Some effects of later-occurring information on the perception of stop consonant and semivowel

In three experiments, we determined how perception of the syllable-initial distinction between the stop consonant [b] and the semivowel [w], when cued by duration of formant transitions, is affected by parts of the sound pattern that occur later in time. For the first experiment, we constructed four series of syllables, similar in that each had initial formant transitions ranging from one short enough for [ba] to one long enough for [wa], hut different in overall syllable duration. The consequence in perception was that, as syllable duration increased, the [b-w] boundary moved toward transitions of longer duration. Then, in the second experiment, we increased the duration of the sound by adding a second syllable, [da], (thus creating [bada-wada]), and observed that lengthening the second syllable also shifted the perceived [b-w] boundary in the first syllable toward transitions of longer duration; however, this effect was small by comparison with that produced when the first syllable was lengthened equivalently. In the third experiment, we found that altering the structure of the syllable had an effect that is not to be accounted for by the concomitant change in syllable duration: lengthening the syllable by adding syllable-final transitions appropriate for the stop consonant [d] (thus creating [bad-wad]) caused the perceived [b-w] boundary to shift toward transitions of shorter duration, an effect precisely opposite to that produced when the syllable was lengthened to the same extent by adding steady-state vowel. We suggest that, in all these cases, the later-occurring information specifies rate of articulation and that the effect on the earlier-occurring cue reflects an appropriate perceptual normalization.     

Information in speech: observations on the perception of [s]-stop clusters

A series of experiments is reported that investigated the pattern of acoustic information specifying place and manner of stop consonants in medial position after [s]. In both production and perception, information for stop place includes the spectrum of the fricative at offset, the duration of the silent closure interval, the spectral relationship between the frequency of the stop release burst and the following periodically excited formants, and the spectral and temporal characteristics of the first formant transition. Similarly, the information for stop manner includes the duration of silent closure, the frequency of the first formant at the release, the magnitude of the first formant transition, and the proximity of the second and third formants at release. A relationship was shown to exist in perception between the spectral characteristics of the first formant and the duration of the silent closure required to hear a stop. This appears to reciprocate the covariation of these parameters in production across different places of articulation and different vocalic contexts. The existence of perceptual sensitivity to a wide range of the acoustic consequences of production questions the efficacy of accounts of speech perception in terms of the fractionation of the signal into elemental acoustic cues, which are then integrated to yield a phonetic percept. It is argued that it is inappropriate to ascribe a psychological status to cues whose only reality is their operational role as physical parameters whose manipulation can change the phenotic interpretation of a signal. It is suggested that the metric of the information for phonetic perception cannot be that of the cues; rather, a metric should be sought in which acoustic and articulatory dynamics are isomorphic.     

Acoustic feature analysis in the perception of voicing contrasts

Various types of acoustic cues have been shown to signal voicing contrasts in initial prestressed stop consonants. Two of the most important of these cues are the time interval between the release burst and the onset of voicing (voice onset time) and the duration of voiced transitions. In the present experiment, a selective adaptation procedure was used to investigate possible feature extraction mechanisms which underlie the perception of voicing. Subjects first identified either of two series of test stimuli, [ba-p^ha) or [ga-k^ha]. Next they listened to repeated presentations of an adapting stimulus and then again identified the original test series. Adapting stimuli having the same value of voice onset time but different voiced transition durations produced differential (and sometimes opposite) effects on the identification of the test stimuli. In every case, the adapting stimulus with the greater duration of voiced transitions led to larger reduction in voiced responses or to a smaller increase in voiceless responses. These results are incompatible with models of voicing perception based strictly on detectors for voice onset time. The results also suggest that the adaptation effect occurs at a point in the system prior to the actual phonetic decision.     

