Biomedical subjects

C J Darwin

Publications and source records attributed to C J Darwin.

At least 19 recordsLinked to original sources

PubMed2005

Simultaneous grouping and auditory continuity.

Are the conditions for illusory auditory continuity entirely local in frequency, or are judgments of continuity made on auditory objects? Listeners made continuous/pulsating judgments on a variety of complex tones that repeatedly alternated with a 100- to 500-Hz bandpass noise. A sufficiently quiet complex tone was heard as continuous when all its harmonics fell within the frequency range of the noise. Adding harmonics outside the noise's frequency range substantially reduced the impression of continuity, which was largely restored when these additional components were given a different fundamental frequency. Judgments of auditory continuity thus appear to be based on entire simultaneously grouped objects, rather than being determined solely by local criteria based on individual frequency channels.

Auditory Perception↗

PubMed2004

Limits to the role of a common fundamental frequency in the fusion of two sounds with different spatial cues.

Two experiments establish constraints on the ability of a common fundamental frequency (F0) to perceptually fuse low-pass filtered and complementary high-pass filtered speech presented to different ears. In experiment 1 the filter cut-off is set at 1 kHz. When the filters are sharp, giving little overlap in frequency between the two sounds, listeners report hearing two sounds even when the sounds at the two ears are on the same F0. Shallower filters give more fusion. In experiment 2, the filters' cut-off frequency is varied together with their slope. Fusion becomes more frequent when the signals at the two ears share low-frequency components. This constraint mirrors the natural filtering by head-shadow of sound sources presented to one side. The mechanisms underlying perceptual fusion may thus be similar to those underlying auditory localization.

Acoustic Stimulation↗

PubMed2001

Formant-frequency matching between sounds with different bandwidths and on different fundamental frequencies.

The two experiments described here use a formant-matching task to investigate what abstract representations of sound are available to listeners. The first experiment examines how veridically and reliably listeners can adjust the formant frequency of a single-formant sound to match the timbre of a target single-formant sound that has a different bandwidth and either the same or a different fundamental frequency (F0). Comparison with previous results [Dissard and Darwin, J. Acoust. Soc. Am. 106, 960-969 (2000)] shows that (i) for sounds on the same F0, introducing a difference in bandwidth increases the variability of matches regardless of whether the harmonics close to the formant are resolved or unresolved; (ii) for sounds on different F0's, introducing a difference in bandwidth only increases variability for sounds that have unresolved harmonics close to the formant. The second experiment shows that match variability for sounds differing in F0, but with the same bandwidth and with resolved harmonics near the formant peak, is not influenced by the harmonic spacing or by the alignment of harmonics with the formant peak. Overall, these results indicate that match variability increases when the match cannot be made on the basis of the excitation pattern, but match variability does not appear to depend on whether ideal matching performance requires simply interpolation of a spectral envelope or also the extraction of the envelope's peak frequency.

Adult↗

PubMed2000

Extracting spectral envelopes: formant frequency matching between sounds on different and modulated fundamental frequencies.

The four experiments reported here measure listeners' accuracy and consistency in adjusting a formant frequency of one- or two-formant complex sounds to match the timbre of a target sound. By presenting the target and the adjustable sound on different fundamental frequencies, listeners are prevented from performing the task by comparing the absolute or relative levels of resolved spectral components. Experiment 1 uses two-formant vowellike sounds. When the two sounds have the same F0, the variability of matches (within-subject standard deviation) for either the first or the second formant is around 1%-3%, which is comparable to existing data on formant frequency discrimination thresholds. With a difference in F0, variability increases to around 8% for first-formant matches, but to only about 4% for second-formant matches. Experiment 2 uses sounds with a single formant at 1100 or 1200 Hz with both sounds on either low or high fundamental frequencies. The increase in variability produced by a difference in F0 is greater for high F0's (where the harmonics close to the formant peak are resolved) than it is for low F0's (where they are unresolved). Listeners also showed systematic errors in their mean matches to sounds with different high F0's. The direction of the systematic errors was towards the most intense harmonic. Experiments 3 and 4 showed that introduction of a vibratolike frequency modulation (FM) on F0 reduces the variability of matches, but does not reduce the systematic error. The experiments demonstrate, for the specific frequencies and FM used, that there is a perceptual cost to interpolating a spectral envelope across resolved harmonics.

