Coarticulation in sequences of two nonhomorganic stop consonants: perceptual and acoustic evidence

This study investigated whether any perceptually useful coarticulatory information is carried by the release burst of the first of two successive, nonhomorganic stop consonants. The CV portions of natural VCCV utterances were replaced with matched synthetic stimuli from a continuum spanning the three places of stop articulation. There was a sizable effect of coarticulatory cues in the natural-speech portion on the perception of the second stop consonant. Moreover, when the natural VC portions including the final release burst were presented in isolation, listeners were significantly better than chance at guessing the identity of the following, "missing" syllable-initial stop. The hypothesis that the release burst of a syllable-final stop contains significant coarticulatory information about the place of articulation of a following, nonhomorganic stop was further confirmed in acoustic analyses which revealed significant effects of CV context on the spectral properties of the release bursts. The relationship between acoustic stimulus properties and listeners' perceptual responses was not straightforward, however.     

Infant discrimination of two- and five-formant voiced stop consonants differing in place of articulation

According to recent theoretical accounts of place of articulation perception, global, invariant properties of the stop CV syllable onset spectrum serve as primary, innate cues to place of articulation, whereas contextually variable formant transitions constitute secondary, learned cues. By this view, one might expect that young infants would find the discrimination of place of articulation contrasts signaled by formant transition differences more difficult than those cued by gross spectral differences. Using an operant head-turning paradigm, we found that 6-month-old infants were able to discriminate two-formant stimuli contrasting in place of articulation as well as they did five-formant + burst stimuli. Apparently, neither the global properties of the onset spectrum nor simply the additional acoustic information contained in the five-formant + burst stimuli afford the infant any advantage in the discrimination task. Rather, formant transition information provides a sufficient basis for discriminating place of articulation differences.     

Perception of the [m]-[n] distinction in VC syllables

This study complements earlier experiments on the perception of the [m]-[n] distinction in CV syllables [B. H. Repp, J. Acoust. Soc. Am. 79, 1987-1999 (1986); B. H. Repp, J. Acoust. Soc. Am. 82, 1525-1538 (1987)]. Six talkers produced VC syllables consisting of [m] or [n] preceded by [i, a, u]. In listening experiments, these syllables were truncated from the beginning and/or from the end, or waveform portions surrounding the point of closure were replaced with noise, so as to map out the distribution of the place of articulation information for consonant perception. These manipulations revealed that the vocalic formant transitions alone conveyed about as much place of articulation information as did the nasal murmur alone, and both signal portions were about as informative in VC as in CV syllables. Nevertheless, full VC syllables were less accurately identified than full CV syllables, especially in female speech. The reason for this was hypothesized to be the relative absence of a salient spectral change between the vowel and the murmur in VC syllables. This hypothesis was supported by the relative ineffectiveness of two additional manipulations meant to disrupt the perception of relational spectral information (channel separation or temporal separation of vowel and murmur) and by subjects' poor identification scores for brief excerpts including the point of maximal spectral change. While, in CV syllables, the abrupt spectral change from the murmur to the vowel provides important additional place of articulation information, for VC syllables it seems as if the format transitions in the vowel and the murmur spectrum functioned as independent cues.     

Perception of the [m]-[n] distinction in consonant-vowel (CV) and vowel-consonant (VC) syllables produced by child and adult talkers