Feature analyzers for the phonetic dimensionstop vs. continuant

Perception of sounds along the phonetic dimensionstop vs. continuant was studied by means of a selective adaptation procedure. Subjects first identified a series of synthetic consonant-vowel syllables whose formant transitions varied in duration, slope, and amplitude characteristics. They were perceived as either [ba] or [wa]. After the initial identification test, an adapting stimulus was presented repeatedly, and then the subjects again identified the original test series. Adapting with a stop (either [ba] or [da]) led to a decrease in the number of test stimuli identified as [ba], whereas adapting with the continuant sound [wa] led to an increase in the number of [ba] identification responses. Removing the vowel portion of an adapting stimulus greatly reduced the identification shift only when the resulting stimulus was no longer perceived as speech-like. A reduction in the number of [ba] identifications occurred even when a nonspeech “stop” (the sound of a plucked string) was used as the adapting stimulus, suggesting that phonetic processing is not a necessary condition for an adaptation effect.     

Formant transition duration and amplitude rise time as cues to the stop/glide distinction.

There is some disagreement in the literature about the relative contribution of formant transition duration and amplitude rise time in signalling the contrast between stops and glides. In this study, listeners identified sets of /ba/ and /wa/ stimuli in which transition duration and rise time varied orthogonally. Both variables affected labelling performance in the expected direction (i.e. the proportion of /b/ responses increased with shorter transition durations and shorter rise times). However, transition duration served as the primary cue to the stop/glide distinction, whereas rise time played a secondary, contrast-enhancing role. A qualitatively similar pattern of results was obtained when listeners made abrupt-onset/gradual-onset judgements of single sine-wave stimuli that modelled the rise times, frequency trajectories, and durations of the first formant in the /ba/-/wa/ stimuli. The similarities between the speech and non-speech conditions suggest that significant auditory commonalities underlie performance in the two cases.     

Vowel-contingent feature detection

Do vowel-contingent selective adaptation effects for place of articulation depend on vowel identity, or on the particular formant frequencies used? An experiment is reported here which tested the adaptation effects of consonants with exactly the same formant transitions before different diphthongs. In this experiment, the phonetic identity of the vowel and the formant frequencies of the consonant are not confounded as they have been in previous studies. In the contingent adaptation condition, no phoneme boundary shifts were observed, and this is interpreted along with previous evidencefor such phoneme boundary shifts when phonetic identity of the vowel and formant frequencies are confounded as indicating that adaptation operates on the spectral representation of the stimulus. Other evidence consistent with this conclusion is that adaptation with alternating adaptors from each end of the test series produced negligible shifts, and that a single adaptor from a diphthong series different from that of the test series produced phoneme boundary shifts as large as those with a single adaptor from the same test series.     

Auditory and phonetic processes in speech perception: Evidence from a dichotic study

The distinction between auditory and phonetic processes in speech perception was used in the design and analysis of an experiment. Earlier studies had shown that dichotically presented stop consonants are more often identified correctly when they share place of production (e.g., /ba-pa/) or voicing (e.g., /ba-da/) than when neither feature is shared (e.g., /ba-ta/). The present experiment was intended to determine whether the effect has an auditory or a phonetic basis. Increments in performance due to feature-sharing were compared for synthetic stop-vowel syllables in which formant transitions were the sole cues to place of production under two experimental conditions: (1) when the vowel was the same for both syllables in a dichotic pair, as in our earlier studies, and (2) when the vowels differed. Since the increment in performance due to sharing place was not diminished when vowels differed (i.e., when formant transitions did not coincide), it was concluded that the effect has a phonetic rather than an auditory basis. Right ear advantages were also measured and were found to interact with both place of production and vowel conditions. Taken together, the two sets of results suggest that inhibition of the ipsilateral signal in the perception of dichotically presented speech occurs during phonetic analysis.     

Processing of place information in stop consonants

One of the basic questior, s that models of speech perception must answer concerns the conditions under which various cues will be extracted from a stimulus and the nature of the mechanisms which mediate this process. Two selective adaptation experiments were carried out to explore this question for the phonetic feature of place of articulation in both syllableinitial and syllable-final positions. In the first experiment, CV and VC stimuli were constructed with complete overlap in their second- and third-formant transitions. Despite this essentially complete overlap, no adaptation effects were found for a VC adaptor and a CV test series (or vice versa). In the second experiment, various vowel, vowel-like, and VC-like adaptors were used. The VC-like adaptors did have a significant effect on the CV category boundary, while the vowel and vowel-like stimuli did not. These results are interpreted within both one- and twolevel models of selective adaptation. These models are distinguished by whether selective adaptation is assumed to affect a single auditory level of processing or to affect both an auditory level and a later phonetic level. However, both models incorporate detectors at the auditory level which respond whenever particular formant transitions are present. These auditory detectors are not sensitive to the position of the consonant transition information within the syllable.     

