Identification of vowels in “vowelless” syllables

Traditionally, it has been held that the primary information for vowel identification is provided by formant frequencies in the quasi-steady-state portion of the spoken syllable. Recent search has advanced an alternative view that emphasizes the role of temporal factors and dynamic (time-varying) spectral information in determining the perception of vowels. Nine vowels spoken in /b/ + vowel + /b/ syllables were recorded. The syllables were modified electronically in several ways to suppress various sources of spectral and durational information. Two vowel-perception experiments were performed, testing subjects’ ability to identify vowels in these modified syllables. Results of both experiments revealed the importance of dynamic spectral information at syllable onset and offset (in its proper temporal relation) in permitting vowel identification. On the other hand, steady-state spectral information, deprived of its durational variation, was a poor basis for identification. Results constitute a challenge to traditional accounts of vowel perception and point toward important sources of dynamic information.     

Infant Directed Speech in Natural Interaction—Norwegian Vowel Quantity and Quality

An interactive face-to-face setting is used to study natural infant directed speech (IDS) compared to adult directed speech (ADS). With distinctive vowel quantity and vowel quality, Norwegian IDS was used in a natural quasi-experimental design. Six Norwegian mothers were recorded over a period of 6 months alone with their infants and in an adult conversation. Vowel duration and spectral attributes of the vowels /a:/, /i:/ and /u:/, and their short counterparts /a/ /i/ and /u/ were analysed. Repeated measures analyses show that effects of vowel quantity did not differ between ADS and IDS, and for back vowel qualities, the vowel space was shifted upwards in IDS compared to ADS suggesting that fronted articulations in natural IDS may visually enhance speech to infants.     

Neural processing of acoustic duration and phonological German vowel length: Time courses of evoked fields in response to speech and nonspeech signals

Recent experiments showed that the perception of vowel length by German listeners exhibits the characteristics of categorical perception. The present study sought to find the neural activity reflecting categorical vowel length and the short-long boundary by examining the processing of non-contrastive durations and categorical length using MEG. Using disyllabic words with varying /a/-durations and temporally-matched nonspeech stimuli, we found that each syllable elicited an M50/M100-complex. The M50-amplitude to the second syllable varied along the durational continuum, possibly reflecting the mapping of duration onto a rhythm representation. Categorical length was reflected by an additional response elicited when vowel duration exceeded the short-long boundary. This was interpreted to reflect the integration of an additional timing unit for long in contrast to short vowels. Unlike to speech, responses to short nonspeech durations lacked a M100 to the first and M50 to the second syllable, indicating different integration windows for speech and nonspeech signals.     

Spectral fissioning in phonemic transformations

Listeners presented with a repeated sequence of brief (30- to 100-msec) steady-state vowels hearphonemic transformations—they cannot identify the vowels, but they perceive two simultaneous utterances that differ in both phonemic content and timbre (Warren, Bashford, & Gardner, 1990). These utterances consist of either English words or syllables that occur in English words. In the present study, we attempted to determine whether the two percepts represent alternative interpretations of the same formant structures, or whether different portions of the vowels are used for each verbal organization. It was found that separate spectral regions are employed for each verbal form; Components below 1500 Hz were generally used for one form, and components above 1500 Hz for the other. Hypotheses are offered concerning the processes responsible for the verbal organization of the vowel sequences and for the splitting into two spectrally limited forms. It appears that the tendency to organize spectral regions separately competes with, and can overcome, the tendency to integrate the different spectral components of speech into a single auditory image. A contralateral induction paradigm was used in a procedure designed to quantitatively evaluate these opposing forces of spectral fission and fusion.     

Speaking rate,voice-onset time,and quantity: The search for higher-order invariants for two Icelandic speech cues

The temporal structure of speech has been shown to be highly variable. Speaking rate, stress, and other factors influence the duration of individual speech sounds. The highly elastic nature of speech would seem to pose a problem for the listener, especially with respect to the perception of temporal speech cues such as voice-onset time (VOT) and quantity: How does the listener disentangle those temporal changes whicqh are linguistically significant from those which are extrinsic to the linguistic message? This paper reports data on the behavior of two Icelandic speech cues at different speaking rates. The results show that manipulations of rate have the effect of slightly blurring the distinction between unaspirated and aspirated stops. Despite great changes in the absolute durations of vowels and consonants, the two categories of syllables-V:C and VC:-are nonetheless kept totally distinct. In two perceptual experiments, it is shown that while the ratio of vowel to rhyme duration is the primary cue to quantity and remains invariant at different rates, no such ratio can be defined for VOT. These results imply that quantity is the only one of these two speech cues that is selfnormalizing for rate. Models of rate-dependent speech processing need to address this difference.     

