VOICE ONSET TIME AS a FUNCTION of WORD STRESS in SPEECH DEVELOPMENT by DEBORAH ANNE LLEWELLYN, B.S

VOICE ONSET TIME AS A FUNCTION OF WORD STRESS IN SPEECH DEVELOPMENT by DEBORAH ANNE LLEWELLYN, B.S. A THESIS IN COMMUNICATION DISORDERS Submitted to the Graduate Faculty of Texas Tech University Health Sciences Center in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN COMMUNICATION DISORDERS

Approveg,

Accepted

May, 1994 /}f_;_CJ7fY'

With great sincerity, I would like to thank Dr. Curt Hamre for servtng as chairperson of my thesis committee. His unending patience. scholarly advice, and writing expertise will always be remembered and appreciated. In addition, Dr. William

Ham's contributions were essential to the successful completion of this project.

I would like to extend my thanks to Jennifer Johnston and Dr. Wang and his staff of the University Medical Center for offering the use of their equipment and facilities for analyzing my data.

I am blessed with a beautiful family and guided by my \vonderful parents. Frank and Joan Kiefel. With their love and support, I was able to focus on my education and work to\vard achieving my goals. Finally, with special love and gratitude I want to acknowledge my husband, Mark Llewellyn, who has helped me keep things tn perspective and kept me smiling throughout my days at Texas Tech University.

0 0 11 TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... ii

ABSTRACT ...... iv CHAPTER

I. INTRODUCTION ...... 1

II. LITERATURE REVIEW ...... 2

Respiratory, Articulatory, and Phonatory Differences ...... 2

Segmental and Suprasegmental Aspects of Speech ...... 6

Temporal Compensation ...... 7

Development of Speech Timing ...... 9

Speech Differences in Stutterers ...... 12

Prosody and Stuttering Loci ...... 13

VOT: A Closer Look ...... 16

Statement of the Problem ...... 17

III. METHODS AND PROCEDURES ...... IS

Subjects ...... 18

Speech Sample ...... 18

Instrumentation ...... 19

Procedures ...... 19

Statistical Design ...... 20

Reliability ...... 20

IV. RESULTS ...... 21

V. DISCUSSION ...... 22

Research Implications ...... 25

REFERENCES ...... 29

111 ABSTRACT

In adults, it has been noted that VOT is longer in stressed syllables. This study examined the influence of stress on VOT in children. Three groups of subjects were analyzed: an adult group, a younger t:,Tfoup of children ( 6:3-7:9 years), and an older group of children (9: 11-10:9 years). For all groups, VOT was significantly longer in stressed syllables. Further, VOT was significantly longer for 6-7 year olds compared to adults, but 10 year olds' VOT was not significantly different from adults' or 6-7 year olds'.

Implications for the development of temporal control are discussed.

IV CHAPTER I

INTRODUCTION

Much research has been conducted attempting to explain the nature of stuttering.

While some researchers have focused on peripheral differences, including respiratory, articulatory, and phonatory aspects of speech, other researchers have studied more central, linguistic differences. This investigation examines these points and proposes one that combines the t\vo.

Voice onset time ( VOT), the time between the release of the consonant and the onset of phonation for the proceeding vowel, is one measure in \vhich researchers have observed differences between stutterers and nonstutterers. On the contrary, other researchers have not observed these differences. One explanation for this controversy is that previous research may have focused too much on this segmental aspect of speech without considering variables which can affect it, namely word stress. Thus, it seems important to investigate the influence of stress on VOT in children who stutter.

However, before this can be examined in a stuttering population, normative data must be collected. Consequently, this investigation examines the intluence of stress on VOT in normal children. VOT in a younger and an older group of children will be analyzed and compared to adult data. It is hypothesized that VOT will be longer in stressed syllables than in unstressed syllables in all three subject groups. Additionally, VOT values will be longer in children with these differences being more marked in the younger group of children. Alternatively, if no VOT differences are noted between the stressed and unstressed syllables, then stress may not affect VOT in children as it has been noted to in adults.

1 CHAPTER II

LITERATURE REVIEW

Van Riper (1982) defined stuttering as "a disruption of the simultaneous and

successive programming of muscular movements" required to produce a speech sound or

its link to the following sound (p. 415). He further stated that:

The integrity of a spoken word demands great precision in the timing of its components. When, for any reason, that timing is awry and askew, a temporally distorted word is produced, and when this happens, the speaker has evidenced a core stuttering behavior. (p. 415)

This disruption in the muscle movement system may stem from a lack of control or

coordination in any of the systems involved in speech production. Much research has

been conducted concerning the differences between stutterers and nonstutterers in

various parameters of speech. These measures have included differences in respiration,

articulation, phonation, and prosody.

Respiratory, Articulatory, and Phonatory Differences

Respiratory abnormalities have been documented m stutterers by several

investigators including Travis ( 1927), Fossler ( 1930), Murray ( 1932), and Henrickson

( 1936 ). Adams ( 1975) summarized the results of these studies which noted: (I) fixated

respiratory structures, (2) inspiratory gasps during expiration, (3) shallow breathing, (4)

asynchronous respiratory movements, including co-contraction of antagonistic respiratory

muscles, and (5) respiratory tremors, also described as diaphragmatic flutters. These

respiratory abnormalities were noted during actual stuttering events~ however, the exact relationship between these respiratory abnormalities and stuttering events remains

unknown. Research has also shown that stutterers may exhibit kinematic differences in articulation. These differences in articulatory dynamics bet\veen stutterers and nonstutterers have been studied by Zimmerman ( 1980a). In his study, stutterers and nonstutterers repeated the isolated syllables /pap;, /marnJ, and /bab/ ten times while he measured the movements, positions, and timing of the lips and jaw using high-speed cineradiography. Only the stutterers' fluent productions were examined. He found that stutterers exhibited longer durations of movement onset, longer transition times to positions, increased time to reach the peak velocity of movements, and increased duration for lip and jaw postures. Further, more asynchronous movements between the lips and jaws were evidenced by the stutterers than by the nonstutterers.

