Integration of spectral and temporal cues in discrimination of nonspeech sounds: a psychoacoustic analysis

This study presents a psychoacoustic analysis of the integration of spectral and temporal cues in the discrimination of simple nonspeech sounds. The experimental task was a same-different discrimination between a standard and a comparison pair of tones. Each pair consists of two 80-ms, 1500-Hz tone bursts separated by a 60-ms interval. The just-discriminable (d' = 2.0) increment in duration delta t, of one of the bursts was measured as a function of increments in the frequency delta f, of the other burst. A trade off between the values of delta t and delta f required to perform at d' = 2.0 was observed, which suggests that listeners integrate the evidence from the two dimensions. Integration occurred with both sub- and supra-threshold values of delta t or delta f, regardless of the order in which the cues were presented. The performance associated to the integration of cues was found to be determined by the discriminability of delta t plus that of delta f, and thus, it is within the psychophysical limits of auditory processing. To a first approximation the results agreed with the prediction of orthogonal vector summation of evidence stemming from signal detection theory. It is proposed that the ability to integrate spectral and temporal cues is in the repertoire of auditory processing capabilities. This integration does not appear to depend on perceiving sounds as members of phonetic classes.     

Temporal integration of trains of tone pulses by normal and by cochlearly impaired listeners

Two experiments investigated the temporal integration of trains of tone pulses by normal and by cochlearly impaired listeners. In the first experiment, thresholds were measured for a single 5-ms, 4-kHz tone pulse, and for ten such tone pulses as a function of interpulse interval (delta t). For normal listeners, temporal integration, defined as the threshold difference between one and ten pulses, was about 8 dB for delta t less than 20 ms, and about 5 dB at longer delta t's. For impaired listeners, temporal integration was only about 2-3 dB across the range of delta t's (5-160 ms) studied. A second experiment measured psychometric functions (log d' versus log signal power) for a single pulse and for ten pulses with delta t's of 5 ms and 80 ms. The normal listeners' functions had slopes close to unity in all three conditions, with a few exceptions. The impaired listeners' functions had slopes close to unity for ten pulses with delta t = 5 ms, but had slopes significantly greater than unity for delta t = 80 ms, and for a single pulse. At delta t = 80 ms, the increase in d' relative to the condition with a single tone was similar (a factor of square root of 10) for both impaired and normal listeners, but the threshold difference was smaller for the impaired listeners due to their steeper psychometric functions. For impaired listeners, then, temporal integration at delta t = 80 ms was normal in terms of a change in d' but abnormal when measured as a threshold difference.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of frequency and duration on psychometric functions for detection of increments and decrements in sinusoids in noise

Psychometric functions for detecting increments or decrements in level of sinusoidal pedestals were measured for increment and decrement durations of 5, 10, 20, 50, 100, and 200 ms and for frequencies of 250, 1000, and 4000 Hz. The sinusoids were presented in background noise intended to mask spectral splatter. A three-interval, three-alternative procedure was used. The results indicated that, for increments, the detectability index d' was approximately proportional to delta I/I. For decrements, d' was approximately proportional to delta L. The slopes of the psychometric functions increased (indicating better performance) with increasing frequency for both increments and decrements. For increments, the slopes increased with increasing increment duration up to 200 ms at 250 and 1000 Hz, but at 4000 Hz they increased only up to 50 ms. For decrements, the slopes increased for durations up to 50 ms, and then remained roughly constant, for all frequencies. For a center frequency of 250 Hz, the slopes of the psychometric functions for increment detection increased with duration more rapidly than predicted by a "multiple-looks" hypothesis, i.e., more rapidly than the square root of duration, for durations up to 50 ms. For center frequencies of 1000 and 4000 Hz, the slopes increased less rapidly than predicted by a multiple-looks hypothesis, for durations greater than about 20 ms. The slopes of the psychometric functions for decrement detection increased with decrement duration at a rate slightly greater than the square root of duration, for durations up to 50 ms, at all three frequencies. For greater durations, the increase in slope was less than proportional to the square root of duration. The results were analyzed using a model incorporating a simulated auditory filter, a compressive nonlinearity, a sliding temporal integrator, and a decision device based on a template mechanism. The model took into account the effects of both the external noise and an assumed internal noise. The model was able to account for the major features of the data for both increment and decrement detection.     