Syllable similarity: The effects of differences in vowels, consonants, and order

In five experiments with synthetic and natural speech syllables, a rating task we used to study the effects of differences in vowels, consonants, and segment order on judged syllable similarity. The results of Experiments I-IV support neither a purely phonemic model of speech representation, in which vowel, consonant, and order are represented independently, nor a purely syllabic model, in which the three factors are integrated. Instead, the data indicate that subjects compare representations in which adjacent vowel and consonant are independent of one another but are not independent of their positions in the syllable. Experiment V provided no support for the hypothesis that this position-sensitive coding is due to acoustic differences in formant transitions.     

Perception of dynamic information for vowels in syllable onsets and offsets.

It has been demonstrated using the "silent-center" (SC) syllable paradigm that there is sufficient information in syllable onsets and offsets, taken together, to support accurate identification of vowels spoken in both citation-form syllables and syllables spoken in sentence context. Using edited natural speech stimuli, the present study examined the identification of American English vowels when increasing amounts of syllable onsets alone or syllable offsets alone were presented in their original sentence context. The stimuli were /d/-vowel-/d/ syllables spoken in a short carrier sentence by a male speaker. Listeners attempted to identify the vowels in experimental conditions that differed in the number of pitch periods presented and whether the pitch periods were from syllable onsets or syllable offsets. In general, syllable onsets were more informative than syllable offsets, although neither onsets nor offsets alone specified vowel identity as well as onsets and offsets together (SC syllables). Vowels differed widely in ease of identification; the diphthongized long vowels /e/, /ae/, /o/ were especially difficult to identify from syllable offsets. Identification of vowels as "front" or "back" was accurate, even from short samples of the syllable; however, vowel "height" was quite difficult to determine, again, especially from syllable offsets. The results emphasize the perceptual importance of time-varying acoustic parameters, which are the direct consequence of the articulatory dynamics involved in producing syllables.     

Characteristics of velocity profiles of speech movements

The control of individual speech gestures was investigated by examining laryngeal and tongue movements during vowel and consonant production. A number of linguistic manipulations known to alter the durational characteristics of speech (i.e., speech rate, lexical stress, and phonemic identity) were tested. In all cases a consistent pattern was observed in the kinematics of the laryngeal and tongue gestures. The ratio of maximum instantaneous velocity to movement amplitude, a kinematic index of mass-normalized stiffness, was found to increase systematically as movement duration decreased. Specifically, the ratio of maximum velocity to movement amplitude varied as a function of a parameter, C, times the reciprocal of movement duration. The conformity of the data to this relation indicates that durational change is accomplished by scalar adjustment of a base velocity form. These findings are consistent with the idea that kinematic change is produced by the specification of articulator stiffness.     

Comparison of associations to vowel speech sounds by English and Spanish speakers

Many psycholinguists have studied associations to vowel speech sounds. It appears that associations involving brightness and size are related to the manner in which the vowels are articulated. That is, high front vowels are judged to be bright and small, and low back vowels are judged to be dim and large. In an extension of a study by Greenberg and Jenkins (1966), 40 English-speaking and 40 Spanish-speaking adults rated nine audiotaped vowel sounds on 23 dimensions. The front-back distinction was again found for both groups. In addition, ratings for all nine vowels were similar for the two groups, which has implications for the cross-cultural universality of these associations.     

The phantom in the phoneme: Invariant cues for stop consonants

The stop consonants /b, d, g, p, t, k/were recorded before/i/,/a/,/u/. The energy spectrum for each stop consonant was removed from its original vowel and spliced onto a different steady-state vowel. Results of a recognition test revealed that consonants were accurately recognized in all cases except when /k/ or/g[ was spliced from/i/to/u/. Further demonstrations suggested that/k/ and /g/ do have invariant characteristics before/i/, /a/, and /u/. These results support the general notion that stop consonants may be recognized before different vowels in normal speech in terms of invariant acoustic features.     

