Deborah A. Fantini

The processing of envelope information in comodulation masking release (CMR) and envelope discrimination

In comodulation masking release (CMR), thresholds for a signal masked by a narrow-band noise are reduced when additional noise is present. To demonstrate CMR, the additional noise must have similar amplitude envelope fluctuations over time as the primary noise band masking the signal. The specific source of information that provides the reduction in masked threshold remains unclear, although it is the focus of recent interest. The envelope of the on-frequency masking band (OFB) changes with the addition of the signal at threshold: in the present work, the importance of these changes in providing the CMR has been investigated. First, CMR thresholds were collected in sinusoidally amplitude-modulated noises which varied either in the phase relationship of their envelopes or in their modulation depth. Then envelope discrimination (ED) thresholds were collected for changes in envelope phase disparity (i.e., envelope correlation) and for changes in modulation depth per se. The patterns of CMR thresholds with envelope phase disparity and with modulation depth are not similar to those of the ED thresholds. A computer simulation was conducted in which the stimulus waveforms were processed through an auditory model which comprised bandpass auditory filters, a square-law nonlinearity, and a sliding temporal window. The envelopes were then extracted from the processed waveforms to determine whether the envelope changes that occur in the signal intervals in the CMR and ED tasks may be similar. The results of this analysis indicate that discriminability of envelope correlation due to the addition of the signal at threshold in the CMR task was insufficient to explain CMR. However, the discriminability of changes in modulation depth due to the addition of the signal is in agreement with thresholds obtained from the CMR task.

The relation between loudness and intensity difference limens for tones in quiet and noise backgrounds

Recent studies of the relation between loudness and intensity difference limens (DLs) suggest that, if two tones of the same frequency are equally loud, they will have equal relative DLs [R. S. Schlauch and C.C. Wier, J. Speech Hear. Res. 30, 13-20 (1987); J.J. Zwislocki and H.N. Jordan, J. Acoust. Soc. Am. 79, 772-780 (1986)]. To test this hypothesis, loudness matches and intensity DLs for a 1000-Hz pure tone in quiet and in a 40-dB SPL spectrum level broadband noise were obtained for four subjects with normal hearing. The DLs were obtained in both gated- and continuous-pedestal conditions. Contrary to previous reports, equally loud tones do not yield equal relative DLs at several midintensities in the gated condition and at many intensities in the continuous condition. While the equal-loudness, equal-relative-DL hypothesis is not supported by the data, the relation between loudness and intensity discrimination appears to be well described by a model reported by Houtsma et al. [J. Acoust. Soc. Am. 68, 807-813 (1980)].

Frequency discrimination of tones presented in filtered noise.

Previous research (Emmerich et al., 1983) in which tones were presented in the center of the notches in band-reject noise backgrounds suggests that information from frequency regions remote from the nominal signal frequency is useful in frequency discrimination. The present work extends the earlier findings by presenting tones on either side of a notch so that only one (or the other) tail of the excitation patterns of the tones would fall into the notch. In addition, tones were presented in high-pass noise, low-pass noise, and various combinations of the two. The results again indicate that remote information affects frequency discrimination, and they are also consistent with the hypothesis that the low-frequency tail of the excitation pattern is more useful for frequency discrimination than is the high-frequency tail.

Frequency discrimination and signal detection in band‐reject noise

An experiment was conducted in order to compare the importance of information from frequency regions remote from the nominal signal frequencies for frequency discrimination and signal detection. In both tasks, signals were presented within the ‘‘notch’’ of band‐reject noise, and different notch widths were employed. The results indicate that information is integrated over a wider range in frequency discrimination than in signal detection. Further, experiments in which a noise floor was present as well as band‐reject noise, indicate that disrupting the information from regions remote from the nominal signal frequencies impairs frequency discrimination even in the absence of any significant impairment of signal detection performance.

Edge effects on frequency discrimination of tones presented in low‐ and high‐pass noise backgrounds

Previous research has indicated that frequency discrimination performance is poorer for tones presented near the sharp spectral edge of a low-pass noise than for tones presented near the edge of a high-pass noise, or for tones in the same low-pass noise with high-pass noise added [Emmerich et al., J. Acoust. Soc. Am. 80, 1668-1672 (1986)]. The present study extends these findings in order to investigate how the steepness of the spectral edges of low- and high-pass maskers influences the discriminability of tones presented near these edges. Frequency discrimination was measured in each of three high- and low-pass noise backgrounds (which differed in the steepness of their filter skirts). The following results were obtained: (1) In the low-pass noise background, frequency discrimination performance improved as the filter skirt became more gradual; (2) in the high-pass noise background, performance first improved and then became poorer as the filter skirt became shallower; and (3) performance in low-pass noise was poorer than that in high-pass noise for the two steepest slopes employed (96 and 72 dB/oct) but not for the shallower slope (36 dB/oct). Results are discussed in the context of lateral suppression and edge pitch effects, and of a trade-off between possible edge effects and masking.