Evidence of laryngeal differences has been found through voicing studies and electromyography (EMG) research. Voice onset time (VOT) is the time between the release of the consonant and the onset of phonation for the proceeding vowel.

Zimmerman ( 1980a) found longer (slower) VOTs for stutterers than nonstutterers.

Slower VOTs have also been observed in studies by Agnello and Wingate ( 1972) (in

Adams, Freeman, & Conture, 1984, pp. 89-104) in which CV utterances were analyzed from matched groups of stutterers and nonstutterers. Agnello ( 1975) reported longer

VOTs for the test syllables /pa, bal repeated three times each by 23 adult stutterers and 23 adult nonstutterers. Hillman and Gilbert ( 1977) also noted longer VOTs for /p,t,kl extracted from intervocalic segments of unstressed contexts in the Rainbow passage

(Fairbanks, 1960) read by matched groups of 10 stutterers and 10 nonstutterers. Finally,

Adams ( 1987) found longer and more variable VOTs in preschool stutterers than nonstutterers for target words beginning \\-ith /b, p, t, kl elicited in isolation, in sentences, and in conversation than preschool nonstutterers. Longer VOTs for child stutterers were also noted by Wendell (1973), Seebach and Caruso (1979), and Caruso et al. ( 1981) (cited in Adams, 1987). Other studies, however. have noted differences either only under some conditions

or not at all. For example, Healey and Gutkin ( 1984) noted significantly longer VOTs for

voiced but not voiceless stops taken from target syllables located at the beginning of a

carrier phrase. Also, Metz, Conture, and Caruso ( 1979) noted longer VOTs for six of the

eighteen consonants and consonant clusters that they analyzed including /b, p, br, pr, 1\v,

bl/. A study by Watson and Alfonso ( 1982) failed to support these findings of VOT

differences. In their study, eight stutterers and eight age- and gender-matched

nonstutterers produced three contiguous sequences of VCVC in which the consonants

included /p,t,k/~ no significant differences in VOT were observed between the two

groups. Borden, Baer, and Kenney ( 1985) controlled for fluency during speech samples

through adaptation trials and noted no significant differences in VOT between

nonstutterers' and stutterers' repeated productions of /tu/ embedded in the number

sequences 3425 and 4253. Finally, Zebrowski, Conture, and Cudahy ( 1985) noted no

significant differences in VOT for /p/ and /b/ obtained from stuttering and nonstuttering

children's productions of CV and CVC words embedded in the carrier phrase "Say .r

again."

Further, there is evidence that stutterers have slower voice initiation times ( VIT)

and speech initiation time (SIT). VIT is the time lapse between some nonspeech

stimulus like a pure tone or flash of light and the start of voicing. Adams and Hayden

( 1976) compared adult stutterers to age- and gender-matched nonstutterers on their VITs between a I 000 Hz pure tone and a phonated /a/. They observed that the stutterers were slower than their matched nonstutterers in VIT. Starkweather, Hirschman, and

Tannenbaum ( 1976) had stutterers and matched nonstutterers produce twenty-six syllables that varied in place and manner of articulation in response to a green light.

Similarly, the group of stutterers evidenced longer VITs than the nonstutterers. Other research by Reardon ( 1977), Cross, Shadden, and Luper ( 1979), Cross and Luper ( 1979),

4 Cross, and Cooke ( 1979), Reich, Till, and Goldsmith ( 1981) (cited in Adams, Freeman,

& Conture, 19 84, pp. 89-104) and Horii ( 1984) has supported these findings.

Speech initiation time (SIT) is the lapse between a nonspeech stimulus and the production of a word or phrase beginning with a voiced sound. Prosek, Montogomery,

Walden, and Schwartz ( 1979) had stutterers and nonstutterers produce sixteen vowel consonant ( VC) words in response to a flash of light, a I 000 Hz pure tone, or spoken cue.

In their study, stutterers exhibited longer SITs than the nonstutterers in all stimulus conditions. Reich, Till, and Goldsmith ( 1981) and Hayden, Adams, and Jordahl (1982)

(in Adams, et. al., 1984, pp. 89-104) also observed longer SITs for stutterers in their test conditions. A possible interpretation of these VOT, VIT, and SIT studies was offered by

Adams ( 1981 ). He suggested that stutterers may be slower in organizing and sending normal neural signals to laryngeal muscles. Additionally, stutterers may send inappropriate neural signals, causing voicing delays.

Additional acoustic evidence of laryngeal differences in male preschool stutterers was noted by Hall and Yairi ( 1992). Fluent productions of CV, VC, CVC, and CCVC utterances extracted from spontaneous speech were used to analyze fundamental frequency, jitter, and shimmer. While no differences were noted in fundamental frequency and jitter bet\veen stutterers and matched nonstutterers, shimmer Yalues were significantly higher for the stutterers. These researchers suggested that this finding confirms that laryngeal involvement in stutterers is evident even in young children.

Finally, electromyographic (EMG) studies of stutterers' laryngeal muscles by

Freeman and Ushijima ( 1975) and Shapiro ( 1980) yielded three findings. First, these researchers observed that higher levels of muscular activity were evidenced during stuttered speech. Second, laryngeal co-contraction between abductor and adductor muscles was noted. Finally, abnormal muscle activity was noted even in the fluent speech of stutterers which suggests that stutterers frequently experience moments of

5 laryngeal discoordination during speech. Depending on the nature, duration, and timing of the discoordination, a stuttering event may or may not be perceived. Freeman and

Ushijima speculated that, during the imperceivable moments, an actual "physiological

stuttering" may be occurring.

Segmental and Suprasegmental Aspects of Speech

The focus on coordination or timing differences in parameters such as respiration,

articulation, and phonation suggests a possible physiological abnormality in stutterers.

Wingate ( 1988, p. 239), however, stated that focusing on these peripheral aspects of

speech production may be too limiting and superficial to explain stuttering. He suggested

that stuttering is a "... defect that extends beyond the level of motor execution." Further,

he described stuttering as a "prosodic defect."

According to Lehiste ( 1970) and Wingate ( 1984 ), "prosody" is difficult to define.

