Clinical Linguistics & Phonetics, February 2015; 29(2): 115–130 ß 2015 Informa UK Ltd. ISSN: 0269-9206 print / 1464-5076 online DOI: 10.3109/02699206.2014.966393

Lexical tone and stuttering loci in Mandarin: Evidence from preschool children who stutter FANG-CHI CHOU1,2, PATRICIA ZEBROWSKI1, & SHU-LAN YANG3 1

Department of Communication Sciences and Disorders, The University of Iowa, Iowa City, IA, USA, Department of Audiology and Speech-Language Pathology, Mackay Medical College, New Taipei City, Taiwan, and 3Department of Special Education, National Pingtung University of Education, Pingtung City, Taiwan 2

(Received 3 January 2014; revised 12 September 2014; accepted 14 September 2014)

Abstract The purpose of this study was to examine the relationship between stuttering loci and lexical tone in Mandarin-speaking preschoolers. Conversational samples from 20 Taiwanese children who stutter (CWS; M ¼ 4:9; range ¼ 3:2–6:4) were analysed for frequency and type of speech disfluency and lexical tone associated with stuttering-like disfluencies (SLDs). Results indicated that SLDs were significantly more likely to be produced on Mandarin syllables carrying Tone 3 and Tone 4 syllables compared to syllables carrying either Tone 1 or Tone 2. Post-hoc analyses revealed: (1) no significant differences in the stuttering frequencies between Tone 1 and Tone 2, or between Tone 3 and Tone 4, and (2) a higher incidence of stuttering on syllables carrying Tone 3 and Tone 4 embedded in conflicting (as opposed to compatible) tonal contexts. Results suggest that the higher incidence of stuttering on Mandarin syllables carrying either Tone 3 or 4 may be attributed to the increased level of speech motor demand underlying rapid F0 change both within and across syllables.

Keywords: Lexical tone, Mandarin, preschoolers, stuttering loci

Introduction Numerous studies have examined the impact of linguistic factors on stuttering loci in the speech of both children and adults who stutter. Research has shown that stuttering instances are more likely to be found on (1) stressed syllables, (2) long words, (3) content words for adults and school-age children, (4) function words for preschoolers, (5) the initial words of sentences, (6) the first syllables of words, and (7) words beginning with consonants (e.g., Au-Yeung, Howell, & Pilgrim, 1998; Bloodstein & Gantwerk, 1967; Brown, 1945; Helmreich & Bloodstein, 1973; Wingate, 1984, 1988). Although descriptions of the stuttering occurrence in relation to linguistic factors

Correspondence: Fang-Chi Chou, Department of Audiology and Speech-Language Pathology, Mackay Medical College, New Taipei City, Taiwan. Tel: 886-2-26360303-1517. E-mail: [email protected]

116 F.-C. Chou et al. have been extensively investigated, most have involved speakers of non-tonal languages such as English (e.g., Au-Yeung et al, 1998) and German (e.g., Natke, Grosser, Snadrieser, & Kalveram, 2002); only a few have involved speakers of tonal languages such as Mandarin (e.g., Yang, 2002). Linguistic stress is a prosodic feature of language that has long been implicated in the location of stuttered disruptions within an utterance (i.e. stuttering loci; Brown, 1938; Wingate, 1984, 1988). Both theoretical models and direct observation of stuttering loci in English- and Germanspeaking adults have converged in support of the ‘‘stress effect,’’ namely, that stuttering events are more likely to be produced on stressed than on unstressed syllables (Bergmann, 1986; Brown, 1938; Harrington, 1986; Natke et al., 2002; Prins, Hubbard, & Krauss, 1991; Wingate, 1979, 1984). For the most part, researchers have attributed the stress effect to the rapid changes in fundamental frequency (F0), vocal intensity and syllable duration that characterise syllabic stress, all of which place increased demands on the speech production system (Clark & Yallop, 1995). It has been speculated that these increased motor demands provide a source of variability that disrupts the vulnerable speech motor systems of people who stutter. The result is a high likelihood for stuttering to be produced on stressed syllables (Packman, Onslow, Richard, & Van Doorn, 1996; Zimmermann, 1980; Zimmermann & Hanley, 1983). Recently, researchers have become interested in expanding the work in stuttering and linguistic stress along two fronts: across developmental stages and languages. For example, Natke, Sandrieser, van Ark, Pietrowsky, and Kalveram (2004) observed that the stress effect in young German-speaking children who stutter (CWS) is restricted to function words in the speech of preschoolers who stutter. Moreover, Natke et al. (2004) reported a relationship between disfluency type and four different stress patterns or categories; unstressed, short stressed, intermediate stressed and long stressed. Most sound prolongations or sound/syllable repetitions were frequently produced on short stressed syllables, whereas monosyllabic whole-word repetitions occurred frequently on syllables receiving intermediate stress. To extend the study of the stress effect on stuttering loci beyond Western European to Asian languages requires a comparison of linguistic stress and lexical tone. For example, Mandarin is one of the most widely spoken languages in the world (Gordon, 2005) and has as one of its unique characteristics lexical tone or tone, a distinctive prosodic feature that is comparable to stress in Western European languages such as English and German. As such, Mandarin is considered a ‘‘tone language’’ in that tones are used to distinguish words and morphemes that are identical in their phonological structure (Xu, 1993). Mandarin uses four lexical tones and a neutral tone. Each lexical tone is characterised by a specific F0 dynamic across a syllable: Tone 1(T1; high-level F0), Tone 2 (T2: high-rising F0), Tone 3 (T3: dipping F0), and Tone 4 (T4: high-falling F0). In contrast, the neutral tone (T0) is generally defined as having no specific F0 targets (Chao, 1968). Mandarin tone and English lexical stress have some similarity in linguistic characteristics and physiology. First of all, tone, like stress, distinguishes word meaning. That is, changing the tone of a syllable changes its meaning. For example, the Mandarin word ma produced with T1 (high-level F0) means ‘‘mother,’’ whereas ma produced with T3 (falling-rising F0) means ‘‘horse’’ (Table 1). Second,