Perceptual integration and differentiation of spectral cues for intervocalic stop consonants

The place of articulation of intervocalic stop consonants is conveyed by temporally distributed spectral information, viz, the formant transitions preceding and following the silent closure interval (VC and CV transitions). Experiment 1 shows that more than 200 msec of silent closure is needed to hear VC and CV formant transitions as separate phonemic events (geminate stops). As closure duration is reduced, these cues are integrated into a single phonemic percept, and the VC transitions become increasingly redundant (Experiments 2 and 3). VC and CV transitions conveying different places of articulation, on the other hand, are heard as separate phonemes at closure durations as short as 100 msec. If closure duration is further reduced, a single stop is heard whose place of articulation corresponds to the CV transitions (Experiment 3). Even in the absence of CV transitions, VC transitions carry little perceptual weight at very short closure durations (Experiment 4). Despite their apparent redundancy, however, the VC transitions exert a positive bias on the perception of CV transitions at very short closure durations. At closure durations beyond 100 msec, on the other hand, VC and CV transitions interact contrastively in perception and tend to be heard as different phonemes (Experiments 5 and 6). The results of these experiments suggest two different processes of temporal integration in phonetic perception, one taking place at a precategorical level, the other combining identical phoneme categories within a certain time span.     

Some effects of acoustic attributes of speech on the processing of phonetic feature information

How do acoustic attributes of the speech signal contribute to feature-processing interactions that occur in phonetic classification? In a series of five experiments addressed to this question, listeners performed speeded classification tasks that explicitly required a phonetic decision for each response. Stimuli were natural consonant-vowel syllables differing by multiple phonetic features, although classification responses were based on a single target feature. In control tasks, no variations in nontarget features occurred, whereas in orthogonal tasks nonrelevant feature variations occurred but had to be ignored. Comparison of classification times demonstrated that feature information may either be processed separately as independent cues for each feature or as a single integral segment that jointly specifies several features. The observed form on processing depended on the acoustic manifestations of feature variation in the signal. Stop-consonant place of articulation and voicing cues, conveyed independently by the pattern and excitation source of the initial formant transitions, may be processed separately. However, information for consonant place of articulation and vowel quality, features that interactively affect the shape of initial formant transitions, are processed as an integral segment. Articulatory correlates of each type of processing are discussed in terms of the distinction between source features that vary discretely in speech production and resonance features that can change smoothly and continuously. Implications for perceptual models that include initial segmentation of an input utterance into a phonetic feature representation are also considered.     

The selective adaptation effects of burst-cued stops

One of the most compelling arguments that selective adaptation affects a phonetic level of processing is the demonstration that adaptation with burst-cued stimuli (which have no formant transitions) affects the perception of transition-cued stimuli (which have formant transitions but no bursts). Experiment 1 showed that adaptation with burst-cued [pi] and [ti] affects the perception of a (transition-cued) [mi-ni] test series, as well as a [bi-di] test series. Inasmuch as nasals never contain bursts, this demonstrates that the adaptation effect of burst-cued stops is not limited to those stimuli which normally contain bursts. If adaptation with burst-cued stops affects the perception of transition-cued stops at a more central, phonetic level of processing, their adaptation effect should transfer interaurally completely. Experiment 2 showed that the adaptation effect of both burst and transition-cued stops transfers interaurally only about one-third. These results suggest that the adaptation effect of burstcued stops is mediated by the fatigue of peripheral auditory detectors which are sensitive to both bursts and formant transitions.     

