Do speakers converge rhythmically? A study on segmental timing properties of Grison and Zurich German before and after dialogical interactions

1.1. Rhythmic Accommodation

Three basic questions may arise when studying rhythmic accommodation: (a) Can speech rhythm in terms of segmental timing properties be object of adjustments between speakers? (b) In which communicative contexts is it possible to study rhythmic accommodation? and (c) Why is the research on accommodation in segmental timing a worthwhile pursuit?

With respect to (a) speech rhythm research has provided evidence that the durational characteristics of consonantal and vocalic intervals, as well as amplitude envelope characteristics, vary in response to the interlocutor’s age and cognitive development. For example, studies on the rhythmic characteristics of infant- compared to adult-directed speech have shown that: a) English, Catalan and Spanish mothers present less durational variability of consonantal and vocalic intervals as well as longer vowel duration when speaking to their children compared to addressing adults (Payne et al., 2009Payne, E., Post, B., Astruc, L., Prieto, P., & Vanrell, M. (2009). Rhythmic modification in child directed speech. Oxford University Working Papers in Linguistics, Philology & Phonetics, 12, 123-144.
); b) in Australian English delta modulations corresponding to the prosodic stress is greater in infant- than in adult-directed speech, while theta modulations, tracking syllable patterns, dominated the adult-directed speech modulation spectrum (Leong et al., 2017Leong V., Kalashnikova, M., Burnham, D., & Goswami, U. (2017). The Temporal Modulation Structure of Infant-Directed Speech. Open Mind: Discoveries in Cognitive Science, 1, 78-90.
). Not only do speech rhythm vary depending on the interlocutors’ characteristics, but the presence itself of an interlocutor (i.e., reading partner) has been shown to influence the degree of rhythm entrainment in synchronous reading tasks (Cerda-Oñate et al., 2021Cerda-Oñate, K., Toledo Vega, G., & Ordin, M. (2021). Speech rhythm convergence in a dyadic reading task. Speech Communication, 131, 1-12. http://dx.doi.org/10.1016/j.specom.2021.04.003
). In light of these findings, it seems plausible to assume that speakers can also mutually adapt the production of segmental timing features after exposure to a dialogue partner. On the other hand, in view of evidence showing that the timing properties of different speech intervals (e.g. consonants, vowels, voicing) are resistant to different sources of within speaker variability (speaking style, prosodic and linguistic factors) (Dellwo et al., 2015Dellwo, V., Leemann, A., & Kolly, M.-J. (2015). Rhythmic variability between speakers: Articulatory, prosodic, and linguistic factors. Journal of the Acoustical Society of America, 137(3), 1513-1528. http://dx.doi.org/10.1121/1.4906837
; Leeman et al. 2014Leemann, A., Kolly, M.-J., & Dellwo, V. (2014). Speaker-individuality in suprasegmental temporal features: Implications for forensic voice comparison. Forensic Science International, 238, 59-67. http://dx.doi.org/10.1016/j.forsciint.2014.02.019
), we cannot exclude that the speakers may maintain their segmental durational characteristics in post-dialogue productions. We will test precisely these two competing hypotheses in the present study.

With respect to (b), one of the contexts in which the study of rhythmic accommodation is possible is that of dialects in contact. In this setting, one might examine whether speakers of dialects that are mutually intelligible but present distinct rhythmic features converge rhythmically after being exposed to each other’s dialect. In this respect, the linguistic situation of German-speaking Switzerland is an excellent testing ground for studying cross-dialectal rhythmic accommodation. Swiss German dialects, indeed, do not only differ for segmental features, speech rate and intonation contours (see Leeman, 2012Leemann, A. (2012). Swiss German Intonation Patterns. Amsterdam, Philadelphia: John Benjamins Publishing Company.
for a review), but also for their rhythmic properties. It has been documented that Midland vs Alpine dialects as well as Eastern vs Western dialects can be grouped according to their rhythmic characteristics, measured acoustically in terms of the timing variability of consonantal and vocalic intervals (Leeman et al., 2012Leemann, A., Dellwo, V., Kolly, M. J., & Schmid, S. (2012). Rhythmic variability in Swiss German dialects. 6th International Conference on Speech Prosody, Shanghai, China, 607-610.
).

