On the causes of compensation for coarticulation: evidence for phonological mediation

This study examined whether compensation for coarticulation in fricative-vowel syllables is phonologically mediated or a consequence of auditory processes. Smits (2001a) had shown that compensation occurs for anticipatory lip rounding in a fricative caused by a following rounded vowel in Dutch. In a first experiment, the possibility that compensation is due to general auditory processing was investigated using nonspeech sounds. These did not cause context effects akin to compensation for coarticulation, although nonspeech sounds influenced speech sound identification in an integrative fashion. In a second experiment, a possible phonological basis for compensation for coarticulation was assessed by using audiovisual speech. Visual displays, which induced the perception of a rounded vowel, also influenced compensation for anticipatory lip rounding in the fricative. These results indicate that compensation for anticipatory lip rounding in fricative-vowel syllables is phonologically mediated. This result is discussed in the light of other compensation-for-coarticulation findings and general theories of speech perception.     

A kinematic analysis of anticipatory coarticulation in the speech of anterior aphasic subjects using electromagnetic articulography

An investigation was made into the extent and time course of anticipatory coarticulation in the speech of two normal and two anterior aphasic, German-speaking subjects. Both labial and velar coarticulation gestures were investigated. Subjects produced sentences containing target words contrasting in postconsonantal vowel rounding (e.g., [geli:ge]/[gely:gel]) and in nasality (e.g., [ti:de]/[ti:ne]). Speech kinematics were monitored by means of electromagnetic articulography. The data revealed that for correct productions, aphasic speakers' coarticulatory patterns were more highly variable than those of control subjects. These differences, however, were found chiefly for spatial displacement characteristics, while the temporal aspects of articulator movement necessary for anticipatory coarticulation appeared largely intact. Articulator mistiming did not appear to explain a small corpus of stop/nasal substitution errors produced by one of the aphasic speakers.     

Anticipatory Coarticulation in Aphasia: Effects of Utterance Complexity

The magnitude and extent of anticipatory coarticulation were examined in groups of fluent and nonfluent aphasic patients and normal control subjects. One- and two-syllable target utterances were elicited at slow and fast rates of speech with or without a consonant intervening between the target consonant and vowel, and with or without a preceding schwa, to manipulate utterance complexity. Acoustic analyses (F2 and centroid frequencies) revealed that both groups of aphasic patients exhibited relatively normal patterns of anticipatory coarticulation. However, small but significant differences among the groups emerged in certain conditions. Surprisingly, increased utterance complexity was not found to reduce coarticulatory effects to a greater degree in the nonfluent relative to the fluent aphasic group. Perceptual tests largely confirmed the acoustic analyses.     

Anticipatory coarticulation in aphasia: acoustic and perceptual data

Two experiments investigated speech motor planning in aphasia by contrasting the degree of labial and lingual anticipatory coarticulation evident in normal subjects' speech with that found in the speech of aphasic subjects. In the first experiment, Linear Predictive Coding (LPC) analyses were conducted for the initial consonants of CV [si su ti tu ki ku] and CCV [sti stu ski sku] productions by 6 normal and 10 aphasic (5 anterior, 5 posterior) subjects. For normal subjects' productions, reliable coarticulatory shift was found for almost all measurements, indicating that acoustic correlates for anticipatory coarticulation obtain for [s], [t], and [k] in a prevocalic environment, as well as when [s] is the initial consonant of a CCV syllable. The data for the aphasic subjects were statistically indistinguishable from those of the normal subject group, and there were no differences noted as a function of aphasia type. In the second experiment, a subset of the consonantal stimuli produced by the normal and aphasic subjects was presented to a group of 10 naive listeners for a vowel identification task. Listeners were able to identify the productions of all subjects at a level well above chance. In addition, small but statistically significant Group differences were observed, with the [sV], [skV], and [tV] productions by anterior aphasics showing significantly lower perceptual scores than those of normal subjects.     

Velar movements during speech in a patient with apraxia of speech.

A fiberscopic observation of velar movements during speech was made of a patient diagnosed as having apraxia of speech. Repeated utterances of the same word showed a marked variability in terms of the pattern of velar movements accompanied at times by a phonetic change. In spite of such a variability, the general successional pattern of velar gestures for a given phonetic context approximated the normal pattern. During the production of nasal and nonnasal consonants the velum tended to take “neutral” positions. Anticipatory coarticulation was present, but some deviation from normal patterns was occasionally observed. Based on these observations, possible mechanisms responsible for the syndrome called apraxia of speech were discussed.     

