Effects of masker component phase on the forward masking produced by complex tones in normally hearing and hearing-impaired subjects
Introduction
The power-spectrum model of masking is based on the assumption that the threshold for detecting a signal in simultaneous masking depends on the masker power at the output of the auditory filter giving the highest signal-to-masker ratio (Fletcher, 1940; Patterson and Moore, 1986); the relative phase of the masker components is assumed to have no effect. However, experimental evidence accrued over the past 20 years clearly contradicts this assumption. This has been shown by using as maskers harmonic tone complexes with identical power spectra but with the components added with different starting phases (Alcántara et al., 2003; Carlyon and Datta, 1997; Gockel et al., 2002; Kohlrausch and Sander, 1995; Lentz and Leek, 2001; Mehrgardt and Schroeder, 1983; Oxenham and Dau, 2001; Smith et al., 1986; Summers and Leek, 1998). Many of these studies have used “Schroeder-phase” complexes, so called because component phases are chosen using equations proposed by Schroeder (1970). We will denote a complex with a positive phase curvature, equivalent to a repeated downward frequency sweep, as “Schroeder positive” (S+). A complex with negative phase curvature (upward frequency sweep), will be denoted “Schroeder negative” (S–). The S– complex can produce up to 20 dB more simultaneous masking than the S+ complex (Carlyon and Datta, 1997; Kohlrausch and Sander, 1995; Oxenham and Dau, 2001; Smith et al., 1986; Summers and Leek, 1998). Also, a complex with components added in cosine phase (all components with 90° starting phase) produces less masking than a complex with components added in random phase (Alcántara et al., 2003; Gockel et al., 2003a). These effects have been attributed to differences in the “peakiness” of the waveforms evoked on the basilar membrane in the different phase conditions; S+ and cosine-phase stimuli produce very “peaky” waveforms, whereas S– and random-phase stimuli produce waveforms with low peak factors (Recio and Rhode, 2000; Rhode and Recio, 2001; Summers et al., 2003).
Effects of masker component phase have also been demonstrated in forward masking. Generally, sounds evoking peaky waveforms on the basilar membrane (S+ and cosine-phase complexes) produce less forward masking than sounds that evoke less peaky waveforms (S– and random-phase complexes) (Carlyon and Datta, 1997; Gockel et al., 2003b). The threshold of a sinusoidal signal in forward masking is often assumed to be monotonically related to the excitation evoked by the masker in the frequency region of the signal, averaged over a period of several tens of milliseconds, which will be called “average excitation” (Carlyon and Datta, 1997; Houtgast, 1974; Moore, 2003; Moore and Glasberg, 1983; Plack and Oxenham, 1998). This basic assumption seems reasonable regardless of the mechanism responsible for forward masking, for example, whether it depends on persistence of the response evoked by the masker at some level in the auditory system, or on adaptation (Oxenham, 2001). Since masker component phase affects forward masking, this implies that the average excitation evoked by the masker changes with component phase. Carlyon and Datta (1997) suggested that fast-acting compression in the cochlea (Recio et al., 1998; Robles et al., 1986; Ruggero, 1992; Ruggero et al., 1997) was primarily responsible for the effects of masker component phase on forward masking, although they also considered the possibility that “compression in neural sites may have had some additional effect.” They argued that fast-acting compression would result in lower average excitation for a peaky waveform than for a less peaky waveform.
Gockel et al. (2003b) measured growth-of-masking functions in forward masking for cosine-phase and random-phase harmonic complex maskers that were filtered so as to contain only high unresolved harmonics. They found that, for signal frequencies well within the passband of the maskers, the level of the cosine-phase masker needed to be as much as 35 dB higher than the level of the random-phase masker to produce an equal amount of masking. They argued that this effect was too large to be explained solely in terms of fast-acting basilar-membrane compression.