It refers to the overall melody of a spoken message and is often used synonymously with the term "suprasegmental." Segments are individual speech sounds or phones capable of making meaningful differences in speech. Suprasegmentals describe aspects of the speech signal \Vhich extend above and beyond the segments. These suprasegmental aspects include pitch, loudness, and quantity (rate and duration). In connected speech they interact, yielding what is perceived as stress or emphasis as well as an intonation pattern (Lehiste, 1970~ Wingate, 1984 ). Changes in stress and intonation may also lead to changes in the meaning of a spoken message (Glenn, Glenn, & Forman, p. 198). For example, in the sentence "I bought a car," the underlying meaning changes when different words are stressed. "I bought a car" describes exactly who bought the car~ whereas, "I bought a car" describes how I got the car. Similarly, the meaning of the message corresponds to the utterance intonation pattern. For a question, the pattern ends with rising intonation. However, falling intonation indicates a statement.

6 Research has revealed that stress is influenced by vanous factors. Namelv.

greater stress tends to occur on content words (Smith, 1959, p. 3 ), on initial syllables of

polysyllabic words (Berry, 1953, cited in Wingate, 1988), on longer words (Denes, 1963 ),

on words that occur less frequently in speech (Berry, 1953, Wingate, 1988), and on words

in initial sentence position (Wingate, 1988). Additionally, in English, consonants and

vowels are longer in stressed than in unstressed syllables (Klatt, 1974~ Umeda, 1975),

contributing to longer durations of stressed syllables ( Lehiste, 1970, p. 36 ). Crystal and

House ( 1988) noted that except for voiceless fricatives and nonvocalic sonorants (i.e ..

liquids, nasals, and glides) all categories of speech sounds are lengthened in stressed

syllables. Finally, in some languages, including English, an increase in speech rate is

achieved by reducing or shortening unstressed syllables (Lehiste, 1970, p. 38). Such

timing regularities have been identified as temporal compensation.

Temporal Compensation

Temporal compensation in speech may be defined as " the effect which operates

to modify the durations of internal segments of articulatory units in repeated productions

so that the overall duration of the unit remains relatively constant" ( Di Simoni, l974d, p.

697). Lehiste ( 1970, p. 23) reviewed a study by Kozhevnikov and Chistovich ( 1965) who

found that the durations of adjacent phonemes are strongly negatively correlated when a

sentence is repeated many times at the same articulatory rate. In other \vords, if a

durational error is made in one phoneme, the error is compensated for in the duration of the following phoneme. Chistovich suggested that the "timing of adjacent phonemes is non independent, but rather that the temporal sequence of articulation must be organized, at least in part, at levels higher than the phoneme" (cited in Lehiste, 1970, p. 23 ).

Allen ( 1973) discussed segmental timing control in speech production. He proposed two kinds of timing control, including a central control which influences

7 speech rate and phonological length, and a peripheral control which includes the neuromuscular control of respiratory, laryngeaL and articulatory muscles. He found that this "neural clock" for timing control is susceptible to small errors such that when a speaker intends to speak at a fixed rate, small, perhaps imperceivable. fluctuations in rate are evident. Also, the clock is assumed to be without memory so these errors are mutually independent. Further, larger speech intervals are liable to more clock errors than shorter intervals. These errors accumulate, leading to greater durational variability.

Thus, larger speech intervals (i.e., a paragraph) repeated over and over would contain greater variability in the segmental durations than would shorter speech intervals (i.e., a sentence). As part of his hypothesis, Allen ( 1973) developed a statistical model which measured the "relative variance" of segmental durations within speech.

This leads to the implication that adult speech is "stress-timed" (Allen. 1972).

Stress-timed speech refers to the manner in which adults regularly time the placement of

"perceptual centers" (P-centers) or loci of "stress beats" in a sequence of words or syllables such that the sequence seems rhythmical (Morton, Marcus, & Frankish, 1976).

Allen ( 1972) had subjects listen to sentences and locate the beats associated with the various syllables in the sentences. He noted that the location of the stress beat was closely associated with, yet slightly preceded, the onset of the nuclear vowel of a stressed syllable. The degree to which the stress beat preceded the onset of the vowel was positively correlated with the duration of the initial consonant of that stressed syllable.

Thus, rhythmic stress beats are focused about a "ballistic release of initial consonants into the stressed vowels" (Allen, 1972, p. 72). This release involves the coordination of respiration, phonation, and articulation for appropriate stress to be executed.

Additionally, Fox and Lehiste ( 1987) observed that as vowel duration increased, as in stressed syllables, the location of the stress beat or "P-center" in the syllable moved away

8 from the vowel onset. This suggests that the location of the stress beat in speech is not constant and depends primarily on the speech context.

Weismer and Ingrisano ( 1979) studied the effect of stress on segment duration in a conversational and fast speaking rate. Subjects produced the sentence "Bob hit the big dog" twenty times in each of five stress conditions: emphasis on each of the four content words and an unstressed or neutral stress condition. In this study, the term "segment" was used to describe CV combinations for each of the four content words. Results indicated that each segment had the greatest duration when it was emphasized.

Additionally, it was generally true that segments adjacent to the segment receiving the emphasis tended to be longer in duration than when displaced further from the location of emphasis. Also, reduction in segment duration varied with the position of the sentence such that when the stress was placed at the beginning of the utterance, segment durations near the end of the utterance were reduced relative to their durations in the neutral stress condition. However, this effect was not observed to occur for segment durations near the beginning of the utterance when the stress was placed toward the end of the utterance.

Finally, at the increased speaking rate, these variations noted at the conversational rate were reduced or eliminated and the utterance-final segment \Vas atlected differently--it was lengthened--from segments in other positions.

Development of Speech Timing In order to more fully understand the phenomenon of speech timing, it is important to study its development. Oller and Smith ( 1977) investigated the developmental aspects of final-syllable lengthening for six normal infants between five and twelve months of age during the production of reduplicative babbling. They found that tinal lengthening was much less pronounced in the infants compared to adult data and concluded that its later occurrence was due to learning.

9 Di Simoni ( l974a) studied the development of vowel duration in different consonant environments of three-, six-, and nine-year-old children. He found that vowel duration variability decreased as age increased and that the children began to produce vowel duration differences relative to consonant environment around age three and continued to develop these differences through age nine.