Influence of vocalic context on perception of the [∫]-[s] distinction

When synthetic fricative noises from a [∫]-[s] continuum are followed by [a] or [u] (with appropriate formant transitions), listeners perceive more instances of [s] in the context of [u] than in the context of [a]. Presumably, this reflects a perceptual adjustment for the coarticulatory effect of rounded vowels on preceding fricatives. In Experiment 1, we found that varying the duration of the fricative noise leaves the perceptual context effect unchanged, whereas insertion of a silent interval following the noise reduces the effect substantially. Experiment 2 suggested that it is temporal separation rather than the perception of an intervening stop consonant that is responsible for this reduction, in agreement with recent, analogous observations on anticipatory coarticulation. In Experiment 3, we showed that the vowel context effect disappears when the periodic stimulus portion is synthesized so as to contain no formant transitions. To dissociate the contribution of formant transitions from contextual effects due to vowel quality per se, Experiment 4 employed synthetic fricative noises followed by periodic portions excerpted from naturally produced [∫a], [sa], [∫u], and [su]. The results showed strong and largely independent effects of formant transitions and vowel quality on fricative perception. In addition, we found a strong speaker (male vs. female) normalization effect. All three influences on fricative perception were reduced by temporal separation of noise and periodic stimulus portions. Although no single hypothesis can explain all of our results, they are generally supportive of the view that some knowledge of the dynamics of speech production has a role in speech perception.     

Reading aloud begins when the computation of phonology is complete

Naming latency experiments in which monosyllabic items are read aloud are based on the assumption that the vocal response is not initiated until the phonology of the entire syllable has been computed. Recently, this assumption has been challenged by A. H. Kawamoto, C. T. Kello, R. Jones, and K. Bame (1998), who argued instead that the reading-aloud response begins as soon as the initial phoneme is computed. This view would be refuted by evidence of anticipatory coarticulation effects on the initial phoneme due to the nature of the following vowel in the speeded reading-aloud task. The authors provide such evidence.     

Perception of anticipatory coarticulation effects in vowel-stop consonant-bowel sequences

Articulatory and acoustic studies of speech production have shown that the effects of anticipatory coarticulation may extend across several segments of an utterance. The present experiments show that such effects have perceptual significance. In two experiments, a talker produced consonant (C) and vowel (V) sequences in a sentence frame (e.g., "I say pookee") of the form "I say / C V1 C V2/" in which V1 was /u, ae/ and V2 was /i, a/. Each /i, a/ sentence pair was cross-spliced by exchanging the final syllable /C V2/ so that coarticulatory information prior to the crosspoint was inappropriate for te final vowel (V2) in crossed sentences. Recognition time (RT) for V2 in crossed and intact (as spoken) sentences was obtained from practiced listeners. In both experiments RT was slower in crossed sentences; crossed sentences also attracted more false alarms. The pattern of perceptual results was mirrored in the pattern of precross acoustic differences in experimental sentences (e.g., formants F2 and F3 were higher preceding /i/ than preceding /a/). Pretarget variation in the formants jointly predicted amount of RT interference in crossed sentences. A third experiment found interference (slower RT) and also facilitation (faster RT) from exchanges of pretarget coarticulatory information in sentences. Two final experiments showed that previous results were not dependent on the use of practiced listeners.     

Tonal Coarticulation in Thai after Unilateral Brain Damage

The magnitude and temporal extent of anticipatory and perseverative tonal coarticulation was investigated in Thai-speaking normal and brain-damaged adults. A total of 47 speakers (10 young normal, 10 old normal, 13 nonaphasic right-brain-damaged patients, 14 left-brain-damaged aphasic patients, 9 fluent, 5 nonfluent) produced all 25 possible sequences of two tones from the five lexical tones of Thai embedded in a carrier sentence.F₀contours were analyzed in terms of height and slope at 10% intervals throughout the duration of the two syllables. Acoustic analysis revealed that anticipatory and perseverative tonal coarticulation of tones was markedly reduced in left fluent aphasics, totally absent in left nonfluent aphasics, but reasonably intact in right hemisphere patients. Findings are interpreted to highlight the nature of speech disturbances in nonfluent and fluent aphasia, hemispheric specialization for tone, and tonal coarticulation in Thai.     

USING TIME DELAY TO PROMOTE SPONTANEOUS SPEECH IN AN AUTISTIC CHILD

One of the frequently observed deficits in autistic children is their lack of spontaneous speech. We used a multiple baseline across behaviors to investigate the effectiveness of a time delay procedure for inducing spontaneous speech in a 10-year-old male autistic child during play. We first taught the child to imitate the experimenter's verbal prompts that described the child's motor response. Once the child reached criteria on imitation, we implemented baseline wherein an immediate verbal prompt for speech was provided after each of the child's motor responses. Intervention consisted of a gradual delay in the presentation of the verbal prompts. The time delay effectively increased the child's spontaneous speech on trained items; some generalization to untrained items also occurred, but only within the same behavioral class of car play. Generalization was also observed across settings. Spontaneous speech remained at high levels during the 4-month maintenance for the behavior of car play but decreased for a second behavior. Decreases in the child's response latencies suggest that spontaneous speech may be an anticipatory verbal response.     