Auditory Perception↗

PubMed2000

Effectiveness of spatial cues, prosody, and talker characteristics in selective attention.

The three experiments reported here compare the effectiveness of natural prosodic and vocal-tract size cues at overcoming spatial cues in selective attention. Listeners heard two simultaneous sentences and decided which of two simultaneous target words came from the attended sentence. Experiment 1 used sentences that had natural differences in pitch and in level caused by a change in the location of the main sentence stress. The sentences' pitch contours were moved apart or together in order to separate out effects due to pitch and those due to other prosodic factors such as intensity. Both pitch and the other prosodic factors had an influence on which target word was reported, but the effects were not strong enough to override the spatial difference produced by an interaural time difference of +/- 91 microseconds. In experiment 2, a large (+/- 15%) difference in apparent vocal-tract size between the speakers of the two sentences had an additional and strong effect, which, in conjunction with the original prosodic differences overrode an interaural time difference of +/- 181 microseconds. Experiment 3 showed that vocal-tract size differences of +/- 4% or less had no detectable effect. Overall, the results show that prosodic and vocal-tract size cues can override spatial cues in determining which target word belongs in an attended sentence.

Adult↗

PubMed2000

Effects of reverberation on spatial, prosodic, and vocal-tract size cues to selective attention.

Three experiments explored the resistance to simulated reverberation of various cues for selective attention. Listeners decided which of two simultaneous target words belonged to an attended rather than to a simultaneous unattended sentence. Attended and unattended sentences were spatially separated using interaural time differences (ITDs) of 0, +/-45, +/-91 or +/-181 micros. Experiment 1 used sentences resynthesized on a monotone, with sentence pairs having F0 differences of 0, 1, 2, or 4 semitones. Listeners' weak preference for the target word with the same monotonous F0 as the attended sentence was eliminated by reverberation. Experiment 1 also showed that listeners' ability to use ITD differences was seriously impaired by reverberation although some ability remained for the longest ITD tested. In experiment 2 the sentences were spoken with natural prosody, with sentence stress in different places in the attended and unattended sentences. The overall F0 of each sentence was shifted by a constant amount on a log scale to bring the F0 trajectories of the target words either closer together or further apart. These prosodic manipulations were generally more resistant to reverberation than were the ITD differences. In experiment 3, adding a large difference in vocal-tract size (+/- 15%) to the prosodic cues produced a high level of performance which was very resistant to reverberation. The experiments show that the natural prosody and vocal-tract size differences between talkers that were used retain their efficacy in helping selective attention under conditions of reverberation better than do interaural time differences.

Attention↗

PubMed1999

Auditory objects of attention: the role of interaural time differences.

The role of interaural time difference (ITD) in perceptual grouping and selective attention was explored in 3 experiments. Experiment 1 showed that listeners can use small differences in ITD between 2 sentences to say which of 2 short, constant target words was part of the attended sentence, in the absence of talker or fundamental frequency differences. Experiments 2 and 3 showed that listeners do not explicitly track components that share a common ITD. Their inability to segregate a harmonic from a target vowel by a difference in ITD was not substantially changed by the vowel being placed in a sentence context, where the sentence shared the same ITD as the rest of the vowel. The results indicate that in following a particular auditory sound source over time, listeners attend to perceived auditory objects at particular azimuthal positions rather than attend explicitly to those frequency components that share a common ITD.

Adult↗

PubMed1999

The integration of nonsimultaneous frequency components into a single virtual pitch.