The contribution of the nasal murmur and vocalic formant transition to the perception of the [m]-[n] distinction by adult listeners was investigated for speakers of different ages in both consonant-vowel (CV) and vowel-consonant (VC) syllables. Three children in each of the speaker groups 3, 5, and 7 years old, and three adult females and three adult males produced CV and VC syllables consisting of either [m] or [n] and followed or preceded by [i ae u a], respectively. Two productions of each syllable were edited into seven murmur and transitions segments. Across speaker groups, a segment including the last 25 ms of the murmur and the first 25 ms of the vowel yielded higher perceptual identification of place of articulation than any other segment edited from the CV syllable. In contrast, the corresponding vowel+murmur segment in the VC syllable position improved nasal identification relative to other segment types for only the adult talkers. Overall, the CV syllable was perceptually more distinctive than the VC syllable, but this distinctiveness interacted with speaker group and stimulus duration. As predicted by previous studies and the current results of perceptual testing, acoustic analyses of adult syllable productions showed systematic differences between labial and alveolar places of articulation, but these differences were only marginally observed in the youngest children's speech. Also predicted by the current perceptual results, these acoustic properties differentiating place of articulation of nasal consonants were reliably different for CV syllables compared to VC syllables. A series of comparisons of perceptual data across speaker groups, segment types, and syllable shape provided strong support, in adult speakers, for the "discontinuity hypothesis" [K. N. Stevens, in Phonetic Linguistics: Essays in Honor of Peter Ladefoged, edited by V. A. Fromkin (Academic, London, 1985), pp. 243-255], according to which spectral discontinuities at acoustic boundaries provide critical cues to the perception of place of articulation. In child speakers, the perceptual support for the "discontinuity hypothesis" was weaker and the results indicative of developmental changes in speech production.     

Stimulus factors influencing the identification of voiced stop consonants by normal-hearing and hearing-impaired adults

The effects of mild-to-moderate hearing impairment on the perceptual importance of three acoustic correlates of stop consonant place of articulation were examined. Normal-hearing and hearing-impaired adults identified a stimulus set comprising all possible combinations of the levels of three factors: formant transition type (three levels), spectral tilt type (three levels), and abruptness of frequency change (two levels). The levels of these factors correspond to those appropriate for /b/, /d/, and /g/ in the /ae/ environment. Normal-hearing subjects responded primarily in accord with the place of articulation specified by the formant transitions. Hearing-impaired subjects showed less-than-normal reliance on formant transitions and greater-than-normal reliance on spectral tilt and abruptness of frequency change. These results suggest that hearing impairment affects the perceptual importance of cues to stop consonant identity, increasing the importance of information provided by both temporal characteristics and gross spectral shape and decreasing the importance of information provided by the formant transitions.     

A role for the second subglottal resonance in lexical access

Acoustic coupling between the vocal tract and the lower (subglottal) airway results in the introduction of pole-zero pairs corresponding to resonances of the uncoupled lower airway. If the second formant (F2) passes through the second subglottal resonance a discontinuity in amplitude occurs. This work explores the hypothesis that this F2 discontinuity affects how listeners perceive the distinctive feature [back] in transitions from a front vowel (high F2) to a labial stop (low F2). Two versions of the utterances "apter" and "up there" were synthesized with an F2 discontinuity at different locations in the initial VC transition. Subjects heard portions of the utterances with and without the discontinuity, and were asked to identify whether the utterances were real words or not. Results show that the frequency of the F2 discontinuity in an utterance influences the perception of backness in the vowel. Discontinuities of this sort are proposed to play a role in shaping vowel inventories in the world's languages [K. N. Stevens, J. Phonetics 17, 3-46 (1989)]. The results support a model of lexical access in which articulatory-acoustic discontinuities subserve phonological feature identification.     

Detectability of words and nonwords in two kinds of noise

Recent models of speech perception emphasize the possibility of interactions among different processing levels. There is evidence that the lexical status of an utterance (i.e., whether it is a meaningful word or not) may influence earlier stages of perceptual analysis. To test how far down such "top-down" influences might penetrate, an investigation was conducted to determine whether there is a difference in detectability of words and nonwords masked by amplitude-modulated or unmodulated broadband noise. The results were negative, suggesting either that the stages of perceptual analysis engaged in the detection task are impermeable to lexical top-down effects, or that the lexical level was not sufficiently activated to have any facilitative effect on perception.     

Spectral and duration properties of front vowels as cues to final stop-consonant voicing

The perception of voicing in final velar stop consonants was investigated by systematically varying vowel duration, change in offset frequency of the final first formant (F1) transition, and rate of frequency change in the final F1 transition for several vowel contexts. Consonant-vowel-consonant (CVC) continua were synthesized for each of three vowels, [i,I,ae], which represent a range of relatively low to relatively high-F1 steady-state values. Subjects responded to the stimuli under both an open- and closed-response condition. Results of the study show that both vowel duration and F1 offset properties influence perception of final consonant voicing, with the salience of the F1 offset property higher for vowels with high-F1 steady-state frequencies than low-F1 steady-state frequencies, and the opposite occurring for the vowel duration property. When F1 onset and offset frequencies were controlled, rate of the F1 transition change had inconsistent and minimal effects on perception of final consonant voicing. Thus the findings suggest that it is the termination value of the F1 offset transition rather than rate and/or duration of frequency change, which cues voicing in final velar stop consonants during the transition period preceding closure.     