Auditory duration discrimination in the European starling (Sturnus vulgaris)

Auditory duration discrimination was studied in the European starling (Sturnus vulgaris, n = 3) using a GO/NOGO-procedure. Acoustic signals were presented by the method of constant stimuli. Duration discrimination limens (DDLs) were determined using signal detection theory (threshold criterion d' = 1.8). Weber fractions delta T/T = 0.23 for reference durations of between 800 and 100 ms, respectively. The DDLs for reference durations of 400 and 200 ms did not differ from those of 800 ms. There was no effect of tone frequency. Weber fractions delta T/T for a decrease in duration were independent of the reference durations and the frequencies tested (average delta T/T = 0.19). Duration discrimination is discussed with respect to data from other animals, and to hypotheses on the perception of signal duration.     

Detection of time- and bandlimited increments and decrements in a random-level noise.

The purpose of this study was to compare detection of increments and decrements occurring over limited regions of time and frequency within a 500-ms broadband (0-6000 Hz) noise. Three listeners tracked detection thresholds adaptively in a two-interval, two-alternative forced-choice task. Thresholds were measured for both increments and decrements in level [delta L = 10 log10(1 + delta N0/N0) dB, where N0 is the spectral power density of the noise] as a function of signal duration (T = 30-500 ms) for a range of signal bandwidths (W = 62-6000 Hz) that were logarithmically centered around 2500 Hz. Listeners were forced to rely on temporal- and spectral-profile cues for detection due to randomization of overall presentation level from interval to interval, which rendered overall energy an inconsistent cue. Increments were detectable for all combinations of W and T, whereas decrements were not consistently detectable for W < 500 Hz. Narrow-band decrements were not detectable due to spread of excitation from the spectral edges of the noise into the decrements. Increment and decrement thresholds were similar for W > or = 1000 Hz. Temporal- and spectral-integration effects were observed for both increments and decrements. The exceptions were for random-level conditions in which the signal matched the bandwidth or duration of the standard. A multicue decision process is described qualitatively to explain how the combination of temporal- and spectral-profile cues can produce temporal- and spectral-integration effects in the absence of overall-energy cues.     

Psychometric functions for pure-tone frequency discrimination

The form of the psychometric function (PF) for auditory frequency discrimination is of theoretical interest and practical importance. In this study, PFs for pure-tone frequency discrimination were measured for several standard frequencies (200-8000 Hz) and levels [35-85 dB sound pressure level (SPL)] in normal-hearing listeners. The proportion-correct data were fitted using a cumulative-Gaussian function of the sensitivity index, d', computed as a power transformation of the frequency difference, Δf. The exponent of the power function corresponded to the slope of the PF on log(d')-log(Δf) coordinates. The influence of attentional lapses on PF-slope estimates was investigated. When attentional lapses were not taken into account, the estimated PF slopes on log(d')-log(Δf) coordinates were found to be significantly lower than 1, suggesting a nonlinear relationship between d' and Δf. However, when lapse rate was included as a free parameter in the fits, PF slopes were found not to differ significantly from 1, consistent with a linear relationship between d' and Δf. This was the case across the wide ranges of frequencies and levels tested in this study. Therefore, spectral and temporal models of frequency discrimination must account for a linear relationship between d' and Δf across a wide range of frequencies and levels.     

Auditory temporal integration in the rhesus macaque (Macaca mulatta).

Temporal integration for pure tones was examined in two rhesus macaques. The subjects were required to respond to a brief sound (a tone burst) that deviated from a previous series of sounds (noise bursts) on a trial (a deviant-stimulus detection paradigm). Psychometric functions and thresholds were determined from correct detections (hit proportions) alone, and from d' scores. Two models describing the decline in threshold as a function of stimulus duration, one a power function the other an exponential, were tested against the data. When the decline (slope) in threshold per log stimulus duration is used as a rate measure, our results yield a lower estimate of temporal integration rate in rhesus than did a previous study [Clack, J. Acoust. Soc. Am. 40, 1140-1146 (1966)]. Both studies, however, gave slope estimates of integration rate that were higher than in most other species. Comparison of the models using data from several species, revealed that the exponential, but not the power model, could account for two sources of variation in threshold measurement. One source is due to the range across threshold as a function of duration (the linear rate component), and is described by the constant of proportionality Ik in the model. The other source of variation arises from the rate of decline within this range (the nonlinear rate component), and is described by the time constant tau. In terms of this model, differences in rate estimates between Clack's study and ours (and between rhesus and other species) are primarily due to the linear component. The nonlinear rate component was about equal for our study and Clack's (tau = approximately 150 ms): a time constant that is just slightly larger (indicating a rate of temporal integration slightly slower) than for most other species examined.     