An Acoustic Analysis of Rate of Speech Effects on Vowel Production in Aphasia

The present investigation examined the production of tense and lax vowel duration differences at two speaking rates in the speech of 10 nonfluent aphasics, 8 fluent aphasics, and 10 normal control subjects. Subjects produced four repetitions of each of the vowels [i e æ o u I ε υ Λ] at each speaking rate. Acoustic analyses revealed that subjects in all three groups were able to manipulate overall rate of speech. In addition, normal controls and fluent aphasic subjects produced vowels under the fast rate condition which were significantly shorter than those under the slow rate condition. Despite a change in overall speaking rate, the nonfluent aphasics did not exhibit a significant difference in vowel duration at the two rates of speech, suggesting a deficit in the implementation of this temporal parameter. Both normal controls and fluent aphasic patients produced nonoverlapping distributions of tense and lax vowels at both speaking rates. In contrast, the nonfluent aphasics demonstrated a great deal of overlap in the distribution of tense and lax vowel durations at the fast rate. Results are discussed in relation to the nature of the speech production deficits in nonfluent and fluent aphasic patients.     

Human adults and human infants show a "perceptual magnet effect" for the prototypes of speech categories, monkeys do not

Many perceptual categories exhibit internal structure in which category prototypes play an important role. In the four experiments reported here, the internal structure of phonetic categories was explored in studies involving adults, infants, and monkeys. In Experiment 1, adults rated the category goodness of 64 variants of the vowel i parallel on a scale from 1 to 7. The results showed that there was a certain location in vowel space where listeners rated the i parallel vowels as best instances, or prototypes. The perceived goodness of i parallel vowels declined systematically as stimuli were further removed from the prototypic i parallel vowel. Experiment 2 went beyond this initial demonstration and examined the effect of speech prototypes on perception. Either the prototypic or a nonprototypic i parallel vowels was used as the referent stimulus and adults' generalization to other members of the category was examined. Results showed that the typicality of the speech stimulus strongly affected perception. When the prototype of the category served as the referent vowel, there was significantly greater generalization to other i parallel vowels, relative to the situation in which the nonprototype served as the referent. The notion of a perceptual magnet was introduced. The prototype of the category functioned like a perceptual magnet for other category members; it assimilated neighboring stimuli, effectively pulling them toward the prototype. In Experiment 3, the ontogenetic origins of the perceptual magnet effect were explored by testing 6-month-old infants. The results showed that infants' perception of vowels was also strongly affected by speech prototypes. Infants showed significantly greater generalization when the prototype of the vowel category served as the referent; moreover, their responses were highly correlated with those of adults. In Experiment 4, Rhesus monkeys were tested to examine whether or not the prototype's magnet effect was unique to humans. The animals did not provide any evidence of speech prototypes; they did not exhibit the magnet effect. It is suggested that the internal organization of phonetic categories around prototypic members is an ontogenetically early, species-specific, aspect of the speech code.     

Mommy is only happy! Dutch mothers’ realisation of speech sounds in infant-directed speech expresses emotion,not didactic intent

Exaggeration of the vowel space in infant-directed speech (IDS) is well documented for English, but not consistently replicated in other languages or for other speech-sound contrasts. A second attested, but less discussed, pattern of change in IDS is an overall rise of the formant frequencies, which may reflect an affective speaking style. The present study investigates longitudinally how Dutch mothers change their corner vowels, voiceless fricatives, and pitch when speaking to their infant at 11 and 15 months of age. In comparison to adult-directed speech (ADS), Dutch IDS has a smaller vowel space, higher second and third formant frequencies in the vowels, and a higher spectral frequency in the fricatives. The formants of the vowels and spectral frequency of the fricatives are raised more strongly for infants at 11 than at 15 months, while the pitch is more extreme in IDS to 15-month olds. These results show that enhanced positive affect is the main factor influencing Dutch mothers’ realisation of speech sounds in IDS, especially to younger infants. This study provides evidence that mothers’ expression of emotion in IDS can influence the realisation of speech sounds, and that the loss or gain of speech clarity may be secondary effects of affect.     