In another study, Di Simoni ( 1974c) analyzed the duration of the phonemes of

/sis/ in the utterance "Susie said /sis/ again" produced by 28 children of three, six, and nine years of age. This study noted that older children had faster rates of speech and, though temporal accuracy increased \vith age, it was not fully developed even at nine years of age.

Gilbert and Purves ( 1977) studied the development of temporal coordination of consonant clusters in children's speech. Five adults served as controls and five normal children in each of the following age groups served as experimental subjects: (I) 5~0-5~6,

(2) 7~0-7~6, ( 3) 9~0-9;6, ( 4) 11 ;0-11 ~6. They found that the five- and seven-year-olds could roughly be separated from the nine- and eleven-year-olds and adults based on their

absolute consonant durations. They suggested that as children develop neurologically,

they converge on an adult's temporal program, allo\ving them to achieve cluster control.

Such convergence seems to represent an "accommodation" principle discussed by Locke

(1992, pp. 427-428: 1993, pp. 149-162) as involving more than phonetic shifts. He said

that infants accommodate or assimilate various aspects of speakers' voices and speaking

style. He suggested that the significance of vocal accommodation is that it "incorporates

superficial habits of language into the infant's repertoire along with non-linguistic vocal

behaviors" ( 428).

There also seems to be a pattern for developing VOT. This pattern begins

around l year of age when children produce primarily all voiced stops (Eguchi & Hirsh,

1969). According to Kewley-Port and Preston ( 1974 ), the production of voiceless stops

10 requires more precise timing between glottal vibration and supraglottal articulation. As children begin to produce voiceless stops, research has shown that their VOT values are significantly longer than adult values. However, by age three or four these values begin to converge to adult values ( Menyuk & Klatt, 1975~ Zlatin & Koenigsknecht. 1976:

Gilbert, 1977). VOT values continue to decrease to adult values through ages nine and eleven years~ however, until they are fully converged, VOT values are marked bv significant variability (Eguchi & Hirsh, 1969).

Also, research has suggested that while adults' speech IS more "stress-timed," children's speech tends to be more "syllable-timed" (Allen & Hawkins, 1980, p. 232 ).

This may be because children's speech rate is slower and much of children's early speech consists of reduplicated words which are characterized by relatively equal stress on each syllable. By age four or five. as children are able to increase their speech rate and reduce weak syllables, their speech rhythm is more adult-like and, hence, "stress-timed" (Allen

& Hawkins, 1980, p. 233).

Finally, Tingley and Allen ( 1975) studied the development of speech timing control in children. Their study required twenty-two subjects between the ages of five and eleven years old to repeat the phrase "twinkle, twinkle, little star, how I wonder what you are" thirty times the same way each time and to perform a finger tapping task as a control. They followed the same statistical model for analyzing variability of segment duration developed and described by Allen ( 1973 ). As expected, they found that consistency in speech timing improved with age~ the nine- and eleven-year-old children were best able to maintain temporal accuracy in repeating the phrase, representing more advanced timing control as children mature.

I 1 Speech Differences in Stutterers

Studies which have focused on stuttering as a prosodic and/or timing defect have noted differences in stutterers' segmental and suprasegrnental timing durations and in their stress patterns. Di Simoni ( 1974b) acoustically examined the tluent productions of monosyllabic CVC's and of lisi/, lisa/, /asal, and /asi/ of six stutterers (5 males and I female) and six age- and gender-tnatched nonstutterers. Each utterance was repeated 25 times. Results indicated that stutterers' vowel and consonant durations were significantly longer and more variable than nonstutterers.

Research by Hand and Luper ( 1980) revealed similar results. They studied durational measures of vocalic steady states and transitions in VCV bisyllables that were embedded in carrier phrases and produced by stutterers and nonstutterers.

Spectrographic analysis revealed that stutterers displayed significantly longer steady state durations, but shorter transitions to and from consonants than the nonstutterers. This finding is interesting because according to Wingate's ( 1988, pp. 179-185) "fault-line" hypothesis, stutters occur during the transition from initial phone, usually a consonant

(onset), to the sequential phone, usually a vowel or syllable nucleus (rime).

Finally, Cooper and Allen ( 1977) conducted a study in which ten nonstutterers.

tive stutterers no longer in therapy, and five stutterers still in therapy were required to

repeat easy and hard sentences, paragraphs, and nursery rhymes thirty times each and do

a finger-tapping task as a control while temporal accuracy \Vas measured. The subjects

were instructed to repeat each sentence, paragraph, and nursery rhyme the same way each

time. Results showed that speakers tended to vary syllable durations most when

repeating a task over a very long interval as in the paragraph repetition task. Secondly, a

moderately positive relationship was found between the subjects' temporal accuracy on

speech and non-speech (finger-tapping) tasks. Finally, on most speech and nonspeech

tasks, normal speakers exhibited more accurate timing control than the two stuttering 12 groups, and stutterers released from therapy exhibited more accurate timing control than

stutterers still in therapy. Hence, Cooper and Allen ( 1977) suggested that stutterers'

timing programs may be unstable or, on a higher level, that stutterers are less efficient in

formulating timing programs for their utterances. They ( 1977, p. 66) further suggested

that speakers may have a "critical length" of speech segments that they can temporally

program. This "critical length" is limited by the speaker's own memory constraint.

Beyond this program, a speaker must formulate new temporal programs. Cooper and

Allen suggested that stutterers may have shorter critical lengths than nonstutterers which

cause them to reformulate more frequently during an utterance, causing a breakdown in

speech perceived as stuttering. Further, stutterers no longer in therapy may have learned

to extend their critical length or have learned to more efficiently formulate a temporal

program for a given utterance. This would enable them to be more accurate timers than stutterers still in therapy.

Prosody and Stuttering Loci

The timing irregularities noted in these studies are mainly related to the

segmental level of speech production. However, suprasegmental effects also determine

the loci of stuttering occurrence. Brown ( 1938) noted that stutterers tend to stutter more on words that occur at the initial position of sentences and tnore on accented words.