Rise time and formant transition duration in the discrimination of speech sounds: the Ba-Wa distinction in developmental dyslexia

Across languages, children with developmental dyslexia have a specific difficulty with the neural representation of the sound structure (phonological structure) of speech. One likely cause of their difficulties with phonology is a perceptual difficulty in auditory temporal processing (Tallal, 1980). Tallal (1980) proposed that basic auditory processing of brief, rapidly successive acoustic changes is compromised in dyslexia, thereby affecting phonetic discrimination (e.g. discriminating /b/ from /d/) via impaired discrimination of formant transitions (rapid acoustic changes in frequency and intensity). However, an alternative auditory temporal hypothesis is that the basic auditory processing of the slower amplitude modulation cues in speech is compromised (Goswami et al., 2002). Here, we contrast children's perception of a synthetic speech contrast (ba/wa) when it is based on the speed of the rate of change of frequency information (formant transition duration) versus the speed of the rate of change of amplitude modulation (rise time). We show that children with dyslexia have excellent phonetic discrimination based on formant transition duration, but poor phonetic discrimination based on envelope cues. The results explain why phonetic discrimination may be allophonic in developmental dyslexia (Serniclaes et al., 2004), and suggest new avenues for the remediation of developmental dyslexia.     

Auditory property detectors and processing place features in stop consonants

The effects of selective adaptation on the perception of consonant-vowel (CV) stimuli varying in place of production was studied under two conditions. In the first condition, repeated presentation of a CV syllable produced an adaptation effect resulting in a shift in the locus of the phonetic boundary between [ba] and [da]. This result replicated previously reported findings. However, in the second condition, an adaptation effect was obtained on this same test series when the critical acoustic information (i.e., formant transitions) was present in final position of a VC speech-like syllable. These latter results support an auditory account of selective adaptation based on the spectral similarity of the adapting stimuli and test series rather than a more abstract linguistic account based on phonetic identity.     

Some properties of linguistic feature detectors

A series of experiments, using a selective adaptation procedure, investigated some of the properties of the linguistic feature detectors that mediate the perception of the voiced and voiceless stop consonants. The first experiment showed that these detectors are centrally rather than peripherally located, in that monotic presentation of the adapting stimulus and test stimuli to different ears resulted in large and reliable shifts in the locus of the phonetic boundary. The second experiment revealed that the detectors are part of the specialized speech processor, inasmuch as adaptation of a voicing detector (as measured by a shift in the phonetic boundary) occurred only when the voicing information was presented in a speech context. In the third experiment, the detector mediating perception of the voiced stops was shown to be more resistant to adaptation than the detector mediating perception of the voiceless stops.     

Bidirectional contrast effects in the perception of VC-CV sequences

The perceived places of articulation of two successive stop consonants are not independent: Given some ambiguity in the formant transition cues and a closure duration between 100 and 200 msec, contrastive perceptual interactions in both directions have been observed in identification tasks. Retroactive contrast declines as the closure interval is lengthened and is strongly influenced by the range of closure durations employed, whereas proactive contrast appears to be less sensitive to these factors (Experiment 1). Reduced contrast and no effects of closure duration are obtained in a discrimination task with selective attention to one stimulus portion; this suggests that the effects in identification arise largely at a higher level of (phonetic) perception (Experiment 2). The contrast effects do not seem to represent a perceptual compensation for coarticulatory dependencies between stops produced in sequence, for there appears to be little coarticulation as far as place of articulation is concerned (Experiment 3). The most plausible hypothesis is that the presumed contrast effects do not result from any direct interaction of spectral cues across the closure interval but are due to perceptual information conveyed by the closure itself: Closure durations of 100–200 msec happen to be most appropriate for sequences of two nonhomorganic stops. Here, it seems, is another case in which listeners’ tacit knowledge of canonical speech patterns determines perception.     

Perceptual coherence of speech: stability of silence-cued stop consonants

A series of experiments was conducted to examine the perceptual stability of stop consonants cued by silence alone, as when [s] + silence + [laet] is perceived as splat. Following a replication of this perceptual integration phenomenon (Experiment 1), attempts were made to block it by instructing subjects to disregard the initial [s] and to focus instead on the onset of the following signal, which was varied from [plaet] to [laet]. However, these instructions had little effect at short silence durations (Experiment 2), and they reduced stop percepts for only 2 subjects at longer silence durations (Experiment 3). That is, subjects were generally unable to voluntarily dissociate the [s] noise from the following signal and thus to perceive the silent interval as silence rather than as a carrier of phonetic information. A low-uncertainty paradigm facilitated the task somewhat (Experiment 4). However, when the [s] frication was replaced with broadband noise (Experiment 5), listeners had no trouble at all in the selective-attention task, except at very short silence durations (less than 40 ms). This last finding suggests that, except for the shortest durations, the effect of silence on phonetic perception does not arise at the level of psychoacoustic stimulus interactions. Rather, the results support the hypothesis that perceptual integration of speech components, including silence, is a largely obligatory perceptual function driven by the listener's tacit knowledge of phonetic regularities.     