With respect to (c), it has been argued that assessments of phonetic convergence based on a single (supra)segmental feature hardly capture the complexity of the phenomenon (Pardo et al., 2017Pardo, J. S., Urmanche, A., Wilman, S., & Wiener, J. (2017). Phonetic convergence across multiple measures and model talker. Attention, Perception, & Psychophysics, 79(2), 637-659. http://dx.doi.org/10.3758/s13414-016-1226-0
). Nevertheless, choosing one acoustic attribute over another is still a valid approach when the comprehension of dynamics of sound variation and change is at stake (Pardo et al., 2017), or when decisions must be taken about which aspects of human-human interaction have to be modelled in speech interactive systems to achieve human-likeness (Beňuš, 2014Beňuš, Š. (2014). Social aspects of entrainment in spoken interaction. Cognition Computing, 6, 802-813. http://dx.doi.org/10.1007/s12559-014-9261-4
). Understanding whether rhythmic properties in terms of segmental durational characteristics are object of mutual adaptations can be also crucial for the interpretation of evidence in forensic phonetic speaker comparisons. Any acoustic adjustments between interlocutors might lead to mistake within- for between-speaker variability and produce higher error in recognition rate.

2.1. Material

To study rhythmic accommodation in a dialect contact situation, we used a corpus of speech material in Zurich and Grison German (henceforth ZHG and GRG), two Swiss German dialects exhibiting crucial segmental and suprasegmental differences (cf. 2.2.) that legitimate the assumption of interspeaker adjustments after exposure to the interlocutor’s dialect.

The corpus was designed, collected and annotated by Hanna Ruch to study vowel accommodation between GRG and ZHG (Ruch, 2015Ruch, H. (2015). Vowel convergence and divergence between two Swiss German dialects. 18th International Congress of Phonetic Sciences, Glasgow, UK.
). It included speech samples of:

The diapix tasks were designed to elicit the target words present in picture naming task and story retelling. All tasks were carried out in one single recording session.

2.2. Cross-dialectal phonetic differences

Grison and Zurich German present noticeable differences at several linguistic levels (Eckhardt, 1991Eckhardt, O. (1991). Die Mundart der Stadt Chur. Zürich: Phonogrammarchiv der Universität 624, Zürich.
; Fleischer & Smith, 2006Fleischer, J. & Schmid, S. (2006). Zurich German. Journal of the International Phonetics Association, 36, 243-253. http://dx.doi.org/10.1017/S0025100306002441
; Christen et al., 2010Christen, H., Glaser, E., & Friedli, M. (2010). Kleiner Sprachatlas der deutschen Schweiz. Frauenfeld: Huber Frauenfeld.
; Leeman, 2012Leemann, A. (2012). Swiss German Intonation Patterns. Amsterdam, Philadelphia: John Benjamins Publishing Company.
). Phonetically, these have to do with the quality of front vowels, realization of word-initial and post-vocalic k, speech rate and intonation contours. It is of interest - for the purpose of this study - that GRG and ZHG also exhibit segmental durational differences that lead to a distinct rhythmic organisation of the two dialects. As reported in the literature on acoustic differences between GRG and ZHG (see a.o. Ruch, 2018Ruch, H. (2018). The role of acoustic distance and sociolinguistic knowledge in dialect identification. Frontiers in Psychology, 9, 818. http://dx.doi.org/10.3389/fpsyg.2018.00818
), these differences concern: a) intervocalic sonorants gemination (henceforth ISG) in words ending in -e; b) open syllable lengthening (henceforth OSL); c) vowel reduction in word final position (henceforth RedVow).

Given that segmental timing properties are among the acoustic correlates of speech rhythm, in this paper we will refer to the three cross-dialectal differences in ISG, OSL and RedVow as rhythmic differences. Regarding ISG, GRG intervocalic sonorants can be realized either as geminates or as single consonants, while ZHG allows only the singleton realisation. As for OSL, in GRG open syllables can be either lengthened or not, while in ZHG the lengthening tendency has not been documented. With respect to RedVow, in GRG vowels in word final position are not reduced in quality, and presumably either in duration, while in ZHG word final vowels are always reduced. (Cf. Table 1 for examples of cross-dialectal realizations of ISG, OSL and GR).

Evidence in support that the differences in the quality of final vowels come also with distinct timing patterns has been provided in Leeman et al. (2012)Leemann, A., Dellwo, V., Kolly, M. J., & Schmid, S. (2012). Rhythmic variability in Swiss German dialects. 6th International Conference on Speech Prosody, Shanghai, China, 607-610.
. Here it was shown that the durational variability of vocalic intervals was higher in Midland dialects (to which ZHG belongs to) than in the group of Alpine dialects (to which GRG belongs to), and this was interpreted in view of the tendency of Alpine dialects to retain full vowels in unstressed position.