The integration of nonsimultaneous frequency components into a single virtual pitch was investigated by using a pitch matching task in which a mistuned 4th harmonic (mistuned component) produced pitch shifts in a harmonic series (12 equal-amplitude harmonics of a 155-Hz F0). In experiment 1, the mistuned component could either be simultaneous, stop as the target started (pre-target component), or start as the target stopped (post-target component). Pitch shifts produced by the pre-target components were significantly smaller than those obtained with simultaneous components; in the post-target condition, the size of pitch shifts did not decrease relative to the simultaneous condition. In experiment 2, a silent gap of 20, 40, 80, or 160 ms was introduced between the nonsimultaneous components and the target sound. In the pre-target condition, pitch shifts were reduced to zero for silent gaps of 80 ms or longer; by contrast, a gap of 160 ms was required to eliminate pitch shifts in the post-target condition. The third experiment tested the hypothesis that, when post-target components were presented, the processing of the pitch of the target tone started at the onset of the target, and ended at the gap duration at which pitch shifts decreased to zero. This hypothesis was confirmed by the finding that pitch shifts could not be observed when the target tone had a duration of 410 ms. Taken together, the results of these experiments show that nonsimultaneous components that occur after the onset of the target sound make a larger contribution to the virtual pitch of the target, and over a longer period, than components that precede the onset of the target sound.

Humans↗

PubMed1998

Perceptual segregation of a harmonic from a vowel by interaural time difference in conjunction with mistuning and onset asynchrony.

The two experiments reported here examine how an inter-aural time difference (ITD) interacts with two other cues, mistuning and onset asynchrony, in reducing the contribution of a single frequency component to the perception of a vowel's identity. Previous experiments have shown that although ITD is generally rather ineffective at segregating a simultaneous harmonic frequency component from a vowel, it can produce some segregation when listeners have already been exposed to the isolated segregated component. A difference in ITD increases segregation overall in experiment 1 where the to-be-segregated component can also have a different onset time from the remainder of the vowel, and experiment 2 shows a similar result when the to-be-segregated component is mistuned. However, segregation by ITD is present just as strongly on trials when there is neither mistuning nor a difference in onset-time as on trials where these additional cues are present. Segregation on trials when there is neither mistuning nor a difference in onset-time is however larger in the present experiment which mixed all conditions together than in similar trials in an earlier experiment that had a blocked design [C.J. Darwin and R.W. Hukin, J. Acoust. Soc. Am. 102, 2316-2324 (1997)]. The results show that segregation by ITD increases when other more potent cues are present in the experiment.

Humans↗

PubMed1997

Perceptual segregation of a harmonic from a vowel by interaural time difference and frequency proximity.

The five experiments reported here examine the conditions under which sounds differing in their interaural time difference (ITD) are segregated for the purposes of perceiving a vowel's identity. Experiment 1 confirms previous findings that (i) a difference in ITD provides only a very weak cue for segregating a vowel's 500-Hz harmonic from the remainder of an isolated vowel; (ii) embedding the harmonic in a series of 500-Hz tones produces some segregation, which is enhanced if the harmonic and the tones differ in ITD from the rest of the vowel; and (iii) when these latter sounds are presented in the same block as isolated vowels, they facilitate segregation of the harmonic by ITD in the isolated vowels. The subsequent experiments show that this last effect, across-trial facilitation, is only produced by sounds which cue both the frequency and the ITD of the harmonic; either alone is insufficient. We also show that: (i) a single cue tone at the frequency of the harmonic is sufficient to facilitate the use of ITD in grouping; (ii) sequential organization by frequency proximity dominates over sequential organization by ITD when simultaneous sound sources are present; and (iii) the effectiveness of a cue tone can be abolished by capturing it into a synchronous harmonic complex. The experiments clarify the conditions under which ITDs contribute to the segregation of simultaneous sounds.

Auditory Threshold↗

PubMed1996

Lateralization of a perturbed harmonic: effects of onset asynchrony and mistuning.

The lateralization paradigm of Trahiotis and Stern [C. Trahiotis and R. M. Sern, J. Acoust. Soc. Am. 86, 1285-1293 (1989)] was extended to investigate the influence of a spectrally flanking complex on the lateral position of a perturbed harmonic. When a complex tone consisting of harmonics 2 through 8 or 100 Hz was presented with an interaural time difference (ITD) of 1.5 ms, the complex was heard on the leading side (experiment 1). However, when the 500-Hz component had a later onset time than the other components (experiments 1 and 2) or was mistuned (experiment 3), it was perceived to be in a different lateral position to the complex. The complex still maintained a residual influence on the lateralization of the pure tone even for the largest asynchrony used (experiment 4). Experiment 5 confirmed that the lateralizaiton of the tonal complex was consistent with the aggregation of binaural information across frequency. The results suggest that across-frequency integration of interaural-timing information is influenced by onset-asynchrony and harmonicity.