Table 1. The pitch codes of Mandarin tones. Tone Tone Tone Tone Tone

1 2 3 4

Description

Citation forms

High level High rising Falling-rising/dipping High falling

55 35 214 51

Example Ma– Ma0 Maˇ Ma1

Mother Hemp Horse Scold

Lexical tone and stuttering loci in Mandarin 117 both tone and stress are manifested through the variation of F0 duration and intensity during syllable production (e.g. Clark & Yallop, 1995; Levelt, 1989; Ong & Yang, 1997; Xu, 1997; Whalen & Xu, 1992). Similar to stressed syllables, Mandarin syllables that are produced with different lexical tones are longer and louder and are produced with a varying F0 when compared to unstressed syllables or syllables with T0 (Chao, 1968; Chen & Xu, 2006). Third, although the representation of tone in the phonological encoding process has been a controversial issue in linguistics, some research results have suggested that Mandarin tones are represented and processed similarly to stress in English (e.g. Chen, 1999; Chen, Chen, & Dell, 2002). For example, Chen (1999) modified the phonological processing model developed by Levelt (1989) and proposed that tone, like stress, is initially detached from a syllable and represented as a diacritical feature that is passed down to the phonetic level, at which point it is assigned to the vowel in the syllable. The resulting phonetic configuration is characterised by a distinctive pitch contour or tone (e.g., high-rising). Given the similarities between stress and tone, we speculated that we would observe a relationship between Mandarin lexical tone and stuttering loci that is similar to the stress effect observed in English and German. Specifically, we hypothesised that stuttering events would be more likely to occur on Mandarin syllables produced with tones of relatively higher variation in F0, greater intensity, and longer duration. Mandarin tones To explain the basic nature of each tone and speculate about the possible influence of Mandarin tones on stuttering loci, this section provides a description of the main characteristics of Mandarin tones, including tone contour/F0 contour, tone sandhi and coarticulation, and the acquisition and development of Mandarin tones in children. The present study focused on Taiwan Mandarin, a Mandarin dialect spoken in Taiwan. Given that research has shown that Taiwan Mandarin is distinct from Standard Mandarin spoken in China in tone production (Shih, 1988; Yang, 2013), the major differences will also be mentioned below. Tone contour/F0 contour Mandarin tones are identified by changes in F0, intensity and duration (Chao, 1968; Xu, 1997, Ong & Yang, 1997). Tone contour/F0 contour serves as the primary cue for both the production and perception of tone (Fu & Zeng, 2000; Wong, 2012a). Each tone (T1–T4) has a distinctive tone contour; however, T0 does not. Thus, syllables carrying T0 are generally defined as ‘‘toneless’’ (Yip, 2002). Chao (1968) indicated that the F0 of T0 in Standard Mandarin depends on the tone that precedes it (Chao, 1968). For example, the F0 of T0 will be high when preceded by a syllable carrying T3 (/maiˇ-le/, T3 + T0; ‘‘bought it,’’ ) and the F0 of T0 will be low when preceded by a syllable carrying T4 (/mai1-le/, T4 + T0; ‘‘sold it,’’ ; Chao, 1968). T0-carrying syllables are less frequent in Taiwan Mandarin, and many of the prescriptively T0-carrying syllables are produced with lexical tones (T1–T4), such as /woˇ men/ (‘‘we,’’ ), which is produced as T3 + T0 in Standard Mandarin but T3 + T2 in Taiwan Mandarin (Duanmu, 2007; Huang, 2012). In addition, Huang (2012) reported that, unlike Standard Mandarin, many T0-carrying syllables in Taiwan Mandarin are not toneless but have a mid-low pitch target regardless of the preceding tones; thus, T0 may be considered the fifth lexical tone in Taiwan Mandarin. However, given that the study was based on read speech, Huang’s (2012) research suggests that empirical evidence from studies adopting spontaneous speech is needed to confirm that T0 is not toneless in Taiwan Mandarin. Given that T0-carrying syllables are less frequent in Taiwan Mandarin, the delineation regarding tone contours and other characteristics for Mandarin tones will focus on T1–T4 in the following sections.

118 F.-C. Chou et al. Tone 1 (247 ms)

5

Tone 2 (273 ms) Tone 3 (349 ms)

F0

4 3 2 1

Tone 4 (214 ms) Time

Figure 1. Time-pitch graph for Mandarin tones. Based on data from A grammar of spoken Chinese, by Y. R. Chao, 1968, Berkeley and Los Angeles: University of California Press, and Contextual tone variations in Mandarin, by Y. Xu, 1997, Journal of Phonetics, 25, 61–83.