Since then, researchers have conducted studies in an attempt to clarify these findings.

A study by Jayaram ( 1984) examined the influence of clause position on stuttering...... He had stutterers read sentences in which the placement of a clause was varied. Results indicated that more stutters occurred when the clause was placed at the beginning of the sentence. This supported Bro\vn's ( 1938) results indicating that the beginnings of complex sentences place higher demands on the speech system: demands which stutterers cannot meet, contributing to stuttering episodes.

13 The significance of greater stuttering episodes at sentence initial position may be related more to the phenomenon of stress than actual position 1Wingate, 1988). Thus, the issue of stress has become an important variable to examine in the occurrence of stuttering. Healey and Gutkin ( 1984) analyzed VOTs for words in utterance-initial

position, where timing irregularities would be more likely. They used sentences such as

"Pea is a word" and "Do is a word"; thus, the critical monosyllabic word was not only in

sentence-initial position, but it was also the accented word of the carrier phrase. As

discussed earlier, these researchers observed longer VOTs for voiced but not voiceless

stop consonants.

Bergmann ( 1986) found that while stutterers could accurately place stress on

words within sentences, they were more likely to exhibit stuttering episodes on stressed

words than unstressed words of an utterance. Because execution difficulty for stress

seemed to cause stuttering, he concluded that stuttering may represent a speech execution

impairment.

Prins, Hubbard, and Krause ( 1991) were interested in the occurrence of stuttering

relative to syllabic stress peaks in sentences. They had ten stutterers read the tirst two

sentences of the Rainbow Passage (Fairbanks, 1960). Results indicated that stuttering

events occurred on syllabic stress peaks about t\vo times more frequently than on

unstressed syllables. Shapiro (1970; cited in Wingate, 1988, p. 172) found that stutterers evidenced

divergent stress patterns from nonstutterers and had a more variable occurrence of the

first syllable peak than the nonstutterers. Shapiro suggested that these tindings implied

some "anomaly" in the way in which stutterers deal with the "point of attack" for a word

in early utterance position. Kalveram and Jancke ( 1989) analyzed stutterers' voice onset times for stressed

and unstressed syllables under delayed auditory feedback. In this study, 18 male

14 stutterers and 18 male nonstutterers produced the test stimulus "ta-ta-tas," placing the stress on the second syllable. Subjects received both simultaneous auditory feedback and delayed ( 40 ms) auditory feedback. Results indicated that while the stutterers showed significantly longer VOTs, no other test conditions achieved significance.

Jancke ( 1994) followed this study with an investigation of stutterers' variability during the production of stressed and unstressed syllables under different speech rates.

Voice onset time and vowel duration were measured. Subjects ( 18 adult male stutterers and 16 age- and gender-matched nonstutterers) produced the test words [tatatas],

[papapas ], and [kakakas] in a prescribed rhythm, placing the stress on the second syllable

\Vith a controlled fast and slow speech rate. Results showed that the stutterers exhibited greater VOT variability \Vhich was not influenced by speech rate, syllable position, or stress. Additionally, stutterers showed increased variability for VOT and vowel duration during the first syllable. Also, for all subjects, decreasing speech rate yielded longer

VOTs and longer vowel durations.

Finally, De Nil and Brutten ( 1991) proposed that the discrepancy in VOT data for stutterers may be in the extent to which speech-associated time pressure was involved in the experiment. They claimed that whereas time pressure is inherent in voice reaction time (e.g., VIT) research, subjects are usually allowed to respond at their O\vn pace in

VOT research~ hence, time pressure is not present. Since increasing time pressure seems to increase stuttering occurrences, De Nil and Brutten hypothesized that VOT differences would become more evident under conditions of time pressure. Subjects were ten male eight- to twelve-year-old stutterers and ten age- and gender-matched nonstutterers. To create a time pressure condition, subjects were instructed to read nine \vords individually presented on a computer monitor as fast as possible. This condition \vas compared to a no-time-pressure condition for which subjects were allowed to take as much time as needed to read the words on the monitor. Results indicated no significant differences in

15 the mean VOTs bet\veen the stutterers and nonstutterers in the t\vo conditions. In contrast, stutterers did exhibit significantly greater VOT variability than the nonstutterers, though this difference did not depend on the level of imposed time pressure. These investigators concluded that the difference in results noted in VOT research is due to factors not related to time pressure.

Thus, these studies have suggested that stutterers exhibit abnormal timing of segmental and suprasegmental aspects of speech. These timing abnormalities may be imperceivable or may be evident as a stuttering event, especially in the context of stressed words and syllables.

VOT: A Closer Look

Voice onset time, a segmental measurement, is a temporal feature of stop consonants which plays a major role in distinguishing initial voiced and voiceless stop consonants in English as well as in other languages. Lisker and Abramson ( 1964) discussed stop consonants as falling into three production ranges, including negative ranges, short-lag positive ranges ( VOT of 0 to + 25 msec. ), or long-lag positive ranges

( VOT of +60 to I 00 msec. ). In English, voiced stops typically fall in the negative and short-lag positive ranges and voiceless stops in the long-lag positive range. Production of

VOT contrasts requires the complex timing and coordination of glottal and supraglottal events. At this time, however, it is not known if VOT control stems from a peripheral motor ability (i.e., coordination of respiratory, phonatory, and articulatory muscles) or from a more central area responsible for timing control (i.e .. "neural clock") or from a combination of peripheral and central factors.

As discussed earlier, VOT values vary with age~ however, they may also vary with other factors. For example, Lisker and Abramson (1967) noted that VOT values tend to increase as the place of articulation for the stop consonant moves posteriorly such that

16 VOT for /k, gl is greater than It, dl which is greater than /p, b/. These researchers also observed longer VOTs in isolated words than in sentences and longer VOTs for ,ptk/ stop consonants initiating stressed than unstressed syllables. Voice onset times for , ptkl ranged from 59 msec. to 84 msec. (depending on place of articulation) in stressed syllables and from 38 msec. to 55 msec. in unstressed syllables. Additionally, Klatt

( 1975) noted longer VOTs before sonorants and high vowels than before mid- and low vowels: Port and Rotunno ( 1979) reported longer VOTs for word-initial consonants before voiceless final clusters than before single nasals, and before tense vowels than lax vowels: and, Summerfield and Haggard ( 1972) (in Hillman & Gilbert, 1977) showed that

VOT varies inversely with speaking rate (i.e., VOT decreases with an increase in speaking rate). Finally, Swartz ( 1992) reported significantly shorter VOT \alues for men than \vomen for the phonemes It/ and /d/. Thus, these studies have shown that VOT is not an independent measure~ it interacts dynamically with age, gender, phonetic context, speaking rate, and stress.