Human adults and human infants show a “perceptual magnet effect” for the prototypes of speech categories,monkeys do not

Many perceptual categories exhibit internal structure in which category prototypes play an important role. In the four experiments reported here, the internal structure of phonetic categories was explored in studies involving adults, infants, and monkeys. In Experiment 1, adults rated the category goodness of 64 variants of the vowel /i/ on a scale from 1 to 7. The results showed that there was a certain location in vowel space where listeners rated the /i/ vowels as best instances, or prototypes. The perceived goodness of Iii vowels declined systematically as stimuli were further removed from the prototypic Iii vowel. Experiment 2 went beyond this initial demonstration and examined the effect of speech prototypes on perception. Either the prototypic or a nonprototypic IM vowel was used as the referent stimulus and adults’ generalization to other members of the category was examined. Results showed that the typicality of the speech stimulus strongly affected perception. When the prototype of the category served as the referent vowel, there was significantly greater generalization to other /i/ vowels, relative to the situation in which the nonprototype served as the referent. The notion of aperceptual magnet was introduced. The prototype of the category functioned like a perceptual magnet for other category members; it assimilated neighboring stimuli, effectively pulling them toward the prototype. In Experiment 3, the ontogenetic origins of the perceptual magnet effect were explored by testing 6-month-old infants. The results showed that infants’ perception of vowels was also strongly affected by speech prototypes. Infants showed significantly greater generalization when the prototype of the vowel category served as the referent; moreover, their responses were highly correlated with those of adults. In Experiment 4, Rhesus monkeys were tested to examine whether or not the prototype’s magnet effect was unique to humans. The animals did not provide any evidence of speech prototypes; they did not exhibit the magnet effect. It is suggested that the internal organization of phonetic categories around prototypic members is an ontogenetically early, species-specific, aspect of the speech code     

Cross-modal speech perception in adults and infants using nonspeech auditory stimuli.

Adults and infants were tested for the capacity to detect correspondences between nonspeech sounds and real vowels. The /i/ and /a/ vowels were presented in 3 different ways: auditory speech, silent visual faces articulating the vowels, or mentally imagined vowels. The nonspeech sounds were either pure tones or 3-tone complexes that isolated a single feature of the vowel without allowing the vowel to be identified. Adults perceived an orderly relation between the nonspeech sounds and vowels. They matched high-pitched nonspeech sounds to /i/ vowels and low-pitched nonspeech sounds to /a/ vowels. In contrast, infants could not match nonspeech sounds to the visually presented vowels. Infants' detection of correspondence between auditory and visual speech appears to require the whole speech signal; with development, an isolated feature of the vowel is sufficient for detection of the cross-modal correspondence.     

Phonemic vowel length contrasts in cerebellar disorders

Apraxia of speech and Broca's aphasia both affect voice onset time (VOT) whereas phonemic vowel length distinctions seem to be preserved. Assuming a close cooperation of anterior perisylvian language zones and the cerebellum with respect to speech timing, a similar profile of segment durations must be expected in ataxic dysarthria. In order to test this hypothesis, patients with cerebellar atrophy or cerebellar ischemia were asked to produce sentence utterances including either one of the German lexial items "Rate" (/ra:t(h)e/, 'installment'), "Ratte" (/rat(h)e/, 'rat'), "Gram" (/gra:m/, 'grief'), "Gramm" (/gram/, 'gramm'), "Taten" (/t(h)atn/, 'actions'), or "Daten" (/datn/, 'data'). At the acoustic signal, the duration of the target vowels /a/ and /a:/ as well as the VOT of the word-initial alveolar stops /d/ and /t/ were determined. In addition, a master tape comprising the target words from patients and controls in randomized order was played to three listeners for perceptual evaluation. In accordance with a previous study, first, the cerebellar subjects presented with a reduced categorical separation of the VOT of voiced and unvoiced stop consonants. Second, vowel length distinctions were only compromised in case of the minimal pair "Gram"/"Gramm." In contrast to "Rate"/"Ratte", production of the former lexical items requires coordination of several orofacial structures. Disruption of vowel length contrasts would, thus, depend upon the complexity of the underlying articulatory pattern.     