When Mandarin syllables are produced in isolation, their tone contours are stable. Chao (1968) developed a five-point scale to delineate Mandarin lexical tone contours, called tone citation forms. As shown in Table 1, the pitch register is shown in a range of numbers 1 to 5, where 1 represents the lowest and 5 the highest pitch register. Each lexical tone has a specific tone citation form. For example, the tone citation form for T1 is 55, which indicates that the pitch register remains at level 5 throughout the syllable. On the other hand, T3 is assigned a tone citation form of 214, which means the pitch register starts at 2, lowers to 1, and then rises to 4 across the syllable. A schematic illustration of tone contours is presented in Figure 1. Again, note that T1 (55) is produced with a non-varying tone contour and T2 (35) with a rising tone contour. T3 (214) is a combination of a falling and a rising tone contour, while T4 (51) is produced with a sharp falling tone contour. The duration and intensity of tones co-varies with F0. As shown in Figure 1, across T1 to T4, when produced in single syllables T3 is the longest (349 ms) and T4 is the shortest (214 ms; Xu, 1997). Further, T3 is produced with the lowest intensity of the four tones, whereas T4 is produced with the highest intensity (Ong & Yang, 1997). Tone sandhi and tonal coarticulation Similar to linguistic stress, tones are modified in connected speech while word meaning is maintained. These context-dependent acoustic changes are referred to as ‘‘tone sandhi’’ and ‘‘tonal coarticulation’’. Both tone sandhi and tonal coarticulation reflect the process of tone transition. Tone sandhi refers to post-lexical changes on tone targets or tone contours (Chen, 2000; Xu, 2009). For example, a tone may be produced as other tones or a different tone contour depending on its surrounding tones. There are three primary tone sandhi rules (Li & Thompson, 1981). T3 is the tone most frequently modified by tone sandhi, in that it is directly affected by two of the three basic rules (Li & Thompson, 1981). (a) Tone sandhi rule 1: If a syllable with T3 is followed by a syllable with T1, T2, or T4, the falling-rising tone changes to low- falling (i.e., 214 ! 21; Half-Third Tone sandhi), as in /xiaoˇ xin–/, ‘‘be careful’’. (b) Tone sandhi rule 2: A T3 syllable changes to T2 when it is followed by another T3 syllable, as in /baoˇ xianˇ/, ‘‘insurance’’. (c) Tone sandhi rule 3: A T2 syllable changes to T1 when it is preceded by T1 or T2 and followed by any of the other four lexical tones, as in /cong– you2 bingˇ/, ‘‘green onion pancake’’.

Lexical tone and stuttering loci in Mandarin 119 The pronunciation of T3 is particularly different in Taiwan Mandarin and Standard Mandarin (Shih, 1988). The final rising of T3 is always absent in word-final position in Standard Mandarin because of the half-third tone sandhi (Chao, 1968, Xu, 1993). However, T3 is often produced without the final rising part regardless of word position (Kubler, 1985; Shih, 1988). Tone coarticulation refers to the variations in tone that are strictly conditioned by context without changing tone target. It is a widespread phenomenon occurring in every tone sequence. As long as there is no pause between two adjacent syllables, the F0 contour of a tone in a tone sequence is substantially influenced by its surrounding tones (Xu, 1997). Tone coarticulation can assume either carry-over or anticipatory forms (the carry-over or anticipatory effect; Chen, 2000; Xu, 2004). A tone generally can be divided into an onset and offset portion, each produced with a specific F0 (referred to as onset or offset F0). The magnitude of F0 change within a tone combination depends on the similarity between the onset F0 of a tone and the offset F0 of the preceding tone, or the phonetic context/environment. The carry-over effect refers to the influence from the preceding tone on a target tone; the onset F0 of the target tone is affected by the offset F0 of its preceding tone. In contrast, the anticipatory effect refers to the influence from the following tone on a target tone; the offset F0 of the target tone is affected by the onset F0 of the following tone (Xu, 2004). Further, a tone combination can be described as either ‘‘compatible’’ or ‘‘conflicting’’ according to its phonetic environment or tonal context (Xu, 1994, 1997). A ‘‘compatible context’’ is a phonetic environment in which the adjacent F0 offset and onset values are identical or similar. A ‘‘conflicting context’’ is a phonetic environment in which they are substantially different (Xu, 1994, 1997). For example, a tone combination of T2 (35) + T4 (51) is a compatible context because the offset F0 of T2 is identical to the onset F0 of T4 (e.g., /shi2 sui1/, ‘‘ten years old’’). A tone combination of T4 (51) + T1 (55), however, is a conflicting context because of the large difference between the F0 offset and onset values (e.g., /ba1 gong–/). The resulting F0 variation associated with conflicting tone contexts is the result of rapid laryngeal adjustment; for that reason, conflicting contexts are considered to place increased demands on the speech motor system (Xu, 1994). Based on the definition, tonal combinations with the compatible tonal context are T1 + T1, T1 + T4, T2 + T1, T2 + T4, T3 + T2, T4 + T2, and T4 + T3. Tonal combinations with the conflicting context are T1 + T2, T1 + T3, T2 + T2, T2 + T3, T3 + T1, T3 + T3, T3 + T4, T4 + T1, and T4 + T4 (Xu, 1994; He, 2010). Given tone sandhi rule 2, the tonal combination T3 + T3 is changed to T2 + T3; thus, it should be considered a conflicting tonal context. Tone acquisition and development in children Results from investigations of tone development in Mandarin-speaking children have been equivocal. This conclusion is based on several factors. First, a number of existing studies are characterised by inadequate transcription reliability or insufficient power, or both. Second, there is considerable variation across studies in the criteria used to determine the emergence and stabilisation of tone (Wong, Schwartz, & Jenkins, 2005). Regardless, some consistent trends regarding the mastery age and order of tone production have appeared in the literature. For example, it has been observed that Tones 1 and 4 are acquired before Tones 2 and 3 (Li & Thompson, 1977; Zhu, 2002). Acquisition of T3 is also affected by the use of the second tone sandhi rule (see previous discussion); that is, in cases where T3 and T2 are combined, children frequently exhibit errors both in auditory discrimination between the two and in production of T3 (Wong et al., 2005). In general, tone production is mastered between the ages of two and three in Mandarinspeaking children (Chang, 1991; Zhu & Dodd, 2000). Evidence that refutes this observation has been provided by Wong in a series of studies that compared Mandarin tone acquisition in three- to five-year-olds growing up in Taiwan, and three- year-old children growing up in the United States. Both groups of children were from homes in which Mandarin was the primary