2.3. Method

To understand whether pairs of GRG and ZHG speakers produce the rhythmic features more similarly after participating in the diapix tasks, the following steps were taken:

To determine whether pairs GRG and ZHG speakers converge, diverge or maintain their rhythmic behaviour after the interaction, we calculated:

2.4. Data analysis and statistics

The present study reports on the data extracted from the picture naming task. In view of evidence showing the influence of linguistic factors on accommodation (Goldinger, 1998Goldinger, S. D. (1998). Echoes of echoes? An episodic theory of lexical access. Psychological Review, 105(2), 251-279. http://dx.doi.org/10.1037/0033-295X.105.2.251
; Goldinger & Azuma, 2004Goldinger, S. D. & Azuma, T. (2004). Episodic memory reflected in printed word naming. Psychonomic Bulletin & Review, 11(4), 716-722. http://dx.doi.org/10.3758/BF03196625
; Nielsen, 2011Nielsen, K. (2011). Specificity and abstractness of VOT imitation. Journal of Phonetics, 39(2), 132-142. http://dx.doi.org/10.1016/j.wocn.2010.12.007
), analysing the data from picture naming tasks (henceforth PNT) has given the main advantage of controlling for the effect of the item variability in the assessment of:

The lexical items used in this study and the dialectal features they instantiate are listed in Table 2 in the Standard German spelling.

To test (a), i.e., whether pairs of GRG and ZHG speakers realised the three durational contrasts differently before the interaction, and thus to make sure that there was room for rhythmic accommodation, we ran three separate Linear Mixed Effects Models with the ratio measures (ISG, OSL and RedVow) as dependent variables, dialect (ZHG and GRG) as fixed factor, and speaker and lexical item as random effect (i.e. random intercepts).

In the light of segmental durational differences between GRG and ZHG mentioned above, we make the following hypotheses regarding the rhythmic behaviour of ZHG and GRG speakers before the interaction:

To test (b), i.e., whether pairs of GRG and ZHG speakers produce the three rhythmic features more similarly after the diapix tasks, we compared the Euclidean distances within pair in ISG, OSL and RedVow before and after the interactions. We ran three separate Linear Mixed Effect Models, with Euclidean distance in ISG, OSL and RedVow as dependent variables and Session (1 = before interaction; 2 = after the interaction) as fixed factor. Given that Euclidean distance between pairs may vary before and after the interaction, in the structure of the random effect we first included the random slope of Pairs by Session. However, this model was too complex to be supported by the data. For this reason, we simplified the random effects by including the intercept for the interaction between Session and Pair, instead of the random slope. The random part of the model comprised also the intercept for Item.

We hypothesise that if rhythmic features are object of accommodation, dyads members adjust their rhythmic behaviour such that the Euclidean distance in ISG, OSL and RedVow will be lower after than before the interaction. In view of findings showing the effect that speakers converge more for features that differ mostly between dialects (MacLeod, 2012MacLeod, B. (2012). The Effect of Perceptual Salience on Phonetic Accommodation in Cross-Dialectal Conversation in Spanish. Dissertation. Toronto: University of Toronto.
; Ruch, 2015Ruch, H. (2015). Vowel convergence and divergence between two Swiss German dialects. 18th International Congress of Phonetic Sciences, Glasgow, UK.
; Walker & Campbell-Kibler, 2015Walker, A. & Campbell-Kibler, K. (2015). Repeat what after whom? Exploring variable selectivity in a cross-dialectal shadowing task. Frontiers in Psychology, 6(546). http://dx.doi.org/10.3389/fpsyg.2015.00546
; Clopper & Dossey, 2020Clopper, C. G. & Dossey, E. (2020). Phonetic convergence to Southern American English: Acoustics and perception. The Journal of the Acoustical Society of America, 147(1), 671-671. http://dx.doi.org/10.1121/10.0000555
) and between the speakers and the model talkers (Babel, 2012Babel, M. (2012). Evidence for phonetic and social selectivity in spontaneous phonetic imitation. Journal of Phonetics, 40(1), 177-189.
), we hypothesise that more accommodation is evoked by RedVow than ISG and OSL. RedVow, indeed, is one of the features that best distinguishes the two dialects. ZHG indeed exhibits open syllable lengthening - though in articulatory contexts other than GRG - and presents longer nasal duration in -CCer words. However, given that the realisation of reduced vowels is also a strong dialect marker for ZHG (Ruch, 2018Ruch, H. (2018). The role of acoustic distance and sociolinguistic knowledge in dialect identification. Frontiers in Psychology, 9, 818. http://dx.doi.org/10.3389/fpsyg.2018.00818
), in view of evidence about little convergence for features that are dialect markers (Babel, 2010Babel, M. (2010). Dialect divergence and convergence in New Zealand English. Language in Society, 39(4), 437-56. http://dx.doi.org/10.1017/S0047404510000400
), we cannot exclude that the speakers may diverge or maintain their original behaviour for RedVow.