Can macaques perceive place of articulation from formant transition information?

An important problem in speech perception is to determine how humans extract the perceptually invariant place of articulation information in the speech wave across variable acoustic contexts. Although analyses have been developed that attempted to classify the voiced stops /b/ versus /d/ from stimulus onset information, most of the human perceptual research to date suggests that formant transition information is more important than onset information. The purpose of the present study was to determine if animal subjects, specifically Japanese macaque monkeys, are capable of categorizing /b/ versus /d/ in synthesized consonant-vowel (CV) syllables using only formant transition information. Three monkeys were trained to differentiate CV syllables with a "go-left" versus a "go-right" label. All monkeys first learned to differentiate a /za/ versus /da/ manner contrast and easily transferred to three new vowel contexts /[symbol: see text], epsilon, I/. Next, two of the three monkeys learned to differentiate a /ba/ versus /da/ stop place contrast, but were unable to transfer it to the different vowel contexts. These results suggest that animals may not use the same mechanisms as humans do for classifying place contrasts, and call for further investigation of animal perception of formant transition information versus stimulus onset information in place contrasts.     

Age differences for stop-consonant and vowel perception in adults

The purpose of this study was to determine the role of static, dynamic, and integrated cues for perception in three adult age groups, and to determine whether age has an effect on both consonant and vowel perception, as predicted by the "age-related deficit hypothesis." Eight adult subjects in each of the age ranges of young (ages 20-26), middle aged (ages 52-59), and old (ages 70-76) listened to synthesized syllables composed of combinations of [b d g] and [i u a]. The synthesis parameters included manipulations of the following stimulus variables: formant transition (moving or straight), noise burst (present or absent), and voicing duration (10, 30, or 46 ms). Vowel perception was high across all conditions and there were no significant differences among age groups. Consonant identification showed a definite effect of age. Young and middle-aged adults were significantly better than older adults at identifying consonants from secondary cues only. Older adults relied on the integration of static and dynamic cues to a greater extent than younger and middle-aged listeners for identification of place of articulation of stop consonants. Duration facilitated correct stop-consonant identification in the young and middle-aged groups for the no-burst conditions, but not in the old group. These findings for the duration of stop-consonant transitions indicate reductions in processing speed with age. In general, the results did not support the age-related deficit hypothesis for adult identification of vowels and consonants from dynamic spectral cues.     

Pitch accent in spoken-word recognition in Japanese

Three experiments addressed the question of whether pitch-accent information may be exploited in the process of recognizing spoken words in Tokyo Japanese. In a two-choice classification task, listeners judged from which of two words, differing in accentual structure, isolated syllables had been extracted (e.g., ka from baka HL or gaka LH); most judgments were correct, and listeners' decisions were correlated with the fundamental frequency characteristics of the syllables. In a gating experiment, listeners heard initial fragments of words and guessed what the words were; their guesses overwhelmingly had the same initial accent structure as the gated word even when only the beginning CV of the stimulus (e.g., na- from nagasa HLL or nagashi LHH) was presented. In addition, listeners were more confident in guesses with the same initial accent structure as the stimulus than in guesses with different accent. In a lexical decision experiment, responses to spoken words (e.g., ame HL) were speeded by previous presentation of the same word (e.g., ame HL) but not by previous presentation of a word differing only in accent (e.g., ame LH). Together these findings provide strong evidence that accentual information constrains the activation and selection of candidates for spoken-word recognition.     