120 F.-C. Chou et al. language spoken (Wong, 2008, 2012a, b, 2013; Wong, et al., 2005). Findings revealed that children from both groups followed a similar order of tone acquisition: T4, T1, T2, and T3; however, rather than achieving mastery by age three, results indicated a protracted acquisition process in which three- year-olds from both groups produced tones in monosyllabic words (i.e., isolation) with limited accuracy (Wong, et al., 2005), while five- year-olds continued to produce disyllabic words (tones produced in combination) with a high degree of inaccuracy (Wong, 2008). Tone mastery and the order of acquisition have been attributed to the relative complexity of laryngeal coordination that characterises each, particularly of crictothroid (CT) and sternohyoid (SH) activity (Wong, 2012a, b, 2013). For example, T4 (first acquired; least complex) is produced with a falling contour that results from the initial contraction of one primary muscle (CT) to achieve a high F0, followed by relaxation to sharply lower it (falling contour; Halle, 1994; Sagart, Hall, Boysson-Bardies, and Arabia-Guidet, 1986; Wong, 2012a, 2013). In comparison, the production of T3 (last acquired; most complex) reflects the most complex laryngeal coordination and is therefore considered to place the most demand on the motor system. To produce T3, the SH is activated to rapidly lower the larynx, resulting in a relatively low F0; a sharply rising contour midway through the vowel results from rapid contraction of CT (Wong, 2012a, 2013; Xu, 1997). Studies of tone production have indicated that Mandarin-speaking children exhibit more errors producing tones with relatively low F0 involving SH activation when compared to higher F0 that require activation of the CT muscles (Wong, 2012a). Variations in tonal context add an additional layer of complexity to the production of tones in disyllabic words. Wong (2008) showed that Mandarin-speaking children made more errors when producing disyllabic words with conflicting tonal contexts than words with compatible tonal contexts. For example, children make fewer errors with the T1 (55) + T4 (51) combination (a compatible tonal context) than with the T4 (51) + T1 (55) combination (a conflicting tonal context). In the T1 + T4 combination, the offset F0 value of T1 (5) and the onset F0 value of T4 (5) are identical, requiring no or minimal change in laryngeal function. However, in the T4 + T1 combination, rapid and precise laryngeal adjustments are required to effect the relatively large change in F0 during the transition from T4 (1) offset and the onset of T1 (5). These observations support Wong’s (2012a, 2013) assertion that the order of tone acquisition is constrained by both the physiological complexity involved in tone production both in and out of context and the maturity of the developing speech motor system, particularly with regard to laryngeal control. In summary, research has shown that the stress effect can be observed in individuals who stutter across ages and languages in which linguistic stress is a distinctive prosodic feature. Specifically, stuttering is produced more frequently on syllables carrying stress as a result of larger variations in F0, duration, and intensity when compared to unstressed syllables. Mandarin tone, like English and German stress, involves prosodic changes in F0, duration, and intensity, with tones that involve ‘‘dipping’’ (Tones 3 and 4) or rapid lowering of F0 presenting the greatest challenge to laryngeal-supralaryngeal coordination, particularly for the developing child. For these reasons we speculated that tone, like stress, influences the occurrence of stuttering events and that stuttering will be produced more frequently on Tones 3 and 4, both of which require rapid and precisely timed F0 lowering. Further, since tone, like stress, is produced in context, the contextual effects of tone sandhi and tonal coarticulation on tone and tone variability are inherent in any relationship that may exist between stuttering events and tone. That being said, we hypothesised that the magnitude of pitch variations in context that characterise Tones 2, 3 and 4 suggests that stuttering is more likely to be produced on syllables produced with these tones in comparison to those produced with T1. Toward that end, the purpose of this study was to (1) examine each of the four lexical tones and the neutral tone in Mandarin and their relationship to moments or instances of

Lexical tone and stuttering loci in Mandarin 121 stuttering and (2) examine the relationship between tonal context (i.e. compatible or conflicting coarticulatory contexts) and stuttering loci in young Mandarin-speaking CWS.