To test these hypotheses, we ran one Linear Mixed Effects Model with DDpair as dependent variable and Ratio Type (ISG, OSL and RedVow) as fixed factor. The random part of the model comprised the intercept for the interaction between Pair and Ratio, as well as the intercept for Item. Statistical analyses were performed with RStudio (2009-2019) Version 1.2.1335.

2.5. Results and Discussion

Regarding (a), i.e., cross-dialectal differences in ISG, OSL and RedVow before the interaction, the results from pre-dialogue recordings show a significant main effect of Dialect for the three measures (Table 3).

As shown in Fig. 1, the scores obtained by GRG speakers in the three ratio measures are higher than ZHG speakers.

If the results for ISG and OSL are in line with predictions, what is more surprising is that RedVow is lower in ZHG than in GRG. One plausible explanation for this finding might be that in picture naming task, for which speakers were asked to pronounce words in isolation, ZHG speakers do not drastically reduce the duration of unstressed vowels in word final position, as these vowels are subjected to pre-pausal lengthening. In other words, in ZHG the durational difference between stressed and unstressed vowels in final word position is not that big as one might expect.

With respect to (b), i.e., the accommodation behaviour in ISG, OSL and RedVow, the results of statistical analysis reveal no significant main effect of Session (pre- and post-dialogue recordings) in the Euclidean distances (Table 4).

In other words, the Euclidean distance between dyads members did not change significantly before and after the interactions (Fig. 2).

With respect to the hypothesis that RedVow is more prone to convergence compared to OSL and ISG, against the predictions, no significant differences in degree and direction of accommodation (DDpair) were found between the three ratio measures (Table 5).

Unlike findings on vowel accommodation between GRG and ZHG or between other dialects, showing more convergence for phonetically more distant features (Ruch, 2015Ruch, H. (2015). Vowel convergence and divergence between two Swiss German dialects. 18th International Congress of Phonetic Sciences, Glasgow, UK.
; MacLeod, 2012MacLeod, B. (2012). The Effect of Perceptual Salience on Phonetic Accommodation in Cross-Dialectal Conversation in Spanish. Dissertation. Toronto: University of Toronto.
; Walker & Campbell-Kibler, 2015Walker, A. & Campbell-Kibler, K. (2015). Repeat what after whom? Exploring variable selectivity in a cross-dialectal shadowing task. Frontiers in Psychology, 6(546). http://dx.doi.org/10.3389/fpsyg.2015.00546
; Clopper & Dossey, 2020Clopper, C. G. & Dossey, E. (2020). Phonetic convergence to Southern American English: Acoustics and perception. The Journal of the Acoustical Society of America, 147(1), 671-671. http://dx.doi.org/10.1121/10.0000555
), and more divergence for acoustic attributes perceived as strong dialect markers (Babel, 2010Babel, M. (2010). Dialect divergence and convergence in New Zealand English. Language in Society, 39(4), 437-56. http://dx.doi.org/10.1017/S0047404510000400
; Clopper & Dossey, 2020Clopper, C. G. & Dossey, E. (2020). Phonetic convergence to Southern American English: Acoustics and perception. The Journal of the Acoustical Society of America, 147(1), 671-671. http://dx.doi.org/10.1121/10.0000555
), in the case of ISG, OSL and RedVow, interpretations of accommodation based on phonetic distance or degree of dialect markedness do not seem tenable (Fig 3).

As shown in Fig. 3, RedVow, indeed, was neither more nor less prone to accommodation than OSL and ISG. Conversely, the values of the three measures circle around zero pointing in favour of rhythmic maintenance.

