Fonetische correlaten en communicatieve functies van linguïstische structuu

Caspers, J.

Leiden University Scholary Publications

MELODIC CHARACTERISTICS OF BACKCHANNELS IN DUTCH MAPTASK DIALOGUESJohanneke CaspersPhonetics Laboratory/Holland Institute of Generative LinguisticsPO Box 9515, 2300 RA Leiden, The Netherlands; CASPERS@RULLET.LEIDENUNIV.NLABSTRACTIn natural conversation backchannels (short optional utteranceslike ‘uh-huh’ or ‘yes’) are used to indicate to the currentspeaker that the current listener understands so far and that thespeaker may continue. The question posed in the present paperis whether backchannels distinguish themselves melodicallyfrom lexically identical utterances that have a different func-tion (e.g., the answer to a question). In a corpus of Dutch MapTask dialogues the melodic configurations realized on allbackchannels and all lexically identical non-backchannels weretranscribed using ToDI [1]. Comparison of the two groups ofdata reveal a clear tendency for backchannels to be marked bya non-prominence lending drop in pitch followed by a highboundary tone (69%), whereas the non-backchannels generallycarry a pitch accent (61%).1. INTRODUCTIONIn natural conversation so-called ‘backchannels’ (the term wasintroduced by Yngve [2], cf. [3]) are a common phenomenon:short optional utterances produced by the hearer to signal thats/he is still engaged in the dialogue, prompting the currentspeaker to go on. Communication is an interactional process,involving continuous feedback between interlocutors, andbackchannels are important instances of responsive behavior.As the name indicates, backchannels are not viewed as speak-er turns, but as sounds occurring during the turn of anotherspeaker (and they are normally left out when a conversation isreproduced). For instance, ‘yes’ can be used to indicate thatthe current listener has understood so far and that the speakermay continue. However, if ‘yes’ is an answer to a yes-noquestion, it is not an optional utterance and therefore not abackchannel. It seems possible that the specific dialoguefunction of short utterances like ‘yes’ is reflected in theirsuprasegmental structure.Recent research within the framework of human-machinecommunication has taken a growing interest in the function ofprosody in human-human conversation, with the aim of im-proving the performance of existing spoken dialogue systems.For example, investigations of the prosodic characteristics ofechoic responses in Japanese dialogues [4,5] show that thefunction of these repeated parts of speech, such as acknowl-edgment or repair-request, tends to be reflected in their pros-odic characteristics. Likewise, investigations of Dutch discon-firmations revealed close connections between the function ofthe word "nee" (‘no’, which serves as a ‘go on’ or a ‘go back’signal in the investigated corpus) and it’s prosodic characteris-tics [6]. These findings may induce the assumption that back-channels – by definition affirmative in nature, i.e., ‘go on’signals – have specific prosodic characteristics, such as a shortduration, a short preceding pause, and no marked pitch accent.The question posed in the present paper is whether backchan-nels distinguish themselves melodically from lexically identi-cal ‘real’ turns. To answer this question, backchannels andlexically identical non-backchannels were marked in a corpusof Dutch task-oriented dialogues, and the melodic charac-teristics of both utterance types were compared.2. METHOD2.1. MaterialsUse was made of a corpus of Dutch guided spontaneousconversations, so-called Map Task dialogues (cf. [7]). In thesetask-oriented dialogues, maps provide a handle on an essenti-ally spontaneous conversation. Two roles can be distinguished:an ‘instruction giver’ and an ‘instruction follower’. The formerparticipant has a map with a route drawn on it and s/he has toexplain to the instruction follower which route to draw on hisor her unmarked copy of the map. The participants cannot seeeach other’s maps. Both maps have a number of referencepoints on it (e.g., ‘old pond’, ‘new pond’, ‘green meadow’)and by introducing small differences between these referencepoints it is possible to complicate the dialogue to some extent.The materials used for the present investigation were collectedby Bob Ladd and Astrid Schepman.2.2. AnalysisInter Pausal Units. The