Methods Participants and data corpus Participants were 20 Taiwanese CWS and who were native Mandarin speakers (M ¼ 4:9; range ¼ 3:2–6:4; years: months). These children were recruited for an earlier study (Chou, 2004). In order to be considered to be stuttering, each child had to meet the following criteria: (1) produced at least three within-word disfluencies (also known as stuttering-like disfluencies, or SLDs) per 100 words of conversational speech, and (2) were regarded by parents, teachers, or caregivers to be stuttering, and (3) had no other speech, language, hearing, neurological, or intellectual problems, other than stuttering (Conture, 2001; Yairi & Ambrose, 1992; Yang, 2002). All participants were recruited by speech–language pathologists. To determine their eligibility, all children were administered informal and standardised assessments including (1) the Peabody Picture Vocabulary Test–Chinese Edition (PPVT-Chinese Edition, Lu & Liu, 1994) and (2) the Assessment for Language Disorders for Preschool-Age Children (Ling & Ling, 1993). Additionally, an interview was conducted with parents in which they were asked to complete a series of forms describing the child’s developmental, medical, and stuttering history. One 500-word conversational speech sample was obtained from each child as he or she talked with the first author about his or her family, school activities, friends, or favorite television programs. These samples comprised the data for this study. Each conversational sample was simultaneously audio and video recorded for analysis. All speech samples were transcribed verbatim by the first author and a college student who had been trained in speech sample transcription.

Data coding Stuttering occurrence Only words produced in Mandarin were included in the analysis. Words produced in other languages such as English or other Chinese dialects such as Taiwanese and Hakka were excluded. All of the child’s speech disfluencies were coded for frequency and type. As described by Conture (2001) and Yairi and Ambrose (1999), English within-word or stuttering-like disfluencies (SLDs) include sound/syllable repetitions, audible and inaudible prolongation, and monosyllabic whole-word repetitions. Between-word or other disfluencies (ODs) include interjection, revisions, and phrase repetitions. There are no differences between English and Mandarin in the definition of prolongation, interjection and revision (Yang, 2002); however, with regard to the morphological structure of Mandarin, repetitions are defined as the following: (a) sound repetition is an iteration of a consonant or a vowel (e.g., /m-m-ma–/, (mother)); (b) syllable repetition is an iteration of the same syllable(s) of a mulitsyllabic word (e.g., /pu2- pu2 tao2/, , ‘‘grapes’’, (ji1-ji1 cheng2 che–/, taxi) and – (/ji1cheng2-ji1 cheng2 che /, taxi); (c) monosyllabic whole word repetition is an iteration of a word composed of a monosyllabic morpheme (e.g., /woˇ-woˇ/, - ,‘‘I’’); and (d) phrase repetition is an iteration of two or more connected words in a sentence (e.g., /woˇ yao1-woˇ yao1/, , ‘‘I want, I want’’). The CKIP Chinese Word Segmentation System (Academia Sinica, Taiwan) was adopted for word segmentation for words with less distinct word boundaries. Then, disfluencies were identified and classified.

122 F.-C. Chou et al. Tone Syllables with tone production errors were excluded from analysis. The audio tapes of all speech samples were reviewed first, and incorrectly produced Mandarin syllables were marked on transcripts. All correctly articulated Mandarin syllables were coded for tone (i.e., 1, 2, 3, 4, and 0 were used to represent Tones 1, 2, 3, 4, and the neutral tone, respectively). Interjudge and intrajudge reliability To assess interjudge and intrajudge measurement reliability for disfluency identification, a graduate student (a native Mandarin speaker who specialised in communication sciences and disorders) was trained by the first author in speech disfluency identification. Speech samples obtained from Mandarin-speaking school-age children who stutter were used as the training material. The training sessions were conducted until both judges’ inter- and intrajudge reliability rates of speech disfluency measurement were more than 80%. Four conversational speech samples from the study’s corpus (20%) were randomly selected. The first author and the graduate student independently observed video recordings of these four conversational samples and coded each disfluent word by speech disfluency type. Point-to-point interjudge and intrajudge reliability were calculated using Sander’s (1961) Agreement Index (# of agreements/# of agreements + disagreements). Mean intejudge reliability across the four samples for speech disfluency type was 90.35%. Mean intrajudge reliability for both the first author and the graduate student was 96.4% and 90.35%, respectively. The four conversational speech samples used in disfluency identification were adopted to assess interjudge and intrajudge measurement reliability for tone coding. A licensed speechlanguage pathologist in Taiwan was trained by the first author by using speech samples obtained from Mandarin-speaking school-age children. The training sessions were concluded until both judges’ inter- and intrajudge reliability rates for tone coding were more than 80%. As with disfluency identification, point-to-point interjudge and intrajudge reliability were calculated using Sander’s (1961) Agreement Index (# of agreements/# of agreements + disagreements). Mean interjudge reliability across the four samples for tone coding was 99.5%. Mean intrajudge reliability for both the first author and the speech-language pathologist was 100% and 99.3%, respectively.

Results The results are presented in two sections. First, we will discuss our findings with regard to the distribution of tones and instances of stuttering. Second, we will present a descriptive analysis of the relationship between stuttering and tonal context for both Tones 3 and 4; specifically the influence of carry-over and anticipatory coarticulation on stuttering for both tones, when coarticulation occurred in either compatible or conflicting contexts.