Statement of the Problem With respect to the literature describing timing differences bet\veen children and adults (Oller & Smith, 1977~ Di Simoni, 1974 a,c: Gilbert & Purves, 1977: Eguchi &

Hirsh, 1969~ Kewley-Port & Preston, 1974: Menyuk & Klatt 1975: Zlatin &

Koenigsknecht, 1976: Gilbert, 1977: Allen & Hawkins, 1980: Tingley & Allen, 1975), further studies might determine if the same factors which influence VOT values in adults also influence VOT values in children. The purpose of this study was to investigate the interaction between VOT and word stress in children. Two hypotheses were tested: ( 1)

VOT \viii be significantly longer in stressed syllables than in unstressed syllables for all three subject !:,TfOups and (2) VOT values will be significantly longer in the children \vith greater differences in the younger group of children.

17 CHAPTER III

~THODSANDPROCEDURES

Subjects

Subjects for the study were obtained from friends of this investigator. Three groups of subjects were used: (1) an adult group, ranging in age from 22:6 to 36: II (five males, five females), (2) a younger group of children, ranging in age from 6:3 to 7:9 (four males, six females), and (3) and older group of children, ranging in age from 9:11 to 10:9

(three males, seven females). Ten subjects were included in each group, yielding a total of 30 subjects. All subjects had no reported history of speech, language, or hearing difficulties.

Speech Sample

The subjects were presented demonstration and test sentences each typed on a

3112 x 5 inch index card. They were instructed to read each sentence, placing stress or emphasis on the word in bold-faced print and inhaling before each sentence. For the test stimuli, a voiceless stop consonant \vas chosen because research has shown that the production of voiceless stops requires more precise timing between glottal and supraglottal structures, and that their VOTs produced by children are not fully converged to adult values until about age eleven. Until then, VOTs tend to be longer and more variable. Also, the middle consonant [t] and the central vowel [/\] were used to control for the effect of place on VOT.

18 The demonstration and test sentences used were:

DEMONSTRATION SENTENCE:

I need my pen.

TEST SENTENCES:

Stressed: I bit my tongue today.

Unstressed: I bit my tongue today.

Instrumentation

Subjects' speech samples were recorded into a Sony cassette recorder ( TCM

S66V). A mic-to-mouth distance of 30 em was maintained for each subject. For analysis, the signal was played into and transduced by an omnidirectional microphone.

The Computerized Speech Lab (Kay Elemetrics Corporation) computer software then digitized the signal at a rate of 10,000 Hz and provided broad-band spectrograms of the

[tJ\] portion of each sentence. Cursors were placed at the point of stop release evident by a sudden spike or amplitude variation in the wave and at the initiation of phonation evident by the beginning of the quasi-periodic waveform of the vowel. The time between the cursors was defined as the voice onset time.

Procedures

Each sentence (demonstration and test) was typed on a 3ti2 x 5 inch index card.

The subjects read the sentences from the cards so that each subject would produce his/her own stress pattern and not one imposed by the investigator such as in an imitation task.

The test sentences were presented only after adequate performance on the demonstration sentence. Subjects \vere required to correctly place the stress on the bold-faced word, inhale before each repetition, and repeat the sentences ten times without giggling or starting over. If a subject did not adequately produce a test sentence, it was repeated

19 until ten consecutive and adequate productions were achieved. Subjects were shown the sentences with the unstressed condition first and instructed to read the printed sentence ten times~ making the "darker word louder" and with an inhalation before each repetition.

Sentences with the stressed condition were then presented and subjects were instructed to again read the printed sentence aloud ten times (making the "darker word louder" and with an inhalation before each sentence).

Additionally, in order to prevent a beginning and end effect for each sentence production, the VOT from the third, fifth, and seventh repetitions were analyzed. Thus, a total of six VOTs were analyzed for each subject.

Statistical Design

An analysis of variance (ANOV A) was calculated to assess the interaction of

VOT for normal children producing [t!\] in stressed and unstressed conditions. Three predictions were made for this study: ( 1) VOTs in the stressed syllables \vill be significantly longer for all groups of subjects and (2) VOTs will be significantly longer for both groups of children than for the adults with greater differences in the younger group of children

Reliability

To assess intrajudge reliability, all six syllables from 12 of the 30 subjects ( 72 syllables) were randomly selected, duplicated spectrographically, re-measured by the author, and compared with original measurements. Five of the 12 consisted of adults, four consisted of I 0-year-olds~ and three consisted of 6-7-year-olds. Thus~ 40o/o of the total syllables (72 of 180) were re-measured. The average intrajudge measurement error for VOT was 4.61 msec. Measurement differences ranged from .13 msec to 18.3 msec.

20 CI1APTER IV

RESULTS

The means and standard deviations for the three subject groups are displayed in

Table 1. A repeated measures analysis of variance (ANOVA) indicated a significant main effect for condition (F = 21.34, df = L p < 0.0001) and for group (F = 6.94. df = 2. p< 0.0013). However, no significant effect for the interaction of group and condition was observed (F = 0.89, df= 2, p < .4143). A Scheffe post hoc analysis showed that the mean

VOT was significantly longer for six-seven-year-old children compared to adults.

Ho\vever. ten~year-old children's VOT was not significantly different from adults' or six- seven-year-old children's.

Table 1. I\1eans and standard deviations of [t'\] produced by three subject !,'TOups in stressed and unstressed conditions.

