Categorical and gradient properties of assimilation in alveolar to velar sequences: evidence from...

Journal of Phonetics (2002) 30, 373–396doi:10.1006/jpho.2001.0162Available online at http://www.idealibrary.com on

Categorical and gradient properties of assimilation

in alveolar to velar sequences: evidence from EPG

and EMA data

Lucy Ellis*

Department of Speech and Language Sciences, Queen Margaret University College,Clerwood Terrace, Edinburgh EH12 8TS, U.K.

William J. Hardcastle

Department of Speech and Language Sciences, Queen Margaret University College,Clerwood Terrace, Edinburgh EH12 8TS, U.K.

Received 9th November 2001, and accepted 14th November 2001

Place assimilation in English is now widely considered to be a gradualphonetic, not categorical process. This view is partly based onprevious EPG evidence of partial alveolar assimilations which lackcomplete stop closure on the alveolar ridge but show a residualtongue blade/body gesture. This study reports EPG data from 10speakers producing, at varying rates of speech, two experimentalsequences, /n#k/ and /?#k/ (the latter a lexical velar–velar sequencewith which apparent cases of complete assimilation can becompared). In fast speech, four distinct assimilatory strategies wereidentified. Two subjects never assimilated, four always assimilated inwhat appeared to be a complete fashion and the remaining four werethe most interesting, showing considerable intra-speaker variability.Two of these four produced the expected continuum of assimilatoryforms including partials. Unexpectedly, the other two produced eitherfull alveolars or complete assimilations in the manner of a binaryopposition. Follow-up EMA analysis yielded no evidence of thereduced coronal gestures found to be absent in the EPG-only data fortwo the speakers who, when they assimilated, did so in a completefashion. Although no claims are made regarding higher-orderrepresentations, we interpret this as evidence of marked individualdifferences in assimilation strategy.

r 2002 Published by Elsevier Science Ltd.

*Address correspondence to L. Ellis, Plymouth Institute of Neuroscience, University of Plymouth,Drake Circus, Plymouth, Devon, PL4 8AA, U.K. E-mail: [email protected]

0095–4470/02/$ - see front matter r 2002 Published by Elsevier Science Ltd.

374 L. Ellis and W. J. Hardcastle

1. Introduction

Assimilation describes the optional variation in the phonetic description of a speechsound as it becomes more like an adjacent speech unit. In the case of alveolarassimilation across a word boundary, e.g., red coat may be realized (in StandardSouthern British English) as [rBc g=Rp], that place as [A]l lhaFo] and nice shower as[/naFP PaR=/]. Frequency of assimilation is strongly correlated with speech rate/style.Barry (1992) and Kerswill (1985) found that assimilation of alveolar stops beforevelar stops is more common at faster rates of speech and a more casual style.

Previous electropalatography (EPG) studies (e.g. Hardcastle & Roach, 1977;Wright & Kerswill, 1989; Barry 1992; Nolan, 1992; Hardcastle, 1994; Zsiga, 1995)have noted partially assimilated alveolar stop closure targets. These observationswere usually made in the context of showing that the articulation of alveolar-to-velar sequences does not mirror the discrete phonological modeling of theassimilation process where the alveolar stop is replaced wholesale by a velar stop.The assumption underlying some of these studies, however (some discussed below),is that assimilation is a gradual process for each individual speaker. Since thisapproach seeks to identify all possible types of assimilation across subjects, with lessattention to individual behavior, a continuum of forms is found and theinterpretation naturally follows that assimilation is a gradient phenomenon. Afurther methodological shortcoming of previous studies is that subject and tokennumbers have tended to be small.