The discrimination of vowel duration by infants

Three groups of nine 5-11-month-old infants provided evidence of discrimination of speechlike stimuli differing only in vowel duration. Ease of discrimination was directly related to the magnitude of the ratio of the longer to shorter vowel. Group one infants discriminated three vowel duration contrasts (with ratios of 0.33, 0.67, and 1.0) embedded in a synthetic [mad] syllable; group two discriminated these same duration contrasts within the bisyllable [ samad ], and group three in the trisyllable [ masamad ]. In all cases, the contrasting durations were carried by the last vowel of the synthetic word. These same three infant groups failed to provide evidence of discrimination of a final position released stop consonant contrast ([mat] versus [mad]) cued by voice excitation during closure of the [d] and not the [t]. These results suggest that vowel duration may be a primary cue for infants' perception of the voicing of final position stop consonants.     

Acoustic invariance in speech production: evidence from measurements of the spectral characteristics of stop consonants

On the basis of theoretical considerations and the results of experiments with synthetic consonant-vowel syllables, it has been hypothesized that the short-time spectrum sampled at the onset of a stop consonant should exhibit gross properties that uniquely specify the consonantal place of articulation independent of the following vowel. The aim of this paper is to test this hypothesis by measuring the spectrum sampled at the onsets and offsets of a large number of consonant-vowel (CV) and vowel-consonant (VC) syllables containing both voiced and voiceless stops produced by several speakers. Templates were devised in an attempt to capture three classes of spectral shapes: diffuse-rising, diffuse-falling, and compact, corresponding to alveolar, labial, and velar consonants, respectively. Spectra were derived from the utterances by sampling at the consonantal release of CV syllables and at the implosion and burst release of VC syllables, and these spectra (smoothed by a linear prediction algorithm) were matched against the templates. It was found that about 85% of the spectra at initial consonant release and at final burst release were correctly classified by the templates, although there was some variability across vowel contexts. The spectra sampled at the implosion were not consistently classified. A preliminary examination of spectra sampled at the release of nasal consonants in CV syllables showed a somewhat lower accuracy of classification by the same templates. Overall, the results support an hypothesis that, in natural speech, the acoustic characteristics of stop consonants, specified in terms of the gross spectral shape sampled at the discontinuity in the acoustic signal, show invariant properties independent of the adjacent vowel or of the voicing characteristics of the consonant. The implication is that the auditory system is endowed with detectors that are sensitive to these kinds of gross spectral shapes, and that the existence of these detectors helps the infant to organize the sounds of speech into their natural classes.     

A timing model for word-initial CV syllables in Modern Greek

Measurements of the temporal characteristics of word-initial stressed syllables in CV CV-type words in Modern Greek showed that the timing of the initial consonant in terms of its closure duration and voice onset time (VOT) is dependent on place and manner of articulation. This is contrary to recent accounts of word-initial voiceless consonants in English which propose that closure and VOT together comprise a voiceless interval independent of place and manner of articulation. The results also contribute to the development of a timing model for Modern Greek which generates closure, VOT, and vowel durations for word-initial, stressed CV syllables. The model is made up of a series of rules operating in an ordered fashion on a given word duration to derive first a stressed syllable duration and then all intrasyllabic acoustic intervals.     

Speaker-independent factors affecting the perception of foreign accent in a second language

Previous research on foreign accent perception has largely focused on speaker-dependent factors such as age of learning and length of residence. Factors that are independent of a speaker's language learning history have also been shown to affect perception of second language speech. The present study examined the effects of two such factors--listening context and lexical frequency--on the perception of foreign-accented speech. Listeners rated foreign accent in two listening contexts: auditory-only, where listeners only heard the target stimuli, and auditory + orthography, where listeners were presented with both an auditory signal and an orthographic display of the target word. Results revealed that higher frequency words were consistently rated as less accented than lower frequency words. The effect of the listening context emerged in two interactions: the auditory + orthography context reduced the effects of lexical frequency, but increased the perceived differences between native and non-native speakers. Acoustic measurements revealed some production differences for words of different levels of lexical frequency, though these differences could not account for all of the observed interactions from the perceptual experiment. These results suggest that factors independent of the speakers' actual speech articulations can influence the perception of degree of foreign accent.     