Perceived continuity and pitch shifts for complex tones with unresolved harmonics

Brief complex tone bursts with fundamental frequencies (F0s) of 100, 125, 166.7, and 250 Hz were bandpass filtered between the 22nd and 30th harmonics, to produce waveforms with five regularly occurring envelope peaks ("pitch pulses") that evoked pitches associated with their repetition period. Two such tone bursts were presented sequentially and separated by a silent interval of two periods (2/F0). When the relative phases of the two bursts were varied, such that the interpulse interval (IPI) between the last pulse of the first burst and the first pulse of the second burst was varied, the pitch of the whole sequence was little affected. This is consistent with previous results suggesting that the pitch integration window may be "reset" by a discontinuity. However, when the interval between the two bursts was filled with a noise with the same spectral envelope as the complex, variations in IPI had substantial effects on the pitch of the sequence. It is suggested that the presence of the noise causes the two tones bursts to appear continuous, hence, resetting does not occur, and the pitch mechanism is sensitive to the phase discontinuity across the silent interval.     

Dual-electrode pitch discrimination with sequential interleaved stimulation by cochlear implant users

Cochlear implant users may perceive intermediate place-pitches between those elicited by the individual electrodes when two electrodes are stimulated simultaneously or sequentially. This study examined pitch discrimination between adjacent electrodes using sequential dual-electrode stimulation in terms of the sensitivity index, d', which was obtained by adding d's from intermediate dual-electrode stimuli. Loudness was balanced for each tested pair and the intensities were roved. Twelve ears with the Nucleus 24 or Freedom implants demonstrated a wide range of d', from 0.7 to 9.6. "Virtual channels" can be implemented through nonsimultaneous stimulation, with comparable pitch discrimination to that observed with simultaneous stimulation.     

The relation between the human frequency-following response and the low pitch of complex tones

The relation between the auditory brain stem potential called the frequency-following response (FFR) and the low pitch of complex tones was investigated. Eleven complex stimuli were synthesized such that frequency content varied but waveform envelope periodicity was constant. This was accomplished by repeatedly shifting the components of a harmonic complex tone upward in frequency by delta f of 20 Hz, producing a series of six-component inharmonic complex tones with constant intercomponent spacing of 200 Hz. Pitch-shift functions were derived from pitch matches for these stimuli to a comparison pure tone for each of four normal hearing adults with extensive musical training. The FFRs were recorded for the complex stimuli that were judged most divergent in pitch by each subject and for pure-tone signals that were judged equal in pitch to these complex stimuli. Spectral analyses suggested that the spectral content of the FFRs elicited by the complex stimuli did not vary consistently with component frequency or the first effect of pitch shift. Furthermore, complex and pure-tone signals judged equal in pitch did not elicit FFRs of similar spectral content.     

Discrimination of interval size in short tone sequences

This study investigates the discrimination of small changes of interval size in short sequences of musical tones. Major, minor and neutral thirds were varied in increments of 15 cents. The nine subjects had varying degrees of amateur musical experience-their level of musical training was lower than that of professional musicians. In some experiments the stimuli were presented purely melodically and in others they were presented together with a sustained tone at a higher pitch. Some subjects were able to make use of the additional cues from beats in the latter case. Category widths for identification were measured at around 70 cents and just-noticeable differences in frequency were measured at around 10 cents. Little significant variation of inter-stimulus sensitivity index d' was observed across the stimulus sets, i.e., there was little evidence for "anchors" or "landmarks" within the range of tunings employed. However, for major thirds, discrimination of the 15 cent increment between 400 and 415 cents was reduced compared to discrimination of other 15 cent increments within the stimulus sets.     

