the requisite

uncertainty may be the difficulty in associating near-threshold modulation with its appropriate carrier when several are present (Hall and Grose 1991). That is, this uncertainty makes it difficult for the listener to attend to the probe. A basis in informational masking does not eliminate possible effect of perceptual segregation on MDI; segregation enhances structure which reduces the uncertainty underlying the masking effect.

Auditory information processing can be divided into the general areas of sound-source determination or scene analysis, and information extraction. At best moderate, and at times absent, effects have been observed directly linking low-rate AM to source determination (see Sheft 2007). Regarding the second area, signal variation, or modulation, is the basis of transmitted information. We believe that this aspect of information processing better represents auditory processing of AM. It is likely that MDI is due to difficulty in attending to the relevant information in complex modulated signals, and it is within this context that MDI may represent a form of informational masking.

Acknowledgments. This work was supported by NIDCD R01 Grant Nos. DC005423 and DC006250.

References

Gockel H, Carlyon RP, Deeks JM (2002) Effects of modulator asynchrony of sinusoidal and noise modulators on frequency and amplitude modulation detection interference. J Acoust Soc Am 112:2975–2984

Hall JW, Grose JH (1991) Some effects of auditory grouping factors on modulation detection interference (MDI). J Acoust Soc Am 90:3028–3035

Sek A, Moore BCJ (2002) Mechanisms of modulation gap detection. J Acoust Soc Am 111: 2783–2792

Shailer MJ, Moore BCJ (1993) Effects of modulation rate and rate of envelope change on modulation discrimination interference. J Acoust Soc Am 94:3138–3143

Sheft S (2007) Envelope processing and sound-source perception. In: Yost WA, Fay RR, Popper AN (eds) Auditory perception of sound sources. Springer, Berlin Heidelberg New York

Watson CS (2005) Some comments on informational masking. Acta Acust 91:502–512 Wojtczak M, Viemeister NF (2005) Forward masking of amplitude modulation: basic charac-

teristics. J Acoust Soc Am 118:3198–3210

Comment by Ewert

You’ve shown that energetic masking at the (fundamental) modulation frequency cannot explain your results. Is it possible to rule out energetic masking in the envelope domain if you would use a smoothed spectral representation of the envelope, assuming processing in relatively broadly tuned band-pass modulation filters?

Reply

Use of a modulation filter does not improve the ability of the envelope amplitude spectrum (‘energetic masking’) to account for MDI. For the 10-Hz conditions in which the extent of MDI was relatively constant, the rms amplitude of the output of a 10-Hz modulation filter significantly varies across conditions in response to the masker. Due to the sluggish response of a lowCF modulation filter, the decibel range of the ac-coupled output values is nearly as great as the change in fundamental amplitude in the modulation spectrum, this despite the limited integration across spectral components by the filter. In the 4-Hz conditions, the correspondence between modulation-filter output and MDI is generally no better than that obtained with consideration of the masker-modulator spectrum. In both cases, the largest mismatch is with a steady-state factor of either 0.0 or 1.0.