Manner of articulation is known to be a possible factor in determiningassimilatory outcomes. There is some evidence that nasal alveolar stops are moresusceptible to assimilation than non-nasal alveolar stops (Hardcastle, 1994).However, there is a dearth of data to help characterize how and how often alveolarstops assimilate when they are nasal and not oral. The study presented here providesa partial answer to this insofar as these nasal items are found to be highlyassimilable for some speakers but not for others. In the light of these inter-speakerdifferences, a simple comparison between the frequency of assimilation of nasal andoral stops is less important. It is not the purpose of the study reported here to takeup the phonological issues associated with the features oral vs. nasal. The studyfocuses on place of articulation assimilation and not manner, not least because novelum data was collected.

The motivation for the study presented here can be represented by the singleoverarching question: At what level in the generation and execution of an utterancedoes assimilation occur? In more detail we ask:

(i) Is gradual reduction the dominant assimilation mechanism not only across anumber of speakers but for each individual subject? In other words, how doesreceived wisdom about assimilation compare to the previously un-addresseddetail of intra-speaker patterns of alveolar to velar assimilation?

(ii) How is partial assimilation distributed across speakers?

Accounts of the phenomenon of place assimilation have traditionally taken twoforms. The first of these is typified by the feature geometry description of placeassimilation which invokes a process yielding categorical variation (e.g., McCarthy,1988; Spencer, 1996). An assimilation is expressed in autosegmental terms by thedeletion of the association line to one place node and consequent reassociation to

Alveolar to velar assimilation: EPG/EMA data 375

the following place node. It is assumed that this process is motivated by a high-order phonological rule. The question relevant to this review is how this model canaccommodate partial realizations of alveolars such as have been recorded inprevious studies (Wright & Kerswill, 1989; Barry, 1992; Nolan, 1992; Hardcastle,1994; Zsiga, 1995).

The second approach taken to assimilation is that it is a gradual phoneticphenomenon. One area where this view has become commonplace is in instrumentalstudies of place assimilation (acoustic, EPG and EPG with combined electro-magnetic articulography (EPG/EMA)). Data from EPG studies (Wright & Kerswill,1989; Barry, 1992; Nolan, 1992; Hardcastle, 1994; Zsiga, 1995) have providedevidence of partial alveolar assimilations in alveolar–velar sequences. An example isshown in Fig. 1 (taken from Hardcastle, 1994) which is a realization of an alveolar-velar sequence Fred can. Here, there is an absence of medial closure in the first threerows of the palate but side contact reaches as far forward as row 4 (frames 141–177), indicating the preservation of the supporting tongue blade/body gesture for thealveolar stop. By comparing the first approach with the second it can be seen thatphonological theory and instrumental research have tended to locate assimilationwithin the abstract planning stage and the concrete physical execution stage,respectively.

Wright & Kerswill (1989) take up the issue of the perceptual correlates ofincomplete assimilation in an EPG/perceptual study of alveolar assimilation. Theyfirst collected articulatory data and defined four assimilatory types: full alveolar(complete closure across one or more rows at the top of the EPG display); residualalveolar or partial alveolar (same description as for Fig. 1’s partial alveolar above);zero alveolar (complete assimilation indistinguishable from all-velar controls) andnonalveolar (all-velar control sequences). Examples of these are shown in Fig. 2 forthe phrase road collapsed and rogue collapsed. Their use of the term ‘‘residualalveolar’’ for intermediate types reflects their view that assimilations are perhapsnever complete and that a phonetic ‘‘reflex’’ remains which can be utilized bylisteners to restore the intended target.

Zsiga (1995) has provided EPG and acoustic evidence that while lexical /P/(whether underlying as in fish or derivational as in impression from impress) iscategorical, postlexical [P], brought about from production of the sequence /s#j/ in

Figure 1. EPG patterns showing a partial alveolar assimilation in the phraseFred can.