materials were divided into so-calledInter Pausal Units. An IPU is defined as "a stretch of a singlespeaker’s speech bounded by pauses longer than 100 ms" ([8],p. 299). This means that boundaries were drawn in all posi-tions where a pause longer than 100 ms appeared in thesignal, and in positions where a change of speaker occurred.IPUs can be determined objectively and the boundaries be-tween these units can be labeled as instances of either turn-holding or turn-changing.Backchannels. Every IPU was labeled in turn transitionalterms (cf. [9] for a detailed description of the labeled data),using the definition for backchannels as formulated by Koisoet al. [8]). Backchannels occur either during the speech pro-duced by the current speaker, or in a pause made by thecurrent speaker, see below (the boxes following S1 depict thestretches of speech uttered by the current speaker, S2 indicatesthe speech by the listener, the dotted line marks the timecourse, and the arrow indicates the relevant IPU boundary):S1orS2...... ...... ........ ......In a minority of the cases it was difficult to decide whether aspecific IPU was a backchannel or not, because the optionalityof the utterance was hard to assess. As was already mentionedby Schegloff in his study of backchannels ([10], p. 85), speak-ers sometimes actually wait for their listener to produce acontinuer like ‘uh-huh’, which means that some backchannelsare more or less obligatory in nature. Note that the labeling ofthe backchannels was done on the basis of an orthographictranscription of the materials, without using any acoustic cues;only when the orthography gave insufficient information as towhether or not the preceding IPU should be interpreted as aquestion (and, hence, the IPU under investigation as theanswer to a question), the melody of the preceding IPU wastaken into account.Below a part of a Map Task is represented, containing fiveshort utterances by the instruction follower (S). IPUs number2, 8 and 10 were labeled as backchannels, whereas 4 and 6were labeled as ‘real’ turns, since they form the answer to ayes-no question produced by the instruction giver (X).1. (X) Dus voordat het naar links gaat buigen stopjij met de lijn‘So before it bends to the left you stop theline’2. (S) Ja‘Yes’3. (X) En dan... heb jij ’t beeld van oorlogsheld‘And then... do you have the statue of warhero’4. (S) Huhum‘uh-huh’5. (X) Rechts‘To the right’6. (S) Ja‘Yes’7. (X) Daar ga je recht naar toe vanaf daar‘There you go in a straight line from there’8. (S) Oké‘Okay’9. (X) En daar ga je onderlangs omheen‘And there you go underneath around’10. (S) Ja‘Yes’Intonation Transcription. As a tool for labeling the melodicphenomena, the recently developed ToDI system (‘Transcrip-tion of Dutch Intonation’, [1]) was used. The last pitch accent– when present – before every IPU boundary was transcribed,as well as the tone sequence following this accent up to theboundary. The intonation was labeled on the basis of theauditory impression of the pitch curve only. Before every IPUboundary a boundary tone was transcribed, so that intonationdomain boundaries were determined by pauses or speakerchanges actually occurring in the material, and not by thesyntactic structure of the utterance. Note that, as a result, theboundary tones marked in the current analysis do not necessar-ily correspond to the boundary tones as defined by ToDI.The intonation of overlapping stretches of speech could notalways be labeled properly, which led to a small amount of‘intranscribable’ data ("?"). Another issue was the transcriptionof a melodic configuration that seemed typical for backchan-nels: a clear drop in pitch immediately followed by a rise,without the overt suggestion of prominence, which means thatthese configurations could not be interpreted as pitch accents.ToDI does not offer a suitable label (neither does the Gram-mar of Dutch Intonation, cf. [11]) and therefore I decided togive these specific configurations the label LH% (i.e., a lowtone followed by a high boundary tone). Figure 1 presentsexamples of a default falling pitch accent (H*L) and of anLH% contour on the word "ja" (‘yes’), uttered by the samefemale instruction follower.2.3. Expectations100150200250300350Time (0.1s)Frequency (Hz)Figure 1: Example of H*L L% (left) and LH% (right) con-tours on the word "ja" (‘yes’); above: waveform, below: F0curve (in Hz).A tendency was expected for backchannels to carry LH%contours, whereas the lexically identical non-backchannelswere expected to be marked by clear pitch accents. Eightcomplete Map Task dialogues were analyzed, amounting to o-Table 1: Absolute (and relative) frequency of occurrence of the different melodic shapes of the backchannels encountered in thematerial, broken down by lexical shape.melodic shapelexical shape pitch accent LH% ? total"ja" (‘yes’) 31 (22%) 95 (69%) 12 (9%) 138 (73%)"oké" (‘okay’) 7 (29%) 15 (63%) 2 (8%) 24 (13%)other 3 (11%) 20 (74%) 4 (15%) 27 (14%)total 41 (22%) 130 (69%) 18 (9%) 189 (100%)ver 40 minutes of speech. These materials contained 1552 IPUboundaries, among which were 189 backchannels. There were153 instances of lexically identical ‘real’ speaker turns.3. RESULTSTable 1 presents the lexical shape and the general melodiccharacteristics of the 189 backchannels encountered in thematerial. In 73% of the cases the listener uttered a simple "ja"(‘yes’); a further 13% of the cases existed of the utterance"oké" (‘okay’) and in the remainder of the cases "hmhmm"(‘uh-huh’), "oh" (‘oh’) or "goed" (‘good’) were used. Thebackchannels in the investigated material carry a low tonefollowed by a conspicuous final rise in almost 70% of thecases (LH%), while the remaining cases could either not betranscribed ("?", 9%) or were marked by a pitch accent (H*,H*L, L* or L*H) followed by a low (L%), high (H%) or level(%) boundary tone (22%). For lack of space the differenttypes of pitch accents and boundary tones were collapsed intoone category.Table 2 contains the lexical shape and melodic characteristicsof the non-backchannels that were found in the material. Thetable reveals that the 153 turns resembling backchannels havea similar lexical distribution (71% ‘yes’, 23% ‘okay’ and 6%‘oh’, ‘uh-huh’ or ‘good’). Their melodic characteristics, how-ever, differ clearly: the non-backchannels are marked by apitch accent in 61% of the cases and carry a LH% configura-tion in only 27% of the cases; the remaining 12% could not betranscribed.An ANOVA was performed on the percentage of backchannelsversus non-backchannels with melodic type (the four mainpitch accent types – H*, H*L, L* and L*H – plus the catego-ries LH% and "?") and lexical shape ("ja", "oké" and "other")as factors. There were main effects of melodic type (F(5,336)=15.51, p<.001) and lexical shape (F(2,339)= 3.51, p<.05) on thedistribution of backchannels versus non-backchannels, but nointeraction (F(10,324)= 1.72, ins.). A posthoc analysis shows thatLH% differs significantly from all other melodic types, exceptfrom the configuration L*H (a pitch accent type that occursonly 6 times in the data and closely resembles LH%); therewere no further differences.As expected, the backchannels present in the current materialsare more often marked by LH% than the lexically identicalIPUs that constitute an actual speaker turn, and, vice versa, the‘real’ speaker turns are more often marked by a pitch accent.4. OTHER PROSODIC CUES?The melodic characteristic of a short utterance like ‘yes’ isprobably not the only prosodic cue to its function. As wasreported by Krahmer et al. [6], Dutch ‘go on’ signals aregenerally shorter than ‘go back’ signals, and they are precededby a shorter pause.Investigation of the current data, however, shows no maineffect of the BC-NOBC opposition on the duration of therelevant IPUs (F(1,340)= 2.53, ins.) and no interactions withother independent variables. It thus seems that there is noinherent difference in duration between backchannels and non-backchannels, which probably means that the ‘real’ turns inthe current materials cannot be compared to the ‘go back’signals investigated by [6].Further investigation indicates that the BC-NOBC oppositiondoes influence the duration of the preceding pause: it is indeedshorter when it precedes a backchannel than when it precedesa turn (F(1,340)= 4.23, p<.05). However, this effect is reversedwhen the pause precedes "oké" (there is a main effect oflexical shape, F(2,339)= 3.94, p<.05 and a significant interaction,F(2,336)= 5.64, p<.01). The reason for this reversal may be that‘okay’ signals a more hesitant type of affirmation than ‘yes’ or‘uh-huh’.Summarizing, the investigated materials do not present simpledurational cues to the function of short utterances like ‘yes’.5. CONCLUSION