An investigation of the facilitation of simple auditory reaction time by predictable background stimuli

Two experiments explored a surprising result reported by Emmerich, Pitchford, and Becker (1976): Simple reaction time (RT) to an auditory stimulus can be facilitated by the presence of a tonal background (or masker). In the first experiment, simple RT to a tonal signal was investigated for a variety of background frequencies and loudness levels, and significant facilitation of RT was found for low levels of the background. In the second experiment, no evidence of facilitation was found when the background stimulus was a randomly varying narrow-band noise, although evidence for facilitation was again found with a constant tonal background.

Effect of flanker band number and excitation pattern symmetry on envelope comparisons in masking release

Greater masking release (MR) is observed when narrow-band noise flanker bands (FBs) are lower, rather than higher, than the signal frequency. This may be due to more effective signal coding on the high-frequency side of the excitation pattern (EP) [B. C. J. Moore and U. Jorasz, J. Acoust. Soc. Am. 100, 2373–2379 (1996)]. If so, masking of a high-frequency signal (asymmetric EP) should lead to a greater MR between FBs below and above the signal frequency than for a lower-frequency signal (symmetrical EP). Masked signal thresholds were compared for pure-tone signals (0.5, 1, or 2 kHz) masked by a narrow-band noise masker centered at the signal frequency [on-frequency band (OFB)] and two (three) FBs at 2-ERB intervals, below (above) the signal frequency. Masked signal thresholds were also measured in the absence of FBs. The OFBs and FBs were modulated at a modulation depth, m, of 0.7. FB envelopes were the same as each other and that of the OFB (comodulated, C), the same as each other but independent of OFB (incoherent, I), or not modulated (unmodulated, U). No clear relationship was found between signal EP symmetry and MR magnitude. MR is susceptible to the computation used, i.e., [(I+U)−C] or [I−C] as well as level cues.

Effects of flanking component spectral position and modulation pattern on thresholds for signals presented in the peaks of a modulated tonal masker

Detection of signals added solely to the peaks of an on-frequency modulated masker has never been found to improve after adding flanking components (FCs) at remote frequencies. However, according to theories underlying comodulation masking release (CMR), adding comodulated FCs could provide cues that improve signal detection for signal-peak placement. A reason masking release was not found with peak placement might be because of processes underlying modulation detection interference (MDI). This possibility was further investigated by using FCs that could diminish MDI but still provide signal detection cues associated with theories of CMR. It seemed that a peripheral within-channel and a central across-channel mechanism underlying MDI could hinder signal detection for signal peak placement.

Evidence for cross‐channel processing in detection of envelope phase disparity

The sensitivity to the correlation between the envelopes of two sinusoidally amplitude‐modulated sinusoids was investigated in two experiments. Correlation was manipulated by varying the phase disparity between the envelopes. In each experiment, two carriers (either 1 and 2 kHz or 1 and 3.2 kHz) were modulated at the same frequency (8 or 128 Hz) and added together either in phase or, on signal intervals, with an adaptively determined phase disparity. The starting phase of the envelope of the lower frequency carrier was randomized across trials. In the first experiment, the lower frequency carrier was presented at 65 dB SPL and the higher frequency carrier was presented at levels from 65 to 25 dB SPL. No decrement in threshold was noted until the higher frequency signal was 20–40 dB below the lower frequency signal. In the second experiment, the lower frequency carrier was presented at 65 dB SPL to one ear, and the higher frequency carrier was presented at the same level to the other ear. Thresholds were s...

Perceptual organization in loudness enhancement and decrement

Loudness enhancement is an increase in the loudness of a target tone that occurs when a more intense inducer tone either precedes or follows the target. Loudness decrement is a similar effect in the opposite direction. The present work investigates the role of perceptual organization in this paradigm by employing a capture tone presented simultaneously with the inducer. In a pair of experiments, the inducer, target, and comparison tone were always 1 kHz. The capture tone, when present, was either 2.0 or 0.5 kHz (an octave above or below the target). A 10‐ms inducer tone immediately preceded a 10‐ms target, with a 10‐ms comparison tone presented 850 ms later. Listeners were instructed to ignore the inducer and to judge whether the target or comparison tone was louder. Equal‐loudness matches were estimated using two randomly interleaved adaptive tracks, one converging on the just‐louder threshold and the other on the just‐quieter threshold. Results from both enhancement and decrement experiments indicate th...