Tone distribution and instances of stuttering A total of 13 201 Mandarin syllables were obtained. The distribution of tones was as follows: T1 (17%); T2 (16%); T3 (21%); T4 (32%) and T0 (13%). Of these syllables, 1280 syllables across 20 participants contained stuttering. As shown in Figure 2, the proportion of stuttering events across the five tones was as follows: T1—17.7% (SD ¼ 7.84%), T2—16.6% (SD ¼ 6.52%), T3—28.72% (SD ¼ 9.53%), T4—35.26% (SD ¼ 9.89%) and T0—1.74% (SD ¼ 3.9%). The distribution of tones across the total number of Mandarin syllables compared to the stuttered syllables alone was significantly different (2(4) ¼ 175.61, p50.001), indicating that

Lexical tone and stuttering loci in Mandarin 123 60.0%

Stuering Frequency

50.0% 40.0%

35.26% 28.72%

30.0% 20.0%

17.7%

16.6%

10.0% 1.74% 0.0% T1

T2

T3

T4

T0

Figure 2. Average frequency of stuttered syllables across the five tones (n ¼ 20).

tone had a significant effect on stuttering loci. This result was confirmed by one-way repeated ANOVA (F(2.938,55.822) ¼ 42.86, p50.001, !2 ¼ 0.12. Mauchly’s test indicated that the assumption of sphericity had been violated, (2(9) ¼ 18.7, p ¼ 0.029); therefore, the degrees of freedom were corrected using Greenhous–Geisser estimates of sphericity. Our analysis showed that stuttering frequency was significantly lower on syllables carrying T0, compared to all other tones, and that T3 and T4 were associated with a significantly higher frequency of stuttering than either T1 or T2. No significant differences were found either between T1 and T2 or between T3 and T4.

The role of context on the relationship between tone and stuttering The primary finding of our analysis of tone and stuttering was that syllables carrying either Tone 3 or 4 were significantly more likely to be stuttered when compared to Tones 0, 1 or 2. Further, there was no significant difference between the two. To investigate the role of tonal context on stuttering loci, two post-hoc analyses were conducted. First, we explored the characteristics of stuttered tone combinations on T3 and T4. Both the carry-over and anticipatory effects were investigated. Next, we examined the influence of compatible and conflicting tone contexts on stuttering loci. Characteristics of stuttered tone combinations for Tones 3 and 4 In order to accomplish this, we first divided all stuttered syllables carrying either T3 or T4 into disyllabic and trisyllabic tone combinations. A stuttered syllable in the sentence-initial position was grouped with its following syllable to form a disyllabic tone combination. A stuttered syllable which was not in the sentence-initial position was grouped with its preceding and following syllables to form a trisyllabic tone combination. In the disyllable condition, we examined syllables carrying either T3 or 4 that were followed by Tones 1–4 and the neutral tone, allowing us to view the relationship between stuttering and anticipatory coarticulation. In the trisyllable condition, we examined the relationship between stuttering and both carry-over

124 F.-C. Chou et al. and anticipatory coarticulation when middle syllables carrying T3 or T4 were embedded in trisyllabic sequences.

Tone 3: Disyllabic tone combinations. To examine the anticipatory effect, disyllabic tone combinations were coded and analysed for all possible sequences in which T3 was the first syllable; for example, 31 represents the disyllabic sequence T3 followed by T1 (T3 + T1). A total of 367 T3 tone combinations were obtained. Of these, 186 were disyllables. As expected, the 186 T3 disyllabic sequences included five combinations: 31, 32, 33, 34, and 30; the proportions for these five tone combinations (from least to most) were (30) 6.45%, (31) 11.29%, (32) 20.43%, (33) 26.43% and (34) 35.48%. That is, across all disyllabic combinations, T3 + T4 tone combinations were associated with the highest proportion of stuttering, while T3 + T0 combinations were stuttered the least.

Tone 3: Trisyllabic tone combinations. There were a total of 181 T3 trisyllabic sequences, each coded according to the syllable tone that preceded and followed the middle syllable carrying T3 as follows: 13X, 23X, 33X, 43X and 03X, where X is one of the five tones (T0 through T4). In order to examine the carry-over effects, we examined the proportion of trisyllables that were stuttered when each of five tones preceded a syllable carrying T3. In this way, the influence of all possible compatible and conflicting contexts could be observed. Across the total of 181 stuttered trisyllables, the proportions of tone combinations (from least to most frequently occurring) were: (23X) 8.29%, (03X) 11.05%, (13X) 16.57%, (33X) 21.55% and (43X) 41.44%. Stuttering occurred the most on T3 syllables preceded by syllables carrying T4 and least on T3-carrying syllables that were preceded by syllables carrying T2. To examine anticipatory effects, trisyllabic sequences of T3 were coded as X30, X31, X32, X33 and X34 (i.e. Tone 3-carrying syllables followed by those carrying T4). Across the total of 181 trisyllables, the proportions for five tone combinations (from least to most frequently occurring) were 2.21% (X30), 17.13% (X32), 17.68% (X33), 23.3% (X31) and 33.15% (X34). Overall, stuttering was produced most frequently on T3-carrying syllables that were both preceded and followed by syllables carrying T4.

Tone 4: Disyllabic tone combinations.

A total of 163 T4 disyllabic tone combinations were obtained, and analysis for contextual effects was identical to that for T3 disyllabic combinations. All T4 tone combinations were coded and analysed across five sequencies: 41, 42, 43, 44 and 40. The proportion of stuttering across the five disyllabic sequences (from least to most) was (40) 9.2%, (42) 9.82%, (41) 22.7%, (43) 23.31% and (44) 34.97%, indicating that instances of stuttering occurred most frequently on T4-carrying syllables that were followed by other T4 syllables.