Figure 2. EPG articulation types for full alveolar contact, partial alveolarcontact, zero alveolar contact and underlying velar (taken from Wright &Kerswill, 1989, p. 51).


for example, miss you, is gradient. Featural representation is said to mostappropriately handle the lexical /P/ phenomenon and the principle of ‘‘gesturalblending’’ handles the postlexical phenomenon, said to arise naturally as part of thephonetic grammar of English from the properties of the speech production system.The prevalence of partial assimilation has been widely interpreted as evidence thathigh-order phoneme substitution rules have no part to play in assimilation. Instead,the mechanism is likely to be located either at speakers’ phonetic representation, alevel which permits noncategorical variation such as stop-closure weakening (a viewfavored by Barry (1992) and Kuhnert (1993) in relation to alveolar assimilation), orlocated at the physical execution stage of speech production itself where inertialproperties of the articulators are observed (as favored by Ohala, 1992).


The ‘assimilation-as-gradual’ approach reflects a wider trend towards attributingphenomena thought to be phonological to phonetics, a level previously thought ofas mere implementation of phonological form determined at a higher level. Keating(1996) has stated that ‘‘assimilation in principle can produce categoricalallophonesythis seems to be a rare occurrence. When suspected cases are examinedcarefully, they generally show gradient characteristics’’. In a similar vein but moreforcefully, Fowler (1995) comments that ‘‘feature-spreading theories are nowgenerally agreed to have been disconfirmedybecause close examination of the datashows that coarticulation is clearly gradient in character’’. The use of the termcoarticulation in the latter quotation we take here to encompass what is hereafterdescribed as assimilation (we make no distinction between the two as they are usedinter-changeably in much existing literature on the subject of word-final alveolars).Pierrehumbert & Talkin (1992) present evidence that allophonic variants of /</ and/h/ are determined by the low-level phonetic effects from the prosodic environmentin which they appear. Sproat & Fujimura (1993) propose that the treatment of lightand dark /l/ as categorically distinct phonological elements does not reflect the facts.They argue that in English at no level in the psycholinguistic mapping, fromabstract lexical representations to physical speech movement, is there a stage wherethe separate forms [l] and [O] are distinguished. The quantitative variations they findare attributed to predictable properties of the speech production mechanism and notallophonic rules. Claims that speakers have minimal stored information are takenfarthest in articulatory phonology (Browman & Goldstein, 1989). This modeleliminates mapping between the planning and execution of speech since ‘‘gestures’’are both phonological and phonetic entities. Connected speech phenomena such asassimilation are the result of variations in the temporal overlap between individualgestures. In the case of apparent consonant deletion, e.g., must be /&mest bi/ as[&mesbi] they predict that the alveolar constriction for /t/ is still present in the‘‘fluent’’ version of must be only it has been ‘‘hidden’’ or completely overlapped bythe bilabial closing gesture. Browman & Goldstein (1990, p. 370) also admit a(limited) role for decreasing magnitude of the gestures: ‘‘Additionally, reductions inmagnitude may combine with increased overlap, leading to greater likelihood of agesture being ‘hidden’ by another gesture...Such reductions of movement amplitudeoften, but not always occur in fast speech’’. While overlap is attributed to the slidingin time of gestures on the gestural score, the nature of the mechanism which governsreduction in magnitude is less clear.

Kuhnert (1993, 1996) rejects the idea that gestural blending can account for allassimilatory forms. In an EPG/EMA study of alveolar-to-velar assimilation, shedemonstrates that all five of her German and English speakers produce residualalveolar articulations at least some of the time. EPG data on its own is limitedbecause it gives information on tongue–palate contact only and not on types oflingual movement which do not result in contact with the hard palate. Thisinformation is important in the light of speculation from EPG studies thatapparently complete assimilations from alveolar to velar sequences may involvesome residual movement in the form of reduced tongue tip raising (Wright &Kerswill, 1989). While her result appears to support the gestural approach, Kuhnertcomments that the tokens which show a complete disappearance of the coronalgesture are problematic for articulatory phonology. In this framework, the sliding ofgestures in time is said to account for assimilations, but in cases of complete


disappearance of the coronal gesture, alterations of magnitude must come into play.The mechanism governing gradations in magnitude of gestures is less well explainedin the articulatory phonology framework.