Revisiting stop-consonant perception for two-formant stimuli

The purpose of this study was to reexamine the factors leading to stop-consonant perception for consonant-vowel (CV) stimuli with just two formants over a range of vowels, under both an open- and closed-response condition. Five two-formant CV stimulus continua were synthesized, each covering a range of second-formant (F2) starting frequencies, for vowels corresponding roughly to [i,I,ae,u,a]. In addition, for the [I] and [a] continua, the duration of the first-formant (F1) transition was systematically varied. Three main findings emerged. First, criterion-level labial and alveolar responses were obtained for those stimuli with substantial F2 transitions. Second, for some stimuli, increases in the duration of the F1 transition increased velar responses to criterion level. Third, the response paradigm had a substantial influence on stop-consonant perception across all vowel continua. The results support a model of stop-consonant perception that includes spectral and time-varying spectral properties as integral components of analysis.     

Perceptual integration of the murmur and formant transitions for place of articulation in nasal consonants

This study reassessed the role of the nasal murmur and formant transitions as perceptual cues for place of articulation in nasal consonants across a number of vowel environments. Five types of computer-edited stimuli were generated from natural utterances consisting of [m n] followed by [i e a o u]: (1) full murmurs; (2) transitions plus vowel segments; (3) the last six pulses of the murmur; (4) the six pulses starting from the beginning of the formant transitions; and (5) the six pulses surrounding the nasal release (three pulses before and three pulses after). Results showed that the murmur provided as much information for the perception of place of articulation as did the transitions. Moreover, the highest performance scores for place of articulation were obtained in the six-pulse condition containing both murmur and transition information. The data support the view that it is the combination of nasal murmur plus formant transitions which forms an integrated property for the perception of place of articulation.     

Effects of prosodic boundary on /aC/ sequences: acoustic results

This study presents various acoustic measures used to examine the sequence /a # C/, where "#" represents different prosodic boundaries in French. The 6 consonants studied are /b d g f s S/ (3 stops and 3 fricatives). The prosodic units investigated are the utterance, the intonational phrase, the accentual phrase, and the word. It is found that vowel target values, formant transitions into the stop consonant, and the rate of change in spectral tilt into the fricative, are affected by the strength of the prosodic boundary. F1 becomes higher for /a/ the stronger the prosodic boundary, with the exception of one speaker's utterance data, which show the effects of articulatory declension at the utterance level. Various effects of the stop consonant context are observed, the most notable being a tendency for the vowel /a/ to be displaced in the direction of the F2 consonant "locus" for /d/ (the F2 consonant values for which remain relatively stable across prosodic boundaries) and for /g/ (the F2 consonant values for which are displaced in the direction of the velar locus in weaker prosodic boundaries, together with those of the vowel). Velocity of formant transition may be affected by prosodic boundary (with greater velocity at weaker boundaries), though results are not consistent across speakers. There is also a tendency for the rate of change in spectral tilt moving from the vowel to the fricative to be affected by the presence of a prosodic boundary, with a greater rate of change at the weaker prosodic boundaries. It is suggested that spectral cues, in addition to duration, amplitude, and F0 cues, may alert listeners to the presence of a prosodic boundary.     

Perception of sine-wave analogs of voice onset time stimuli

It has been argued that perception of stop consonant voicing contrasts is based on auditory mechanisms responsible for the resolution of temporal order. As one source of evidence, category boundaries for nonspeech stimuli whose components vary in relative onset time are reasonably close to the labeling boundary for a labial stop voiced-voiceless continuum. However, voicing boundaries change considerably when the onset frequency of the first formant (F1) is varied--either directly or as a side effect of a change in F1 transition duration. Stimuli consisted of a midfrequency sinusoid that was initiated 0-50 ms prior to the onset of a low-frequency sinusoid. Results showed that the labeling boundary for relative onset time increased for longer durations of a low-frequency tone sweep. This effect is analogous to the F1 transition duration effect with synthetic speech. Further, the discrimination of differences in relative onset time was poorer for stimuli with longer frequency sweeps. However, unlike synthetic speech, there were no systematic effects when the frequency of a transitionless lower sinusoid was varied. These findings are discussed in relation to the potential contributions of auditory mechanisms and speech-specific processes in the perception of the voicing contrast.