Tone 4: Trisyllabic tone combinations. To examine carry-over effects on stuttering across T4 trisyllabic tone combinations, 284 trisyllabic sequences were divided into five combinations in which T4 was preceded by T0–4. In this way, as with our analysis of T3 trisyllables, the influence of all possible compatible and conflicting contexts could be observed when T4 was the middle syllable in the sequence (i.e. 14X, 24X, 34X, 44X and 04X). Results showed that the proportions of five tone combinations (from least to most) were (04X) 10.21%, (24X) 14.44%, (34X) 16.2%, (14X) 17.96% and (44X) 41.2%. Overall, stuttering occurred more often in syllables with the tonal combination of 44X. As with T3, in order to examine the relationship between stuttering and anticipatory coarticulation for syllables carrying Tone 4, we coded the 284 trisyllabic tone combinations as follows: X41, X42, X43, X44 and X40. The proportion across all possible T4 trisyllabic combination (from least to most) was (X40) 7.39%, (X42) 14.44%, (X41) 15.14%, (X43) 26.06%

Lexical tone and stuttering loci in Mandarin 125 and (X44) 36.97%. Results indicated that in anticipatory coarticulatory contexts, instances of stuttering were most frequent when T4 was followed by another syllable carrying T4. As with the T3 trisyllables, stuttering was most frequent on T4-carrying syllables that were either preceded or followed by another syllable that carried T4.

Summary.

Our analysis revealed that across the T3 and T4 di- and trisyllabic sequences the tone combinations associated with the highest proportion of stuttering were 34, X34, 43X, 44, X44 and 44X. Of note is that with the exception of 43X, all of these are conflicting tonal contexts. Additionally, both the di- and trisyllable comprised of the T3 + T4 sequence is further affected by the application of tone sandhi in that T3 is produced as 21 when followed by a syllable carrying T4. From a physiological perspective, the application of tone sandhi along with a conflicting context increases the complexity of production for syllables with a T3 + T4 sequence, as the difference in F0 between the offset of T3 and the onset of T4 is large and presumably requires a rapid and precise laryngeal-supralaryngeal adjustment. As for the tonal sequence 43X, 67.5% of stuttered disruptions occurred on the T3 trisyllable combinations 431 and 434, both reflecting a conflicting anticipatory context. The role of compatible and conflicting contexts The distributions of the compatible and conflicting contexts across the total number of tone combinations and stuttered tone combinations were compared by using the chi-square goodness of fit test to examine the existence of a tonal context effect on stuttering loci. Four speech samples, 20% of the data in this study, were randomly selected to be the corpus of data analysis. All syllables were divided into disyllabic and trisyllabic tone combinations. There were two types of disyllabic tone combinations: one was composed of a syllable in the sentence-initial position and its following syllable, whereas the other was composed of a syllable in the sentencefinal position and its preceding syllable. Because no stuttering events were found in the sentencefinal position in the present study, the disyllabic tone combinations composed of syllables in the sentence-final position were excluded. Next, a syllable not in the sentence-initial or final position was grouped with its preceding and following syllables to form a trisyllabic tone combination. To examine if tonal context (compatible versus conflicting) has an effect on stuttering loci, tone combinations were classified into compatible and conflicting context groups (as defined in introductory section). For example, the combinations of 11, 11X and X11 were in the group of compatible context, whereas the combinations of 13, 13X and X13 were in the group of conflicting context. Tonal combinations involving T0 (such as 45, 45X and X45) were excluded the data analysis. The proportions of two tonal contexts in all disyllabic sequences were 34% for compatible context and 65% for conflicting context. A total of 109 stuttered tone combinations were obtained; 41 (37.61%) stuttered tone combinations were in the group of compatible context, and 68 (62.39%) stuttered tone combinations were in the group of conflicting context. No significant difference was found between the distributions of all tone combinations and stuttered tone combinations in disyllabic sequences (2(1) ¼ 0.624, p ¼ 0.37). Results showed that the proportions of five tone combinations (from least to most) were (04X) 10.21%, (24X) 14.44%, (34X) 16.2%, (14X) 17.96% and (44X) 41.2%. Overall, stuttering occurred most often in syllables with the tonal combination of 44X. The carry-over and anticipatory effects of the influence from tonal context were measured in trisyllabic tone combinations. With regard to the carry-over effect of tonal contexts, our results showed that the proportions of two tonal contexts were 36% for compatible context and 63% for conflicting context. The observed proportions of two tone contexts were 28.07% (compatible) and 71.94% (conflicting). As for the anticipatory effect of tonal contexts, 40.91% of the tone

126 F.-C. Chou et al. combinations were compatible, and 59.09% were conflicting. The observed proportion was 27.87% for compatible context and 72.13% for conflicting context. Our results revealed that the distribution of two tone contexts between all and stuttered tone combinations was significantly different in the trisyllabic tone combinations (carry-over: 2(1) ¼ 3.811, p ¼ 0.03; anticipatory: 2(1) ¼ 8.589, p ¼ 0.002), indicating a substantial tonal context effect on trisyllabic tone combinations. Stuttering events are more likely to occur on tone combinations with a conflicting tone context.