Holst & Nolan (1995) in their acoustic study also found a continuumof assimilatory forms in the sequence /s/ to /P/ (as in restocks shelves) where /s/blends with /P/ to yield a ‘‘contour segment’’ (the result of the rapid productionof two place-incompatible targets). Like Kuhnert with alveolar to velar sequences,they found that /s/ to /P/ assimilatory forms and underlying forms (/P/ to /P/)can become indistinguishable. Rather than considering these complete assimilationsto be at one end of a reduction continuum, they invoke a phonological rule.The reason they do not consider the [P] yielded by assimilation as simply themost extreme form of reduction is that it was found to be significantly longerthan canonical [P]: ‘‘The extra duration of [P]-like frictiony must mean thatsome duration ‘belonging’ as it were to [s] has been articulated as [P]Fthat is,in a very real sense, a process of assimilation has occurred’’(p. 328). In a laterpaper reporting on similar data Nolan, Holst & Kuhnert (1996 p. 116) proposethat, in fact, two processes act together to produce the complete assimilationforms: ‘‘the first [s] gesture yhad been replaced by a [P] gesture beforethe phonological representation was given its articulatory representationFinvolving,of course, considerable overlapping of the (now) two [P] gestures bringing theduration of the fricative event down to near, but still greater than, that ofthe singleton [P]’’. Interestingly, articulatory phonology takes the opposite view ofthis extra duration. Here, it is taken as direct evidence that assimilated formsare still distinguishable from underlying [P]Fthat is that there are still two gestures,one for /s/ and one for [P] (Browman, 1995). Holst and Nolan and Nolan et al.,however, argue for the ‘‘co-existence of an optional cognitive phonological processof assimilation. This might be seen as an advance cognitive remedial action tosmooth the articulators task’’. It is not clear, however, how this cognitive strategy istriggered and how it relates to the gestural blending mechanism. Furthermore, inthis analysis the result of the assimilation is a geminate /P7/, something not neededin English otherwise.

If then the ‘‘natural properties of the articulators’’ can be called upon to explainsome gradient assimilatory effects, this must be seen in the context of work whichbrings to light evidence that place assimilation is not universal. Farnetani & Busa(1994) found that in Italian, assimilation in /nk/ clusters is always categorical. Barry(1992) has shown that alveolar–velar assimilation in Russian is nowhere near asextensive as it is in other languages. On the basis of an auditory study of a speakerfrom North East (Durham) England, Kerswill (1987) notes either an absence or nearabsence of place assimilation in contexts where it might be predicted to occur inother varieties of English. This sort of evidence suggests that place assimilation issomething which speakers control.

The following sections describe an electropalatography (EPG) study and a follow-up combined electromagnetic articulography (EPG/EMA) study. The EPG-onlyexperiment involved 10 speakers, each of whom produced 10 repetitions of analveolar–velar sequence /n#k/ and a control /?#k/ sequence with which assimilatedalveolar–velar sequences can be compared. This is described first followed by afollow-up combined EPG/EMA study. The latter involved a re-recording of twospeakers producing the same stimuli as for the first study.


2. Experiment 1FEPG only

2.1. Method

2.1.1. Materials

Test materials comprised two experimental sequences of the structure [yV1C1#C2V2y], where ‘‘#’’ represents a word boundary. The experimental consonantsequences were:

(i) an alveolar nasal stop to a velar plosive /n#k/,(ii) a velar nasal stop to a velar plosive /?#k/.

These were embedded in the sentences It’s hard to believe the ban cuts no ice andI’ve heard the bang comes as a big surprise, respectively. The first captures thepotential assimilation site and the second is a control lexical velar-to-velarsequence the purpose of which was to serve as a comparison for apparentcases of complete alveolar assimilation and as a benchmark for the identificationof partial assimilations. The immediate vowel environment of the consonants waskept consistent. In the two experimental sentences, the preceding vowel V1 wasalways /a/ and the following vowel V2 was always /e/. All experimental words beginwith /b/ to ensure minimal perseverative coarticulatory influence on the productionof the consonantal sequence. Eight filler sentences of no experimental interest wereadded to the test materials as distracters bringing the total number of test sentencesto 10.