Invariants,specifiers, cues: An investigation of locus equations as information for place of articulation

This experiment explored the information for place of articulation provided bylocus equations— equations for a line relating the second formant (F2) of a vowel at midpoint toF2 of the formant at consonant-vowel (CV) syllable onset. Locus equations cue place indirectly by quantifying directly the degree of coarticulatory overlap (coarticulation resistance) between consonant and vowel. Coarticulation resistance is correlated with place, The experiment tested predictions that when coarticulation resistance varies due to properties of the consonant other than place of articulation (in particular due to manner of articulation), locus equations would not accurately reflect consonantal place of articulation. These predictions were confirmed. In addition, discriminant analyses, using locus equation variables as classifiers, were generally unsuccessful in classifying a set of consonants representing six different places of articulation. I conclude that locus equations are unlikely to provide useful place information to listeners.     

Differential diagnosis between apraxia and dysarthria based on acoustic analysis

Acoustic analysis provides objective quantitative measures of speech that enable a comprehensive and accurate understanding of motor disorders and complement the traditional measures. This paper aims to distinguish between normal and pathological speech, more specifically between apraxia of speech and spastic dysarthria in native Spanish speaking patients using acoustic parameters. Participants (4 aphasic with apraxia of speech, 4 with spastic dysarthria, and 15 without speech disorders) performed three different tasks: repeating the syllable sequence [pa-ta-ka], repeating the isolated syllable [pa] and repeating the vowel sequence [i-u]. The results showed that the normative values of motor control, in general, coincide with those obtained in previous research on native English speakers. They also show that damage to motor control processes results in a decrease in the rate of alternating and sequential movements and an increase in the inter-syllabic time for both types of movements. A subset of the acoustic parameters analyzed, those that measure motor planning processes, enable differentiation between normal population and apraxic and dysarthric patients, and between the latter. The differences between the pathological groups support the distinction between motor planning and motor programming as described by van der Merwe's model of sensorimotor processing (1997).     

Precision of working memory for speech sounds

Memory for speech sounds is a key component of models of verbal working memory (WM). But how good is verbal WM? Most investigations assess this using binary report measures to derive a fixed number of items that can be stored. However, recent findings in visual WM have challenged such “quantized” views by employing measures of recall precision with an analogue response scale. WM for speech sounds might rely on both continuous and categorical storage mechanisms. Using a novel speech matching paradigm, we measured WM recall precision for phonemes. Vowel qualities were sampled from a formant space continuum. A probe vowel had to be adjusted to match the vowel quality of a target on a continuous, analogue response scale. Crucially, this provided an index of the variability of a memory representation around its true value and thus allowed us to estimate how memories were distorted from the original sounds. Memory load affected the quality of speech sound recall in two ways. First, there was a gradual decline in recall precision with increasing number of items, consistent with the view that WM representations of speech sounds become noisier with an increase in the number of items held in memory, just as for vision. Based on multidimensional scaling (MDS), the level of noise appeared to be reflected in distortions of the formant space. Second, as memory load increased, there was evidence of greater clustering of participants' responses around particular vowels. A mixture model captured both continuous and categorical responses, demonstrating a shift from continuous to categorical memory with increasing WM load. This suggests that direct acoustic storage can be used for single items, but when more items must be stored, categorical representations must be used.     

Statistical information and coarticulation as cues to word boundaries: a matter of signal quality

We investigated how statistical information in the form of transitional probabilities (TPs) interacts with coarticulation, another sublexical cue to word boundaries, and examined the impact of signal quality on the weighting of these cues. In an artificial-language-learning setting, with phonetically intact speech, coarticulation overruled TPs, suggesting the predominance of subsegmental, low-level information. However, whereas the role of coarticulation in segmentation was highly modulated by signal quality, TPs were very resilient to noise. When coarticulation was rendered unreliable by strongly degrading the input with a 10-dB signal-to-noise ratio (SNR), only statistical information drove segmentation. In a more mildly degraded 22-dB SNR condition, in which some acoustic properties were still available, coarticulation was exploited, although with less reliability than in optimal conditions. These results can be interpreted according to a hierarchical approach (Mattys, White, & Melhorn, 2005) in which both the available segmentation cues and the listening conditions have an important role in speech segmentation.     