Discussion There were four main findings from this study. First, we observed that stuttering occurred significantly more frequently on syllables carrying lexical tones T1, T2, T3 and T4 when compared to T0. Second, comparison across T1–T4 indicated that instances of stuttering were significantly more likely on syllables carrying either T3 or T4 when compared to either T1 or T2, yet there was no difference between the two with regards to likelihood of stuttering. Third, di- or trisyllabic tone combinations containing either T3 or T4 were more likely to be stuttered when syllables carrying these two tones were either preceded (carry-over) or followed (anticipatory) by a syllable that carried T4; most of these tone combinations with high stuttering occurrence had a conflicting tonal context. Finally, our results regarding the influence of tonal contexts on stuttering loci, in part, lend a support that stuttering events were more likely to be found on lexical tones (T1-T4) embedded in a conflicting than in a compatible tonal context: stuttering events occurred more frequently on trisyllabiic tone combinations with a conflicting than with a compatible tonal context. In part, our findings support our hypotheses that stuttering moments would be more likely on syllables carrying Tones 3 and 4, and that coarticulatory context would play a role in this relationship. In addition, our results suggest that similar to the relationship between lexical stress and stuttering, the increased frequency of stuttering on Tones 3 and 4 results from the intricacies of tone contour production within syllables and tone shift between syllables. As with stress, the basic production characteristics of tone are stable, but they vary with the context (Wingate, 1988). We propose that several likely sources of both production and contextual variation may account for the link between tone and stuttering loci in Mandarin. For example, T3 is the most complex in articulation as well as the latest acquired and mastered for children (Wong, 2012a, b, 2013). Results from acoustic analyses have shown that children have more difficulties producing tones with relatively low F0 targets; T3 has the lowest F0 onset of all other tones, and T4 is produced with a sharply falling F0. Further, children make more errors in discriminating T3 from other tones (Wong et al., 2005; Wong, 2012a, b). The developmental trajectory of T3 acquisition and production accuracy is most likely related to its relative complexity. For example, Xu and Sun (2002) reported that the magnitude of pitch change in Mandarin tone production is directly related to the duration and velocity of change. That is, the larger the magnitude, the longer and faster the change in F0 from onset to offset of the tone. F0 control is vested in the coordination between intrinsic and extrinsic laryngeal muscles and subgottal pressure control (Atkinson, 1978; Halle, 1994; Hirano, Ohala, & Vennared, 1969). Preschoolers, unlike adults, have not yet achieved mature coordination of their laryngeal muscles to quickly regulate the vocal fold tension and length for changing or maintaining F0 throughout a syllable with the shortest tone duration (Hirano, Kurita, & Nakachima, 1983). T3 is characterised by the most varied pitch change and longest duration when produced in isolation (i.e. falling-rising), requiring rapid changes and precise coordination of laryngeal adjustments resulting from activation of the CT and SH muscles (Wong, 2012a). Further, recent

Lexical tone and stuttering loci in Mandarin 127 imaging work by Howell, Jiang, Peng, and Lu (2012a) indicated that there appear to be different neural mechanisms underlying rising and falling tones; the rapid transition between a falling and rising tone contour in T3 and the shift in neural activity suspected to be required for this transition may increase production variability as well as its vulnerability to disruption (in the form of both accuracy and disfluency/stuttering). In a follow-up study, Howell, Jiang, Peng, and Lu (2012b) did, in fact, observe that adults who stutter showed different neural control of rising and falling tones than controls and that this difference was most obvious during falling tone production. Finally, the application of tone sandhi rules to T3 production adds an additional layer of complexity (Xu, 2001, 2004). Two of the most common sandhi rules involve T3. The first is halfthird tone sandhi, in which Tone 3 is realised as 21 in non-final position and followed by either Tones 0, 1, 2 or 4. The second calls for T3 produced before another T3 to shift to T2. Overall, every syllable carrying T3 that is not in the final position and is produced in isolation is produced with at least one sandhi rule applied. Our observation that stuttering occurs more frequently on syllables carrying T3 and T4 with a conflicting, as opposed to a compatible, tonal context is in line with research that has shown that children make more articulation errors when producing words with a conflicting rather than compatible context (Wong, 2008). For example, the tone contour for T4 that is produced in isolation has a sharp falling trajectory; in connected speech this distinctive trajectory, rather than the F0 target, is the primary goal so as to maintain word meaning (Xu, 2001, 2004). Recent work has shown that in order to maintain the appropriate F0 trajectory while accommodating to adjacent tones in connected speech requires a complex interaction of both involuntary (articulatory constraints) and voluntary (communicative intent) influences (Xu, 2004). Conflicting tonal contexts with high communicative demands may tax the limits of laryngeal and supralaryngeal movement and coordination. In most cases the developing system is able to perform in contexts where demand exceeds capacity; however, children with slower or less stable language and motor systems may produce errors in tone or fluency disruption in such situations.

Limitations and further research Present findings provide a preliminary understanding of the influence of Mandarin tone on stuttering loci in Mandarin-speaking preschoolers. The findings have shown that the complexity of tone contour production within syllables and tone transition between syllables contribute to the occurrence of stuttering events. However, there are some limitations that may be overcome in future studies. First of all, because of the limited speech sample size, only descriptive data were reported with regard to the effect of tone context on T3 and T4 carrying tone combinations. To completely understand the influence from tone context on stuttering loci, collecting a large speech sample size is recommended for future studies. Second, the present study only considered the influence from one linguistic factor (Mandarin tone) in a specific age group (preschoolers). Stuttering loci, however, can be attributed to multiple linguistic factors (e.g., word position and word category) and vary across different age groups, based on evidence from English and German. Future researchers may want to take these two issues into consideration.

Acknowledgements The authors would like to thank the graduate students and the speech-language pathologist from Taiwan on this project, Shanju Lin and Yuting Huang, for their assistance with reliability measures for disfluency coding and tone coding. We would also like to thank our colleagues Tim

128 F.-C. Chou et al. Arbisi-Kelm, Bryan Brown, Julia Hollister and Naomi Hertsberg from the Stuttering Research Lab at the University of Iowa for their suggestions for this project.

Declaration of interest The authors report no conflict of interest. The authors alone are responsible for the content and writing of this article.

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