Differential responses to acoustic damage and furosemide in auditory brainstem and otoacoustic emission measures

Characteristics of distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs) were measured in Mongolian gerbil before and after the introduction of two different auditory dysfunctions: (1) acoustic damage with a high-intensity tone, or (2) furosemide intoxication. The goal was to find emission parameters and measures that best differentiated between the two dysfunctions, e.g., at a given ABR threshold elevation. Emission input-output or "growth" functions were used (frequencies f1 and f2, f2/f1 = 1.21) with equal levels, L1 = L2, and unequal levels, with L1 = L2 + 20 dB. The best parametric choice was found to be unequal stimulus levels, and the best measure was found to be the change in the emission threshold level, delta x. The emission threshold was defined as the stimulus level required to reach a criterion emission amplitude, in this case -10 dB SPL. (The next best measure was the change in emission amplitude at high stimulus levels, specifically that measured at L1 x L2 = 90 x 70 dB SPL.) For an ABR threshold shift of 20 dB or more, there was essentially no overlap in the emission threshold measures for the two conditions, sound damage or furosemide. The dividing line between the two distributions increased slowly with the change in ABR threshold, delta ABR, and was given by delta x(t) = 0.6 delta ABR + 8 dB. For a given delta ABR, if the shift in emission threshold was more than the calculated dividing line value, delta x(t), the auditory dysfunction was due to acoustic damage, if less, it was due to furosemide.     

An autocorrelation model with place dependence to account for the effect of harmonic number on fundamental frequency discrimination

Fundamental frequency (f0) difference limens (DLs) were measured as a function of f0 for sine- and random-phase harmonic complexes, bandpass filtered with 3-dB cutoff frequencies of 2.5 and 3.5 kHz (low region) or 5 and 7 kHz (high region), and presented at an average 15 dB sensation level (approximately 48 dB SPL) per component in a wideband background noise. Fundamental frequencies ranged from 50 to 300 Hz and 100 to 600 Hz in the low and high spectral regions, respectively. In each spectral region, f0 DLs improved dramatically with increasing f0 as approximately the tenth harmonic appeared in the passband. Generally, f0 DLs for complexes with similar harmonic numbers were similar in the two spectral regions. The dependence of f0 discrimination on harmonic number presents a significant challenge to autocorrelation (AC) models of pitch, in which predictions generally depend more on spectral region than harmonic number. A modification involving a "lag window"is proposed and tested, restricting the AC representation to a limited range of lags relative to each channel's characteristic frequency. This modified unitary pitch model was able to account for the dependence of f0 DLs on harmonic number, although this correct behavior was not based on peripheral harmonic resolvability.     

Perceptual organization of complex-tone sequences: a tradeoff between pitch and timbre?

Sequences of rapidly occurring sounds that differ from each other are often perceptually segregated into "streams" within which the range of differences is smaller [Bregman and Campbell, J. Exp. Psychol. 89, 244-249 (1971)]. Early research on streaming implied it to be pitch dominated, but Wessel [Comput. Music J. 3, 45-52 (1979)] demonstrated that timbre differences could also bring about segregation. In the present study, pitch and timbre attributes were put in competition in four-tone sequences of the form: T2P1-TmP1-T2Pn-TmPn, with the first pair assigned pitch P1 but different timbres T2 and Tm, and the second pair pitch Pn, and similarly contrasted timbres. Six listeners were asked to indicate whether perceived grouping of 49 such sequences was based on pitch proximity, timbre similarity, or ambiguous percepts not dominated by either cue. Results confirm that timbre can segregate sequences and imply that timbre and pitch compete in perceptually organizing complex sequences. Because timbre differences were provided by varying the locus of four equal-amplitude harmonics, and pitch differences were provided by varying their relative spacing, it is suggested that the tradeoffs observed may actually arise due to differences in perceived salience of "spectral pitch" and "virtual pitch" [Terhardt, J. Acoust. Soc. Am. 55, 1061-1069 (1974)] dependent on relative changes in spectral locus and spectral spacing over time.     

Asymmetry of masking between noise and iterated rippled noise: evidence for time-interval processing in the auditory system.