There could be at least two possible explanations for this result: (1) likewise the rhythmic metrics analysed in previous research (e.g., Leeman et al., 2014Leemann, A., Kolly, M.-J., & Dellwo, V. (2014). Speaker-individuality in suprasegmental temporal features: Implications for forensic voice comparison. Forensic Science International, 238, 59-67. http://dx.doi.org/10.1016/j.forsciint.2014.02.019
; Dellwo et al., 2015Dellwo, V., Leemann, A., & Kolly, M.-J. (2015). Rhythmic variability between speakers: Articulatory, prosodic, and linguistic factors. Journal of the Acoustical Society of America, 137(3), 1513-1528. http://dx.doi.org/10.1121/1.4906837
), the three timing measures examined here may be robust against source of within-speaker variability. The exposure to the distinct rhythmic behaviour of the dialogue partner might have not altered the post-dialogue realization of ISG, OSL and RedVow, as instead was observed for vowel formants. We cannot exclude, however, that accommodation in segmental timing properties has happened in the more spontaneous tasks of the corpus which has not been object of the present investigation. For future research, it will be interesting to examine whether the same pattern would replicate when rhythm is examined at the utterance level, using the metrics which have been typically employed in speech rhythm research (see a.o. Ramus, Nespor and Mehler, 1999; Grabe & Low, 2002Grabe, E. & Low, E. L. (2002). Durational variability in speech and the rhythm class hypothesis. In N. Warner & C. Gussenhoven (Eds.), Papers in Laboratory Phonology 7 (pp. 515-546). Berlin: Mouton de Gruyter. http://dx.doi.org/10.1515/9783110197105.515
; Dellwo, 2006Dellwo, V. (2006). Rhythm and speech rate: A variation coefficient for C. In P. Karnowski & I. Szigeti (Eds.), Language and Language-Processing (pp. 231-241). Frankfurt am Main: Peter Lang.
; White and Mattys, 2007White, L. & Mattys, S. L. (2007). Calibrating rhythm: First language and second language studies. Journal of Phonetics, 35(4), 501-522. http://dx.doi.org/10.1016/j.wocn.2007.02.003
; He & Dellwo, 2016He, L., & Dellwo, V. (2016). The role of syllable intensity in between-speaker rhythmic variability. International Journal of Speech, Language, and the Law, 23(2), 243-275. http://dx.doi.org/10.1558/ijsll.v23i2.30345
). (2) Another possible explanation may have to do, instead, with the perceptual salience of the cross-dialectal features, captured by the three rhythmic measures. Given that differences must be perceptible in order to be imitated (Mitterer & Müssler, 2013Mitterer, H. & Müsseler, J. (2013). Regional accent variation in the shadowing task: Evidence for a loose perception-action coupling in speech. Attention, Perception and Psychophysics, 75, 557-575. http://dx.doi.org/10.3758/s13414-012-0407-8
), the interspeakers’ differences in ISG, OSL and RedVow may probably be too subtle to be perceived or retained after the interaction. This would be also in line with findings from Swiss German dialects recognition research that shows that listeners pay attention to segmental features to a higher degree than rhythmic and prosodic features when recognizing the dialectal origin of the speakers (see Leemann, et al., 2018Leemann, A., Kolly, M.-J., Nolan, F., & Y. Li (2018). The role of segments and prosody in the identification of a speaker’s dialect. Journal of Phonetics, 68, 69-84. http://dx.doi.org/10.1016/j.wocn.2018.02.001
; for varieties of English see a.o., Fuchs, 2015Fuchs, R. (2015). You’re not from around here, are you? Dialect discrimination experiment with speakers of British and Indian English. In E. Delais-Roussarie, M. Avanzi, & S. Herment (Eds.), Prosody and Language in Contact (pp. 123-148). Berlin: Springer.
).

The differences in accommodation behaviour of the same ZHG and GRG speakers across segmental and rhythmic measures confirm the complexity and multi-facetedness of vocal accommodation. As pointed out by Sanker (2015)Sanker, C. (2015). Comparison of phonetic convergence in multiple measures. Cornell Working Papers in Phonetics and Phonology 2015, 60-75.
and Cohen Priva and Sanker (2018)Cohen Priva, U. & Sanker, C. (2018). Distinct behaviors in convergence across measures. Annual Conference of the Cognitive Science Society, Madison, WI, 1518-1523.
, patterns of convergence in one measure within a pair or within a speaker cannot be taken to be representative of pairs and speakers’ overall convergence patterns in other measures.