First and second language vowel perception in early bilinguals

Previous research has shown that the perception of speech sounds is strongly influenced by the internal structure of maternal language categories. Specifically, it has been shown that stimuli judged as good exemplars of a phonemic category are more difficult to discriminate from similar sounds than bad exemplars from equally similar sounds. This effect seems to be restricted to phonemes present in the maternal language, and is acquired in the first months of life. The present study investigates the malleability of speech acquisition by analysing the discrimination capacities for L2 phonemes in highly proficient Spanish-Catalan bilinguals born in monolingual families. In Experiment I subjects were required to give goodness of fit judgments to establish the best exemplars corresponding to three different vowel categories (Catalan /e/ and /ε/ Spanish /e/). In Experiments 2 and 3, bilinguals were asked to perform a discrimination task with materials in their maternal language (Exp. 2) and in their second language (Exp. 3). Results reveal that bilinguals show a reduced discrimination capacity only for good exemplars of their maternal language, but not for good exemplars of their second language. The same pattern of results was obtained in Experiment 4, using a within-subjects design and a bias-free discrimination measure (d'). These findings support the hypothesis that phonemic categories are not only acquired early in life, but under some circumstances, the acquisition of new phonemic categories can be seriously compromised, in spite of early and extensive exposure to L2.     

The weckud wetch of the wast: lexical adaptation to a novel accent

Two experiments investigated the mechanism by which listeners adjust their interpretation of accented speech that is similar to a regional dialect of American English. Only a subset of the vowels of English (the front vowels) were shifted during adaptation, which consisted of listening to a 20-min segment of the "Wizard of Oz." Compared to a baseline (unadapted) condition, listeners showed significant adaptation to the accented speech, as indexed by increased word judgments on a lexical decision task. Adaptation also generalized to test words that had not been presented in the accented passage but that contained the shifted vowels. A control experiment showed that the adaptation effect was specific to the direction of the shift in the vowel space and not to a general relaxation of the criterion for what constitutes a good exemplar of the accented vowel category. Taken together, these results provide evidence for a context-specific vowel adaptation mechanism that enables a listener to adjust to the dialect of a particular talker.     

Auditory enhancement and the perception of concurrent vowels

Listeners identified both constituents of double vowels created by summing the waveforms of pairs of synthetic vowels with the same duration and fundamental frequency. Accuracy of identification was significantly above chance. Effects of introducing such double vowels by visual or acoustical precursor stimuli were examined. Precursors specified the identity of one of the two constituent vowels. Performance was scored as the accuracy with which the other vowel was identified. Visual precursors were standard English spellings of one member of the vowel pair; acoustical precursors were 1-sec segments of one member of the vowel pair. Neither visual precursors nor contralateral acoustical precursors improved performance over the condition with no precursor. Thus, knowledge of the identity of one of the constituents of a double vowel does not help listeners to identify the other constituent. A significant improvement in performance did occur with ipsilateral acoustical precursors, consistent with earlier demonstrations that frequency components which undergo changes in spectral amplitude achieve enhanced auditory prominence relative to unchanging components. This outcome demonstrates the joint but independent operation of auditory and perceptual processes underlying the ability of listeners to understand speech despite adversely peaked frequency responses in communication channels.     

Perception of dynamic information for vowels in syllable onsets and offsets

It has been demonstrated using the “silent-center” (SC) syllable paradigm that there is sufficient information in syllable onsets and offsets,taken together, to support accurate identification of vowels spoken in both citation-form syllables and syllables spoken in sentence context. Using edited natural speech stimuli, the present study examined the identification of American English vowels when increasing amounts of syllable onsetsalone or syllable offsetsalone were presented in their original sentence context. The stimuli were /d/-vowel-/d/ syllables spoken in a short carrier sentence by a male speaker. Listeners attempted to identify the vowels in experimental conditions that differed in the number of pitch periods presented and whether the pitch periods were from syllable onsets or syllable off-sets. In general, syllable onsets were more informative than syllable offsets, although neither onsets nor offsets alone specified vowel identity as well as onsets and offsets together (SC syllables). Vowels differed widely in ease of identification; the diphthongized long vowels /e/, /ae/, /o/ were especially difficult to identify from syllable offsets. Identification of vowels as “front” or “back” was accurate, even from short samples of the syllable; however, vowel "height" was quite difficult to determine, again, especially from syllable offsets. The results emphasize the perceptual importance of time-varying acoustic parameters, which are the direct consequence of the articulatory dynamics involved in producing syllables.