This study describes the masking asymmetry between noise and iterated rippled noise (IRN) as a function of spectral region and the IRN delay. Masking asymmetry refers to the fact that noise masks IRN much more effectively than IRN masks noise, even when the stimuli occupy the same spectral region. Detection thresholds for IRN masked by noise and for noise masked by IRN were measured with an adaptive two-alternative, forced choice (2AFC) procedure with signal level as the adaptive parameter. Masker level was randomly varied within a 10-dB range in order to reduce the salience of loudness as a cue for detection. The stimuli were filtered into frequency bands, 2.2-kHz wide, with lower cutoff frequencies ranging from 0.8 to 6.4 kHz. IRN was generated with 16 iterations and with varying delays. The reciprocal of the delay was 16, 32, 64, or 128 Hz. When the reciprocal of the IRN delay was within the pitch range, i.e., above 30 Hz, there was a substantial masking asymmetry between IRN and noise for all filter cutoff frequencies; threshold for IRN masked by noise was about 10 dB larger than threshold for noise masked by IRN. For the 16-Hz IRN, the masking asymmetry decreased progressively with increasing filter cutoff frequency, from about 9 dB for the lowest cutoff frequency to less than 1 dB for the highest cutoff frequency. This suggests that masking asymmetry may be determined by different cues for delays within and below the pitch range. The fact that masking asymmetry exists for conditions that combine very long IRN delays with very high filter cutoff frequencies means that it is unlikely that models based on the excitation patterns of the stimuli would be successful in explaining the threshold data. A range of time-domain models of auditory processing that focus on the time intervals in phase-locked neural activity patterns is reviewed. Most of these models were successful in accounting for the basic masking asymmetry between IRN and noise for conditions within the pitch range, and one of the models produced an exceptionally good fit to the data.     

Temporal discrimination for single components of nonspeech auditory patterns

This paper extends previous research on listeners' abilities to discriminate the details of brief tonal components occurring within sequential auditory patterns (Watson et al., 1975, 1976). Specifically, the ability to discriminate increments in the duration delta t of tonal components was examined. Stimuli consisted of sequences of ten sinusoidal tones: a 40-ms test tone to which delta t was added, plus nine context tones with individual durations fixed at 40 ms or varying between 20 and 140 ms. The level of stimulus uncertainty was varied from high (any of 20 test tones occurring in any of nine factorial contexts), through medium (any of 20 test tones occurring in ten contexts), to minimal levels (one test tone occurring in a single context). The ability to discriminate delta t depended strongly on the level of stimulus uncertainty, and on the listener's experience with the tonal context. Asymptotic thresholds under minimal uncertainty approached 4-6 ms, or 15% of the duration of the test tones; under high uncertainty, they approached 40 ms, or 10% of the total duration of the tonal sequence. Initial thresholds exhibited by inexperienced listeners are two-to-four times greater than the asymptotic thresholds achieved after considerable training (20,000-30,000 trials). Isochronous sequences, with context tones of uniform, 40-ms duration, yield lower thresholds than those with components of varying duration. The frequency and temporal position of the test tones had only minor effects on temporal discrimination. It is proposed that a major determinant of the ability to discriminate the duration of components of sequential patterns is the listener's knowledge about "what to listen for and where." Reduced stimulus uncertainty and extensive practice increase the precision of this knowledge, and result in high-resolution discrimination performance. Increased uncertainty, limited practice, or both, would allow only discrimination of gross changes in the temporal or spectral structure of the sequential patterns.     

Enhancement, adaptation, and the binaural system

In a test sound consisting of a burst of pink noise, an arbitrarily selected target frequency band can be "enhanced" by the previous presentation of a similar noise with a spectral notch in the target frequency region. As a result of the enhancement, the test sound evokes a pitch sensation corresponding to the pitch of the target band. Here, a pitch comparison task was used to assess enhancement. In the first experiment, a stronger enhancement effect was found when the test sound and its precursor had the same interaural time difference (ITD) than when they had opposite ITDs. Two subsequent experiments were concerned with the audibility of an instance of dichotic pitch in binaural test sounds preceded by precursors. They showed that it is possible to enhance a frequency region on the sole basis of ITD manipulations, using spectrally identical test sounds and precursors. However, the observed effects were small. A major goal of this study was to test the hypothesis that enhancement originates at least in part from neural adaptation processes taking place at a central level of the auditory system. The data failed to provide strong support for this hypothesis.     

Influence of spectral locus and F0 changes on the pitch and timbre of complex tones.

