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Biomedical subjects

C J Darwin

Publications and source records attributed to C J Darwin.

At least 19 recordsLinked to original sources
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↗
PubMed1993

The role of timbre in the segregation of simultaneous voices with intersecting F0 contours.

When the fundamental frequency (F0) contours of two speakers' voices intersect, the listener is presented with a problem. The listener must decide which of the F0 contours emerging from the intersection is a continuation of which contour entering the intersection: have the F0 contours crossed or merely approached and parted? In the present experiment, subjects listened to two simultaneous diphthong-like sounds with F0 contours that either approached and diverged or crossed over. The task was to report whether the pitches "crossed" or "bounced" away from each other. Despite the changing timbres of the two sounds, the subjects were able to discriminate crossing and bouncing F0s, provided that the timbres of the vowels differed at the moment when their F0s were the same. When the timbres were the same, the subjects could not make the discrimination and tended to hear a bouncing percept. These results are consistent with the idea that listeners use continuity of timbre rather than continuity of F0 movement to disambiguate F0 intersections.

Acoustic Stimulation↗
PubMed1992

Grouping in pitch perception: effects of onset asynchrony and ear of presentation of a mistuned component.

Three experiments investigated how the onset asynchrony and ear of presentation of a single mistuned frequency component influence its contribution to the pitch of an otherwise harmonic complex tone. Subjects matched the pitch of the target complex by adjusting the pitch of a second similar but strictly periodic complex tone. When the mistuned component (the 4th harmonic of a 155 Hz fundamental) started 160 ms or more before the remaining harmonics but stopped simultaneously with them, it made a reduced contribution to the pitch of the complex. It made no contribution if it started more than 300 ms before. Pitch shifts and their reduction with onset time were larger for short (90 ms) sounds than for long (410 ms). Pitch shifts were slightly larger when the mistuned component was presented to the same ear as the remaining 11 in-tune harmonics than to the opposite ear. Adding a "captor" complex tone with a fundamental of 200 Hz and a missing 3rd harmonic to the contralateral ear did not augment the effect of onset time, even though the captor was synchronous with the mistuned harmonic, the mistuned component was equal in frequency to the missing 3rd harmonic of the captor complex tone and it was played to the same ear as the captor. The results show that a difference in onset time can prevent a resolved frequency component from contributing to the pitch of a complex tone even though it is present throughout that complex tone.

Acoustic Stimulation↗
PubMed1991

Effects of phase changes in low-numbered harmonics on the internal representation of complex sounds.

A series of experiments investigated the effect of phase changes in low-numbered single harmonics in target sounds that were either synthesized steady-state vowels fo periodic signals having only a single formant. A matching procedure was sued in which subjects selected a sound along a continuum differing in first formant frequency in order to get the best match with the target sound; perceptual effects of the phase manipulations in the target were detected as a change in the matched first formant frequency. Stimuli had to contain at least three harmonics to produce the effect, but id did not require a particular starting phase of the components. A suppression phenomenon is discussed, in which phase changes alter the phase-locking characteristics of auditory fibres tuned to low-numbered harmonics.

Adult↗
PubMed1989

Assessment of feature size abnormalities using receiver operating characteristic analysis.

The ability of an observer to detect variations in size of a geometrical image feature have been investigated using receiver operating characteristic (ROC) analysis. Three types of image were constructed using computer graphics: disc-shaped targets of variable radius, model chest radiographs showing a variable heart diameter and model arterial angiograms with variable vessel width. Five factors were investigated: observer experience, variation of detectability with theoretical signal-to-noise ratio, the prior probability of the presence of an abnormality, viewing distance, and uncertainty in the location of an abnormality. In all but one experiment, excellent agreement was found between measured detectabilities and the predictions of signal detection theory, providing an initial practice session was included for each observer. No significant variation in detectability was found using six different prior probabilities and two different viewing distances, and the reduction in detectability for a four-alternative location task was in good agreement with theoretical predictions. The high statistical efficiencies found for the detection of geometrical signals suggest that the levels of observer "internal" noise arising from decision-making processes during an ROC experiment are very low.

Computer Graphics↗
PubMed1989

Vowel quality changes produced by surrounding tone sequences.

In three experiments, we examined whether energy at the same frequency as one of a vowel's harmonics in the F1 region can be captured by a preceding or following sequence of tones. The position of the /I/-/E/ phoneme boundary along an F1 continuum was used to assess the extent of capture. The first two experiments showed that a sequence of tones at 500 Hz (56-msec duration at 10/sec) can perceptually remove added energy at 500 Hz from a steady vowel (F0 = 125 Hz) that forms part of the sequence. The effect is detectable with one preceding tone, asymptotes with four, and is greater when two tones follow the vowel than when none do. Rising and falling sequences of tones (at 62.5-Hz intervals or at whole-tone intervals) differ in their effect. Falling sequences behave much like constant tones at 500 Hz but with less effect, whereas rising sequences show no evidence of removing the added tone. The second experiment replicated the first and also showed that when the vowel is embedded in a rising or a falling sequence of tones that continue after it, the following tones have no effect. The third experiment suggested that the different effects found with rising versus falling sequences are qualitatively predictable on the basis of the additive effects of their constituent tones rather than by virtue of their contour. The experiments indicated that sequences of repeating tones are much more effective at capturing a harmonic from a vowel than are sequences that follow a simple pattern. This result may reflect the operation of a principle of least commitment in auditory grouping.

Attention↗
PubMed1986

Mistuning a harmonic of a vowel: grouping and phase effects on vowel quality.

The harmonic sieve has been proposed as a mechanism for excluding extraneous frequency components from the estimate of the pitch of a complex sound. The experiments reported here examine whether a harmonic sieve could also determine whether a particular harmonic contributes to the phonetic quality of a vowel. Mistuning a harmonic in the first formant region of vowels from an /I/-/e/ continuum gave shifts in the phoneme boundary that could be explained by (i) phase effects for small amounts of mistuning and (ii) a harmonic sievelike grouping mechanism for larger amounts of mistuning. Similar grouping criteria to those suggested for pitch may operate for the determination of first formant frequency in voiced speech.

Humans↗
PubMed1986

The representation of steady-state vowel sounds in the temporal discharge patterns of the guinea pig cochlear nerve and primarylike cochlear nucleus neurons.

We have recorded the responses of fibers in the cochlear nerve and cells in the cochlear nucleus of the anesthetized guinea pig to synthetic vowels [i], [a], and [u] at 60 and 80 dB SPL. Histograms synchronized to the pitch period of the vowel were constructed, and locking of the discharge to individual harmonics was estimated from these by Fourier transformation. In cochlear nerve fibers from the guinea pig, the responses were similar in all respects to those previously described for the cat. In particular, the average-localized-synchronized-rate functions (ALSR), computed from pooled data, had well-defined peaks corresponding to the formant frequencies of the three vowels at both sound levels. Analysis of the components dominating the discharge could also be used to determine the voice pitch and the frequency of the first formants. We have computed similar population measures over a sample of primarylike cochlear nucleus neurons. In these primarylike cochlear nucleus cell responses, the locking to the higher-frequency formants of the vowels is weaker than in the nerve. This results in a severe degradation of the peaks in the ALSR function at the second and third formant frequencies at least for [i] and [u]. This result is somewhat surprising in light of the reports that primarylike cochlear nucleus cells phaselock, as well as do cochlear nerve fibers.

Animals↗