AND DISCUSSIONThe data suggest that speech melody plays a role in signalingthe dialogue function of short utterances like ‘yes’ and ‘okay’.The data show a clear tendency for backchannels to carry aLH% contour (69%) and for segmentally identical ‘real’ turnsto be marked by a pitch accent (61%). The melodic shape thusTable 2: Absolute (and relative) frequency of occurrence of the melodic shape of non-backchannels, broken down by lexicalshape.melodic shapelexical shape pitch accent LH% ? total"ja" (‘yes’) 62 (57%) 34 (32%) 12 (11%) 108 (71%)"oké" (‘okay’) 26 (72%) 6 (17%) 4 (11%) 36 (23%)other 5 (56%) 1 (11%) 3 (33%) 9 (6%)total 93 (61%) 41 (27%) 19 (12%) 153 (100%)seems to support information that can be derived from othersources, such as syntactic and pragmatic structure.However, it should be noted that the materials investigated arequite specific in nature: Map Task dialogues consist mainly ofinstructions, with occasional questions and checks uttered bythe instruction follower. It could be the case that other typesof dialogue elicit backchannels with different melodic charac-teristics. Further investigation of diverse types of spontaneousconversation will have to reveal whether or not backchannelsin general typically carry a non-prominence lending low tonefollowed by a high boundary tone.Furthermore, the LH% configuration does not seem to be anexclusive marker of backchannels, since it was found onapproximately a quarter of the ‘real’ turns as well. It couldwell be the case that LH% is essentially some sort of ‘go on’signal, which suits backchannels in general, but may fit certainnormal speaker turns as well (for example, the answer to ayes-no question, which at the same time serves as a continua-tion signal, cf. line 6 in the example given in subsection 2.2.).Further perception experiments are needed to establish wheth-er the LH% configuration is generally interpreted as a ‘go on’signal in Dutch.6. ACKNOWLEDGMENTSThis research was funded by the Netherlands Organization forScientific Research (NWO) under project #355-75-002.7. REFERENCES1. Gussenhoven, C., Rietveld, T. and Terken, J., “ToDI,Transcription of Dutch Intonation” , URLhttp://lands.let.kun.nl/todi/, 1999.2. Yngve, V., “On getting a word in edgewise”, Papersfrom the Sixth Regional Meeting, Chicago LinguisticSociety, Chicago, 567-577, 1970.3. Duncan, S. and Fiske, D.W., Face-to-face interaction:Research, methods, and theory, Lawrence ErlbaumAssociates, Hillsdale, N.J., 1977.4. Swerts, M., Koiso, H., Shimojima, A., and Katagiri, Y.,“On different functions of repetetive utterances”, Pro-ceedings of the 5th International Conference on SpokenLanguage Processing, Sydney, 483-486, 1998.5. Shimojima, A., Katagiri, Y., Koiso, H., and Swerts, M.,“An experimental study on the informational and groun-ding features of Japanese echoic responses”, Proceed-ings of the ESCA Workshop on Dialogue and Prosody,Eindhoven, 187-192, 1999.6. Krahmer, E., Swerts, M., Theune, M., and Weegels, M.,“Prosodic correlates of disconfirmations”, Proceedingsof the ESCA Workshop on Dialogue and Prosody, Eind-hoven, 169-174, 1999.7. Anderson, A.H., Bader, M., Gurman Bard, E., Boyle,E., Doherty, G., Garrod, S., Isard, S., Kowtko, J., Mc-Allister, J., Miller, J., Sotillo, C., and Thompson, H.S.,“The HCRC Map Task Corpus”, Language and Speech34, 351-366, 1991.8. Koiso, H., Horiuchi, Y., Tutiya, S., Ichikawa, A., andDen, Y., “An analysis of turn-taking and backchannelsbased on prosodic and syntactic features in JapaneseMap Task dialogs”, Language and Speech 41, 295-321,1998.9. Caspers, J., “Looking for melodic turn-holding configu-rations in Dutch”, in H. de Hoop and T. van der Wou-den (eds), Linguistics in the Netherlands 2000, JohnBenjamins, Amsterdam, to appear.10. Schegloff, E.A., “Discourse as an interactional achieve-ment: some uses of ‘uh huh’ and other things that comebetween sentences”, in D. Tannen (ed) Analyzing Dis-course: Text and talk, Georgetown University Press,Washington DC, 71-93, 1981.11. Hart, J. ’t, Collier, R., and Cohen, A., A PerceptualStudy of Intonation, Cambridge University Press, Cam-bridge, 1990.

Melodic characteristics of backchannels in Dutch Map Task dialogues

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Melodic characteristics of backchannels in Dutch Map Task dialogues

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