Gockel et al. (2003b) suggested that suppression may have an influence on the forward masking produced by cosine-phase maskers. A cosine-phase masker evokes a highly peaky waveform on the basilar membrane, and the peaks are approximately synchronous at different places on the basilar membrane. For example, in response to a cosine-phase masker with a low fundamental frequency, the largest peaks in the response occur within a 1-ms time interval within each period for characteristic frequencies within a 5-ERBN range around 1 kHz (Dau et al., 2000; Uppenkamp et al., 2001), where ERBN stands for the average equivalent rectangular bandwidth of the auditory filter determined at moderate sound levels for young normally hearing listeners (Glasberg and Moore, 1990; Moore, 2003). As a result, the peaks at places tuned away from the signal frequency may suppress peaks at the place tuned to the signal frequency, and this may reduce the forward masking produced by the peaks. Also, as the masker dips would be roughly synchronous across different places, suppression would be weak during the dips, as the strength of suppression depends partly on overall level (Javel, 1981; Recio and Rhode, 2000; Sellick et al., 1982). If suppression is at its strongest when the masker itself is at its highest short-term level, and is weakest when the masker is at its lowest short-term level, this could produce a strong reduction of the effective level of a peaky masker, and thus contribute to differences in effectiveness of cosine-phase and random-phase complexes as forward maskers.
Cochlear hearing loss is often associated with impaired function of outer hair cells, which leads to reduced effectiveness of the “active mechanism” within the cochlea (Moore, 1995; Ruggero and Rich, 1991). This in turn results in both a loss of compression on the basilar membrane (Moore, 1995; Moore et al., 1999; Oxenham and Plack, 1997; Ruggero and Rich, 1991) and a loss of suppression (Leshowitz and Lindstrom, 1977; Mills and Schmeidt, 1983; Moore and Glasberg, 1986; Penner, 1980; Ruggero et al., 1996; Wightman et al., 1977). Most of the above studies suggest that hearing losses of 50–60 dB are usually sufficient to eliminate compression and suppression completely. In this study, we measured the influence of masker component phase on forward masking using both normally hearing subjects and subjects with moderate cochlear hearing loss. If the effects of masker component phase on forward masking depend solely on compression and suppression, then no phase effects should be observed for hearing-impaired subjects at signal frequencies where the hearing loss is greater than about 50–60 dB. However, if other factors are involved, a phase effect may still be observed.
Section snippets
Subjects
There were two groups of subjects, and each subject was tested using one ear only. The first group was composed of five normally hearing subjects. Three (AK, ET and JT) were aged 21–22 years, and the other two (BG and MA) were aged 59 and 51 years, respectively. All of these subjects had absolute thresholds better than 20 dB HL at the standard audiometric frequencies. The second group was composed of five subjects with moderate cochlear hearing loss. Audiograms for the test ears of the
Results
Initially, separate four-way analyses of variance (ANOVAs) were conducted for each subject, both on the masked thresholds, and with thresholds expressed as amount of masking. The factors were masker level, masker phase, signal frequency, and masker offset point. In no case was the effect of masker offset point significant. This is consistent with previous results (Carlyon and Datta, 1997; Gockel et al., 2003b) and it indicates that forward masking is determined by the average effect of the
Effects of masker phase
If the effects of masker component phase on forward masking depended solely on a combination of cochlear compression and suppression, then no effects of phase would be expected for subjects with cochlear hearing loss at signal frequencies where the loss exceeds about 50–60 dB. The phase effects for the hearing-impaired subjects were markedly smaller than those for the normally hearing subjects. This is consistent with the idea that peripheral compression and suppression make a substantial
Testing the role of the efferent system
To test the possible role of the efferent system, we explored the effect of manipulating the duration of the signal and the masker-signal interval. If the phase effect depends on the efferent system having a different effect on the masker and the signal, the phase effect should decrease if the signal duration is made very short, and might increase if the signal duration and/or the masker-signal interval is made longer. The following conditions were tested: signal with 5-ms onset/offset ramps
Summary and conclusions
For normally hearing subjects, we found that complex tones with components added in cosine phase produced less forward masking and led to shallower growth-of-masking functions than complex tones with components added in random phase. This is consistent with previous work (Carlyon and Datta, 1997; Gockel et al., 2003b). To test the role of peripheral compression and suppression in producing the phase effect, we used the same complex tones to measure growth-of-masking functions in forward
Acknowledgments
This work was supported by the Medical Research Council (UK). We thank José Alcántara, Hedwig Gockel, Fred Wightman and one anonymous reviewer for helpful comments on an earlier version of this paper.
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