Acoustic cues and psychological processes in the perception of natural stop consonants

These studies examined the perceptual role of various components of naturally produced stop consonants (/b, d, g, p, t, k/) in CV syllables. In the first experiment, the context-sensitive voiced formant transitions were removed with a computer-splicing technique. Identification accuracy was 84% when the consonant was presented with the same vowel as had been used to produce it. Performance fell to 66% when the consonant was juxtaposed with a different vowel. The second experiment not only deleted the voiced formant transition, but also replaced the aspiration with silence. Here, identification accuracy dropped substantially, especially for voiceless stops, which had contained devoiced formant transitions in the replaced interval. The pattern of errors suggested that listeners try to extract the missing locus of the consonant from the vowel transition, and in the absence of a vowel transition, they try to extrapolate it from the second formant of the steady-state vowel.     

The relative weighting of acoustic properties in the perception of [s]+stop clusters by children and adults

We examined the perceptual weighting by children and adults of the acoustic properties specifying complete closure of the vocal tract following a syllable-initial [s]. Experiment 1 was a novel manipulation of previously examined acoustic properties (duration of a silent gap and first formant transition) and showed that children weight the first formant transition more than adults. Experiment 2, an acoustic analysis of naturally producedsay andstay, revealed that, contrary to expectations, a burst can be present instay and that first formant transitions do not necessarily distinguishsay andstay in natural tokens. Experiment 3 manipulated natural speech portions to create stimuli that varied primarily in the duration of the silent gap and in the presence or absence of a stop burst, and showed that children weight these stop bursts less than adults. Taken together, the perception experiments support claims that children integrate multiple acoustic properties as adults do, but that they weight dynamic properties of the signal more than adults and weight static properties less.     

The role of duration and rapid temporal processing on the lateral perception of consonants and vowels

This study explored the extent to which rapid temporal processing and duration contribute to the right-ear advantage (REA) and presumably left-hemisphere processing for stop consonants and the lack of clear-cut laterality effects for vowels. Three sets of synthetic stimuli were constructed: consonant vowel stimuli [ba da ga bi di gi bu du gu] of 300 msec duration (full stimuli) and two shortened stimuli consisting either of a noise burst and 40-msec transitions (40-msec stimuli), or a noise burst and 20-msec transitions (20-msec stimuli). Stimuli were presented dichotically for consonant, vowel, and syllable identification. Results indicated a significant REA for consonants in the full and 40-msec conditions and a non-significant REA in the 20-msec condition. Nevertheless, the magnitude of laterality did not change across the three conditions. These results suggest that although transition information including duration contributes to lateralization for stop consonants, it is the presence of abrupt onsets which crucially determines lateralized processing. For vowels, there was a significant REA only in the full stimulus condition, and a significant decrement in the magnitude of the laterality effect in the two shortened stimulus conditions. These results suggest that for vowel perception, it is the nature of the acoustic cue used for phonetic identification and not duration that seems to be the critical determinant of lateralization effects.     

Grouping frequency components of vowels: When is a harmonic not a harmonic?

When one harmonic of a vowel starts before and stops after the others, its contribution to the vowel's phonetic quality is reduced. Two experiments demonstrate that this reduction cannot be attributed entirely to adaptation. The first experiment shows that a harmonic that starts at the same time as a short vowel but continues after the vowel has ended contributes almost as little to the vowel's phonetic quality as a harmonic that starts before but stops at the same time as the vowel. The second experiment shows that the small contribution to vowel quality of a harmonic that starts before a vowel can be increased by adding an additional tone that will in turn form a perceptual group with that part of the harmonic preceding the vowel. The experiments demonstrate that some perceptual grouping operations are performed before the first formant of a vowel is estimated from the amplitudes of its component harmonics.