Harmonic complex tones comprising components in different spectral regions may differ considerably in timbre. While the pitch of "residue" tones of this type has been studied extensively, their timbral properties have received little attention. Discrimination of F0 for such tones is typically poorer than for complex tones with "corresponding" harmonics [A. Faulkner, J. Acoust. Soc. Am. 78, 1993-2004 (1985)]. The F0 DLs may be higher because timbre differences impair pitch discrimination. The present experiment explores effects of changes in spectral locus and F0 of harmonic complex tones on both pitch and timbre. Six normally hearing listeners indicated if the second tone of a two-tone sequence was: (1) same, (2) higher in pitch, (3) lower in pitch, (4) same in pitch but different in "something else," (5) higher in pitch and different in "something else," or (6) lower in pitch and different in "something else" than the first. ("Something else" is assumed to represent timbre.) The tones varied in spectral loci of four equal-amplitude harmonics m, m + 1, m + 2, and m + 3 (m = 1,2,3,4,5,6) and ranged in F0 from 200 to 200 +/- 2n Hz (n = 0,1,2,4,8,16,32). Results show that changes in F0 primarily affect pitch, and changes in spectral locus primarily affect timbre. However, a change in spectral locus can also influence pitch. The direction of locus change was reported as the direction of pitch change, despite no change in F0 or changes in F0 in the opposite direction for delta F0 < or = 0-2%. This implies that listeners may be attending to the "spectral pitch" of components, or to changes in a timbral attribute like "sharpness," which are construed as changes in overall pitch in the absence of strong F0 cues. For delta F0 > or = 2%, the direction of reported pitch change accord with the direction of F0 change, but the locus change continued to be reported as a timbre change. Rather than spectral-pitch matching of corresponding components, a context-dependent spectral evaluation process is thus implied in discernment of changes in pitch and timbre. Relative magnitudes of change in derived features of the spectrum such as harmonic number and F0, and absolute features such as spectral frequencies are compared. What is called "spectral pitch," contributes to the overall pitch, but also appears to be an important dimension of the multidimensional percept, timbre.     

Gap detection for similar and dissimilar gap markers

Detection thresholds for temporal gaps between markers of dissimilar frequency are usually elevated with respect to thresholds for gaps between markers of similar frequency. Because gaps between markers of dissimilar frequency represent both a spectrally based perceptual discontinuity as well as a temporal discontinuity, it is not clear what factors underlie the threshold elevation. This study sought to examine the effects of perceptual dissimilarities on gap detection. The first experiment measured gap detection for configurations of narrow-band gap markers comprised of pure tones, frequency-modulated tones, and amplitude-modulated tones. The results showed that gap thresholds for frequency-disparate pure-tone markers were elevated with respect to isofrequency tonal markers, but that perceptual discontinuities between markers restricted to the same frequency region did not uniformly elevate threshold. The second experiment measured gap detection for configurations of markers where the leading and trailing markers could differ along the dimensions of bandwidth, duration, and pitch. The results showed that, in most cases, gap detection deteriorated when the bandwidth of the two markers differed, even when the spectral content of the narrower-band marker was completely subsumed by the spectral content of the wider-band marker. This finding suggests that gap detection is sensitive to spectral dissimilarity between markers in addition to spectral discontinuity. The effects of marker duration depended on the marker bandwidth. Pitch differences across spectrally similar markers had no effect.     

Spectral integration in bands of modulated or unmodulated noise

Spectral integration was measured for pure-tone signals masked by unmodulated or modulated noise bands centered at the signal frequencies. The bands were typically 100 Hz wide, and when modulated, they were sinusoidally amplitude modulated at a rate of 8 Hz and a depth of 100%. In experiment 1, thresholds were first measured for each individual pure tone of a triplet in the presence of its respective masker band, and then for those three tones added together at their respective threshold levels, masked by their respective masker bands. Four sets of triplets were used: 250, 1000, 4000 Hz; 354, 1000, 2828 Hz; 500, 1000, 2000 Hz; and 800, 1000, 1200 Hz. When the masker bands were unmodulated, the amount of spectral integration was about 2.4 dB for all triplets, consistent with the integration expected based on the multiband energy detector model. When the bands were modulated, the amount of integration depended upon the spacing between masker bands; for the two widest spacings, the integration was between about 0 and 3 dB, whereas for the two closest spacings, the integration was approximately 5 dB. Experiments 2 and 3 addressed the cause of this greater spectral integration in the presence of the modulated masker bands with closer spacing. The second experiment demonstrated that sensitivity (d') was proportional to signal power regardless of whether the background noise was modulated or not, and thus the greater integration in dB in the presence of the modulated noise bands could not be accounted for by shallower psychometric functions in those conditions. Instead, the third experiment showed that the greater integration was likely due to the fact that the masker bands were comodulated. In other words, it was probably due to cues related to comodulation masking release when all three bands (and signals) were present.