GROUP CONDITION N MEAN so

adults stressed 30 80.670 msec 27.515 msec

adults unstressed 30 56.593 msec 23.152 msec

10 year olds stressed 30 86.220 msec 28.082 msec

10 year olds unstressed 30 69.277 msec 16.012 msec

6-7 vear_, olds stressed 30 91.843 msec 29.410 msec

6-7 year olds unstressed 30 80.123 tnsec 26.599 msec

21 CHAPTER V

DISCUSSION

The results of this study indicate that VOT was significantly longer for all subject groups for the stressed syllables. Additionally, the six-seven-year-old children evidenced

significantly longer VOTs from the adults~ however, the ten-year-old children were not

significantly different from the six-seven-year-old children nor the adults. These results

suggest that for six-seven-year-old children and for ten-year-old children VOT is

influenced by stress. Thus, as in adults, VOT is significantly longer in stressed syllables

for children. Further, the results suggest that the younger group of children exhibited an

immature timing control for speech production. However, the older group of children tended to produce a more adult-like timing pattern.

Comparing these results to previous studies, several similarities and differences

were observed. First, the mean VOT values obtained for the adult subject group were

similar to the ranges noted by Lisker and Abramson ( 1967). The mean VOT for stressed

syllables for adults in this study was 80.670 msec. According to Lisker and Abramson

( 1967), VOT in stressed syllables ranged from 59 msec to 84 msec. For unstressed

syllables, the mean VOT for adults in this study was 56.593 msec which fell slightly

above the range of 38 msec to 55 msec obtained by Lisker and Abramson (1967). Thus,

results of this study agree with results and values of Lisker and Abramson's study.

Eguchi and Hirsh ( 1969), Kewley-Port and Preston ( 1974 ), and Gilbert ( 1977)

noted greater VOT variability in children. Additionally, Di Simoni ( 1974a) and Tingley

and Allen ( 1975), using other temporal measurements, reported greater variability in

children. No statistical test of variance between subject groups was completed for this

study. However, the standard deviation for all subjects were comparable to standard

deviations reported by Zlatin and Koenigsknecht ( 1976). For voiceless stops, they noted 22 standard deviations ranging from 23.67 to 25.66 msec for adults and from 23.23 to

25.86 msec for six-year-old children. The odd standard deviation for the ten year olds for the unstressed condition was somewhat lower on casual inspection ( 16.012 msec) compared to 23.142 msec for the adults in the unstressed condition and 26.599 msec for the six-seven-year-olds. There is no apparent explanation for this finding. It was observed in this study that all subjects seemed to exhibit wide variation with a tendency to produce greater variability in the stressed syllable condition. Wide variability among all subjects was also reported by Eguchi and Hirsh ( 1969), Lisker and Abramson ( 1965), and Preston, Yeni-Komshian and Stark ( 1965) (in Eguchi & Hirsh, 1969). Kewley-Port and Preston (1974) commented on this variability and proposed that the production of voiceless stop consonants is more difficult than voiced stop consonants. According to these investigators,

For the long voicing lag stops, English /tJ can serve as a representative example... 90o/o of the It! stops [fall] within a 50 ms VOT interval. This indicates that the adult articulation of It! involves the very careful control of timing between the supraglottal and glottal articulators, which ... are separately innervated. This precise timing constraint is not necessary tor the short lag stops. Although the VOT range is about 20 ms, adduction of the vocal folds can be achieved any time during apical closure. For those languages containing voicing lead stops ... the range of VOT values [is] approximately 90 ms. Thus, timing between glottal and supraglottal articulators seems more carefully controlled for long voicing lag stops than for voicing lead or short voicing lag stops. (p. 204)

Kewley-Port and Preston also provided evidence from EMG studies that supported their hypothesis. These researchers suggested that this hypothesis helps explain the greater

VOT variability noted for voiceless stop consonants. If the production of voiceless stops is more complex. perhaps the addition of the prosodic component of stress in the present study further increases complexity. This would explain the slightly increased VOT variability noted for stressed syllables. 23 The results from this investigation also agree with studies by Gilbert ( 1977) and

Menyuk and Klatt ( 1975) with respect to longer VOT values for voiceless stop consonants for children. On the contrary, Zlatin and Koenigsknecht ( 1976) noted shorter

VOTs for children than for adults. Differences may be due to methodology: Gilbert

(1977) extracted [d] and [t] for VOT analysis from three year olds spontaneous speech~

Menyuk and Klatt ( 1975) studied three and four year olds in a word and sentence repetition (i.e., imitation) task~ Zlatin and Koenigsknecht ( 1976) analyzed VOTs for two and six year olds via a picture identification task~ and, in this study, [t/\] was elicited from six-seven and ten year olds in a reading task. The children's different VOT values

(whether longer or shorter) may be due to the increased complexity of voiceless stop consonants. Children's VOTs may be longer as noted in this and other studies because they are overcompensating for the increased difficulty of voiceless stop consonants.

Kewley-Port and Preston ( 1974) have hypothesized that voiceless stops require more precise timing between glottal and supraglottal articulators. In developing this timing control, children may overestimate the voicing lag for voiceless stops in an attempt to produce voicing contrasts in their speech. Likewise, there may be a time in which children underestimate the lag time, explaining the shorter VOTs reported by Zlatin and

Koenigsknecht ( 1976 ). Further studies may explore these possibilities.

In this study, the ten-year-old children did not show significantly different VOTs from the six-seven-year-old children nor from the adults. Their VOT values tended to fall in the middle of the ranges from the six-seven-year-old children and adults. This finding lends support to other research (Di Simoni~ 1974 a,c, Gilbert & Purves, 1977, and

Tingley & Allen, 1975) that has noted more advanced timing control as children mature.

This study supports the idea that as children develop neurologically, they "converge" on

(Gilbert & Purves, 1977) or "accommodate" (Locke, 1992, 1993) to adults' temporal

24 program, allowing children, in this study, to achieve greater temporal accuracy for voice

onset time.

Research Implications

The results of this investigation indicate that children and adults are able to

produce VOT contrasts relative to stressed syllables. It is noteworthv that while all

subject groups produced significantly longer VOTs in stressed syllables, the difference

between mean VOT for stressed and unstressed syllables seemed to decrease as age

decreased. Allen and Hawkins ( 1980) noted that, while adults' speech is more "stress

timed," children's speech tends to be more "syllable-timed." They further claimed that by

age four or five children are able to increase their speech rate and reduce weak syllables.