The contribution of apraxic speech to working memory deficits in Parkinson's disease

This article links two formerly separate areas of research associated with Parkinson's disease (PD): speech and memory. It is proposed that speech deficits occur in PD not merely at the level of muscular control, as is commonly termed dysarthria, but also at the level of speech planning and programming, more aptly described as a form of apraxia. It is further argued that PD patient groups exhibit small deficits in verbal span, and the link between apraxic speech and verbal span is elucidated via Baddeley's (1986) model of working memory. An experiment is described in which aspects of speech of 36 PD and 43 healthy control subjects were rated and classified, and measures of span and articulation rate for words of different syllable lengths were taken. Twenty-three PD subjects had dysarthric speech, while 14 of them had apraxic speech, which was associated with lower memory span scores for longer words. It is concluded that apraxic speech can be a source of reduced memory span in PD. In addition to implications for rehabilitation and therapeutic work with PD sufferers, these findings advance our theoretical understanding of the Parkinsonian syndrome.     

The role of production abilities in the perception of consonant category in infants

The influence of motor knowledge on speech perception is well established, but the functional role of the motor system is still poorly understood. The present study explores the hypothesis that speech production abilities may help infants discover phonetic categories in the speech stream, in spite of coarticulation effects. To this aim, we examined the influence of babbling abilities on consonant categorization in 6‐ and 9‐month‐old infants. Using an intersensory matching procedure, we investigated the infants’ capacity to associate auditory information about a consonant in various vowel contexts with visual information about the same consonant, and to map auditory and visual information onto a common phoneme representation. Moreover, a parental questionnaire evaluated the infants’ consonantal repertoire. In a first experiment using /b/–/d/ consonants, we found that infants who displayed babbling abilities and produced the /b/ and/or the /d/ consonants in repetitive sequences were able to correctly perform intersensory matching, while non‐babblers were not. In a second experiment using the /v/–/z/ pair, which is as visually contrasted as the /b/–/d/ pair but which is usually not produced at the tested ages, no significant matching was observed, for any group of infants, babbling or not. These results demonstrate, for the first time, that the emergence of babbling could play a role in the extraction of vowel‐independent representations for consonant place of articulation. They have important implications for speech perception theories, as they highlight the role of sensorimotor interactions in the development of phoneme representations during the first year of life.     

Word Length and Vowel Duration in Apraxia of Speech: The Use of Relative Measures

Previous research has established that the duration of stressed word stem vowels is shorter in polysyllabic words than in monosyllabic words for normal speakers and for speakers with aphasia and apraxia of speech (AOS). However, the results are inconsistent across studies with regard to the magnitude and pattern of the duration reduction for apraxic speakers. We hypothesized that this inconsistency may be explained based on different relative measures of duration reduction. A speech sample was obtained from 10 aphasic speakers with AOS, 10 aphasic speakers without AOS, and 10 normal controls. As predicted, the use of two different relative measures resulted in different vowel reduction patterns, both of which were consistent with previous reports. The results further indicate that the production of polysyllabic words is particularly taxing in AOS and is associated with a substantial reduction of speaking rate compared to other aphasic and normal speakers.     

Verbal reaction time patterns in aphasic adults: consideration for apraxia of speech

Verbal reaction time patterns were compared in aphasic adults presenting anterior and posterior left hemisphere lesions. Reaction Times were measured from simultaneous recording of the subjects' verbal responses and electromyographic activity from three oral-facial sites. Total Reaction Time was fractionated into Premotor Time and Motor Time components to assess latencies associated with motor speech planning and execution. The results suggested that only anterior lesions result in deficits in motor speech planning and/or execution while posterior lesion patients perform no differently than normal. The evidence supports traditional concepts regarding apraxia of speech as being associated with frontal lobe lesions.     

Homotopic language reorganization in the right hemisphere after early left hemisphere injury

Speech production is inextricably linked to speech perception, yet they are usually investigated in isolation. In this study, we employed a verbal-repetition task to identify the neural substrates of speech processing with two ends active simultaneously using functional MRI. Subjects verbally repeated auditory stimuli containing an ambiguous vowel sound that could be perceived as either a word or a pseudoword depending on the interpretation of the vowel. We found verbal repetition commonly activated the audition-articulation interface bilaterally at Sylvian fissures and superior temporal sulci. Contrasting word-versus-pseudoword trials revealed neural activities unique to word repetition in the left posterior middle temporal areas and activities unique to pseudoword repetition in the left inferior frontal gyrus. These findings imply that the tasks are carried out using different speech codes: an articulation-based code of pseudowords and an acoustic-phonetic code of words. It also supports the dual-stream model and imitative learning of vocabulary.