Adolescent↗

PubMed1995

Absence of effect of coherent frequency modulation on grouping a mistuned harmonic with a vowel.

When a single harmonic close to the first formant frequency is mistuned by about 8%, that harmonic makes a reduced contribution to the vowel's first formant frequency as measured by a shift in the phoneme boundary along an F1 continuum between /I/ and /epsilon/ [C.J. Darwin and R.B Gardner, J. Acoust. Soc. Am. 79, 838-45 (1986)]. In the present experiments, phoneme boundaries along an /I/-/epsilon/ continuum were measured for vowels differing in F1 whose fourth harmonic (500 Hz) was mistuned by 0, +/- 3, +/- 6, or +/- 9%. All the harmonics of a vowel (including the mistuned one) were given either no FM or coherent FM at a rate of 6 Hz and modulation depth of +/- 5%. The results replicated the previous findings, but found no evidence for coherent FM preventing the segregation of the mistuned harmonic from the vowel.

Humans↗

PubMed1995

Grouping in pitch perception: evidence for sequential constraints.

Evidence is presented that sequential auditory grouping constraints apply to the perception of pitch. Experiment 1 shows that the pitch changes produced by mistuning the fourth harmonic of a 90-ms 12-harmonic 155-Hz fundamental complex tone are substantially reduced when the complex is preceded by four 90-ms tones at the same frequency as the mistuned component. Both the pitch changes and their reduction by the tonal sequence precursor remain when the mistuned component and the precursor are presented contralateral to the remaining components. Experiment 2 shows that reducing the level of the same mistuned component reduces the size of the pitch change, but only if the mistuned component is presented ipsilaterally. To the extent that adaptation can be equated with a physical reduction in level, this result provides further evidence against peripheral adaptation playing a significant role in the auditory grouping of harmonics in pitch perception.

Functional Laterality↗

PubMed1995

Comparison of the effect of onset asynchrony on auditory grouping in pitch matching and vowel identification.

Previous experiments have shown that when a slightly mistuned harmonic of a complex tone starts more than about 80 msec before the remaining components, it makes a reduced contribution to the pitch of the complex. This contribution decreases to zero by about 300-msec onset asynchrony. In vowel perception, however, analogous experiments have shown that a much shorter asynchrony (around 40 msec) is enough to ensure that a component does not influence a vowel's phonemic category. The three experiments reported here demonstrate that this difference in the utility of onset time as a grouping cue does not arise because of differences in stimulus structure, but rather is due to the perceptual task. They show that the onset asynchrony needed in a pitch-matching experiment to remove the contribution that a mistuned component makes to the pitch of a vowel is the same as that needed to remove the contribution to the pitch of a flat-spectrum complex tone. They further show that a much smaller onset asynchrony is needed to perceptually remove the same harmonic from a vowel for the calculation of vowel quality. The implication of this result for models of auditory grouping is discussed.

Adult↗

PubMed1994

Perceptual and computational separation of simultaneous vowels: cues arising from low-frequency beating.

Identification of simultaneous speech sounds, such as pairs of steady-state vowels (double vowels), is more accurate when there is a difference in fundamental frequency (F0). Accuracy of identification for double vowels increases with increasing F0 difference (delta F0) asymptoting above 1 semitone. The experiment described here attempts to distinguish two mechanisms underlying this effect: first, perceptual separation by grouping together harmonic components of a common F0; and, second, exploitation of the fluctuations in the spectral envelope of the composite stimulus that result from beating between unresolved components. The beating is mainly caused by interactions between corresponding harmonics of the two vowels with a small delta F0. Identification accuracy for normal, harmonically excited double vowels was compared with that for double vowels composed from the same components, but whose constituent vowels were excited by a mixture of the two harmonic series. These double vowels were designed to produce similar beating patterns to the normal double vowels. Both harmonically and inharmonically excited constituents improved identification with increasing delta F0, but the increase was larger for harmonically excited vowels. A computational model based upon psychophysical measurements of auditory frequency and temporal resolution correctly predicted an increase in accuracy of identification with increasing delta F0 which was attributable to beating. The results are interpreted in terms of a spectral change cue in the identification of double vowels with delta F0's which complements grouping by F0, and which plays a dominant role for delta F0's smaller than 1 semitone.