With these skills, their speech rhythm "converges" to a more adult-like speech rhythm,

becoming "stress-timed." Thus, future studies may investigate the influence of stress on

VOT in younger groups of children. Perhaps children who are younger than four years

who exhibit syllable-timed speech would be unable to produce significantly longer voice

onset times in stressed syllables. In such an experiment, a research design as employed

in this study could not be applied since the children would not have learned to read. An

imitation type design would have to be used~ however, with such a method the

investigator's own VOT may influence the children's VOT through immediate

convergence. It may be possible to use a deferred imitation task in which the children

repeat the investigator's stimulus following an imposed period of time. In order to do this, research would be needed regarding the length of time after which children's speech

is no longer influenced by immediate convergence. Future studies may also investigate gender differences in VOT. While this study

in,cluded males and females, it did not separately analyze VOT according to gender.

Swartz (1992) noted that men exhibit significantly shorter VOT values for the phonemes 25 [t] and [d]. This finding may or may not be relevant to the fact that many more males than females stutter. Studies may explore the interaction of gender and VOT in children to determine if a relationship exists.

Additionally, future studies may explore the influence of stress on VOT in stutterers. Researchers have noted that occurrences of stuttering coincide with syllable stress. Bergmann ( 1986) discussed three reasons why stuttering might be related to stressed syllables. First, he stated that overall timing patterns of sentences are determined by stressed syllables, affecting overall fluency. Thus, if stuttering is a timing disorder as Van Riper ( 1982) and Cooper and Allen ( 1977) suggested, then stuttering episodes may be evident on stressed syllables since they determine overall timing patterns. Second, Bergmann stated that greater articulatory precision is required for stressed syllables, posing greater constraints in motor execution for speakers. Finally,

Bergmann cited research that suggested emotional expression in speech is executed mainly through stressed syllables and that its expression is important in not only linguistic but also pragmatic aspects of communication. Because of the importance of stress in communication, its expression requires accurate control and coordination of respiration, phonation, and articulation. If stutterers exhibit difficulty achieving the motor precision required for stressed syllables, then a breakdown evidenced as stuttering may occur. As discussed, achieving control of VOT also requires complex motor timing and coordination and stutterers have been reported to exhibit longer VOTs than nonstutterers. Other studies, however, have noted little or no significant differences in VOTs between stutterers and nonstutterers.

De Nil and Brutten ( 1991) proposed that this was due to time pressure, though the results of their study failed to support this hypothesis. Another explanation may be that previous stuttering research has focused too much on this segmental feature without considering the variables that may influence it. Since segmental and suprasegmental aspects of

26 speech are highly interdependent, it is important to consider both, especially when conducting stuttering research. Three VOT studies that were reviewed have attempted to consider these two aspects of speech. Healey and Gutkin ( 1984) analyzed words placed in the initial position where they would also be the accented \vord of the sentence and found significant results for voiced but not voiceless stop consonants. However, thev did not specifically examine VOT and stress since their study did not include the critical words in an unstressed condition. Additionally, Kalveram and Jancke ( 1989) analyzed

VOT in stressed and unstressed conditions. Though stutterers exhibited significantly longer VOTs in both stressed and unstressed conditions, no significant differences were noted for the stutterers between the stressed and unstressed conditions. Jancke (1994) extended this study to consider speech rate, noting greater VOT variability for stutterers independent of speech rate, syllable position, and syllable stress. Their (Kalveram &

Jancke, 1989; Jancke, 1994) test stimuli, however, were unnatural, nonsense syllables.

Thus, in addition to considering segmental and suprasegmental aspects of speech, it is important that test stimuli be real words and sentences rather than nonsense syllables that lack propositional content as well as linguistic complexity.

Thus, a future study might investigate the influence of stress on voice onset time in stutterers, using stressed and unstressed conditions and real-word stimuli. Since both

VOT and stress execution require a high level of precision and coordination between semantic and prosodic requirements as well as motor level respiration, phonation, and articulation, it may be hypothesized that VOT differences between the stutterers and nonstutterers will be greater on stressed rather than unstressed syllables. Alternatively, if no VOT differences are noted between the two conditions, then other variables might be responsible for the VOT differences observed by researchers. Additionally, since VOT is a developmental feature and stuttering is a developmental disorder, then studying the disorder in children still acquiring VOT control appears to be an important avenue for

27 research. Perhaps children who stutter have greater difficulty developing or achieving the timing control required to produce VOT contrasts.

One limitation of this study was that speech rate was not controlled. Summerfield and Haggard ( 1972, in Hillman & Gilbert, 1977) and Jancke ( 1994) noted longer VOTs with slower speech rates. Future studies may consider controlling for this variable.

A second limitation of this study was the manner in which data were collected and analyzed. For the convenience of the subjects, speech samples were recorded into a cassette tape recorder. Using this method, aliases are introduced into the data analysis procedure, decreasing the study's internal validity. While a direct line from the recorder to the computer was attempted, the computer would not accept data in this manner.

Future studies may replicate this investigation, however, one could increase internal validity by directly entering the sample into the computer.

Another threat to internal validity may have been introduced by this investigator when analyzing data. To minimize this effect, this investigator documented only the beginning and ending points used to determine VOT. Later, the difference bet\veen the two points was subtracted in order to find VOT. Thus, VOT values were unknown during the actual time of spectrogram analysis. Additionally, this investigator followed the same procedure in re-measuring 40°/o of the samples to determine intrajudge reliability. As stated previously, intrajudge reliability was 4.61 msec which was judged to be fairly low. Additional studies should include a measure of interjudge reliability to decrease the compounding effects of researcher bias. In summary, this research sought to investigate the influence of stress on VOT in children. It was noted that children and adults produced significantly longer voice onset times in stressed syllables. Also, the younger group of children produced significantly longer VOTs than the adults; however, VOT for the older group of children was not significantly different from the younger group of children nor the adults. 28 REFERENCES

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