Attention↗

PubMed1994

Effects of frequency and amplitude modulation on the pitch of a complex tone with a mistuned harmonic.

It has previously been found that when a single low-numbered harmonic of a complex tone is progressively mistuned, for mistunings up to about 3%, the pitch of the complex changes in the direction of the mistuning but for larger mistunings (by about 8%) the pitch returns to its original value. This result is compatible with the operation of a mechanism such as a graded harmonic sieve, which can reject from the calculation of pitch those frequency components that are implausibly distant from a harmonic frequency. The first experiment shows that the tolerance of such a sieve is increased when all the components of the complex tone (including the mistuned component) share a common pattern of frequency modulation at a rate of 6 Hz. The second experiment shows that the tolerance of the sieve is not increased when the components share a common pattern of amplitude modulation at 17 Hz. The third experiment replicates these findings and further shows that the increase in sieve tolerance for FM, but not for AM, occurs at both 6 and at 17 Hz.

Acoustic Stimulation↗

PubMed1993

Perceptual separation of simultaneous vowels: within and across-formant grouping by F0.

Six experiments explored why the identification of the two members of a pair of diotic, simultaneous, steady-state vowels improves with a difference in fundamental frequency (delta F0). Experiment 1 confirmed earlier reports that a delta F0 improves identification of 200-ms but not 50-ms duration "double vowels"; identification improves up to 1 semitone delta F0 and then asymptotes. In such stimuli, all the formants of a given vowel are excited by the same F0, providing listeners with a potential grouping cue. Subsequent experiments asked whether the improvement in identification with delta F0 for the longer vowels was due to listeners using the consistent F0 within each vowel of a pair to group formants appropriately. Individual vowels were synthesized with a different F0 in the region of the first formant peak from in the region of the higher formant peaks. Such vowels were then paired so that the first formant of one vowel bore the same F0 as the higher formants of the other vowel. These across-formant inconsistencies in F0 did not substantially reduce the previous improvement in identification rates with increasing delta F0's of up to 4 semitones (experiment 2). The subjects' improvement with increasing delta F0 in the inconsistent condition was not produced by identifying vowels on the basis of information in the first-formant or higher-formant regions alone, since stimuli which contained either of these regions in isolation were difficult for subjects to identify. In addition, the inconsistent condition did produce poorer identification for larger delta F0's (experiment 3). The improvement in identification with delta F0 found for the inconsistent stimuli persisted when the delta F0 between vowel pairs was confined to the first formant region (experiment 4) but not when it was confined to the higher formants (experiment 6). The results replicate at different overall presentation levels (experiment 5). The experiments show that at small delta F0's only the first-formant region contributes to improvements in identification accuracy, whereas with larger delta F0's the higher formant region may also contribute. This difference may be related to other results that demonstrate the superiority of resolved rather than unresolved harmonics in coding pitch.

Acoustic Stimulation↗

PubMed1993

Effects of onset asynchrony on pitch perception: adaptation or grouping?

A previous paper by Darwin and Ciocca [J. Acoust. Soc. Am. 91, 3381-3390 (1992)] showed that a slightly mistuned frequency component of a (target) harmonic complex produced smaller pitch shifts in the target if it started 160 ms or more before the other components than if all the components were simultaneous. Three experiments investigated whether this effect of onset asynchrony is due to peripheral adaptation to the leading portion of the mistuned component or to perceptual grouping. The first two experiments showed that the effect of asynchrony could be influenced by grouping mechanisms without changing the amount of adaptation produced by the leading portion of the mistuned component. In the first experiment, the effect of asynchrony was reduced by the presence of an additional (captor) complex which was harmonically related to the mistuned component and synchronous with just its leading portion. In experiment 2, the effect of asynchrony was increased by presenting a captor that was synchronous with the entire mistuned component. This capturing effect was independent of the harmonic relation between the captor and the mistuned component at 40-ms asynchrony; at 160 ms the effect of asynchrony increased further only if the captor and the mistuned component were harmonically related. In the third experiment, the expected amount of adaptation was increased (relative to that produced by a single sine precursor) by presenting several components that were close in frequency to the mistuned component and synchronous with its leading portion.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