2.1.2. Subjects

Ten adult speakers of English (two male, eight female) with no reported speech orlanguage pathology or hearing impairment were recorded. All speakers wereexperienced users of EPG palates. The subject group represented a range of BritishEnglish accents. Four spoke Standard Southern British English (subjects D, H, Iand J); one subject was originally from the North East of England (subject F); onewas originally from Eastern Australia (subject G); one was from Northern Ireland(subject C) and the remaining three subjects were Scottish. Two of these three werefrom the West of Scotland, Glasgow (subjects A and B) and the other was fromFife, near Edinburgh (subject E).

2.1.3. Data collectionData was acquired using the reading electropeletography EPG 3 system whichrecords an acoustic signal along with details of the timing and location of tonguecontact with the hard palate. Elicitation of the data fell into two parts to obtainslow careful speech and secondly, fast casual speech. For both parts, 10 repetitionsof all 10 test sentences were required from subjects. For the first part, subjects werepresented with a randomized list of 100 single sentences and asked to read themindividually in a slow and careful style. For the second part, subjects were given thesame sentences but this time ordered into groups of three with filler sentencespurposely distributed to avoid clustering of experimental sentences. Subjects wereinstructed to read out the sentences in each group fast and casually one after theother avoiding pauses in between. A time limit of 5 s was imposed on the production


of each group of three sentences in an attempt to impose a minimum speech rateacross all speakers. Subjects wore their palates for an hour prior to the recording inorder to become accustomed to the feel of the device.

2.1.4. Spatial analysisClassificatory criteria were used to identify and describe the various realizations ofthe alveolar stop. Possible assimilatory forms identified for the purposes of thisstudy are: nonassimilations, partial assimilations, and apparent complete assimila-tions (the latter item with the proviso that more sophisticated movement-trackingdevices may reveal some of the alveolar raising gesture short of contact with thepalate).

The procedure for identifying of partial assimilations from EPG data has receiveda lot of attention in the literature (Kerswill, 1985; Barry, 1992; Kuhnert, 1993, 1996;Hardcastle, 1994). The method adopted in this study follows the work of Hardcastle(1994) and Hardcastle & Roach (1977) upon which the methodology of somesubsequent studies in this area (e.g., Nolan, 1992) have been based. In order tobe identified as a nonassimilation, a token must show contact in the alveolar regionof the palate (this is shown in Fig. 3 below, horizontal lines). The importantquestion is how much contact must be absent before an alveolar stop becomes anassimilation, albeit partial, in cases where there is a break in continuous contactedelectrodes horizontally. Fig. 4 shows EPG patterns yielded from the present studyfor four types of fast speech /n#k/ realization, in the order of lessening alveolarcontact for target stop closure. Individual frames are 10ms apart and tongue–palatecontact is indicated by filled circles. The top of each diagram is the alveolar regionand the bottom is the velar region. The focus of attention in these examples is therealization of the stop closure gesture and not its duration, total contact or degreeof overlap with the velar gesture. Following the method adopted by this study, thefirst example, Fig. 4(a), shows ‘‘full’’ alveolar stop closure. The second, Fig. 4(b),shows a realization with some loss of contact in the alveolar region (starting atframe 424) resulting in an incomplete spread of electrodes. This token is classed as anonassimilation since there is still contact in the mid-sagittal area (this area is shownby the dashed line in Fig. 3). This is an established criterion (Hardcastle & Roach,1977; Nolan, 1992) to indicate that, crucially, the coronal raising gesture is still

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0

Alveolar region

Palatal region

Velar region

Figure 3. Division of EPG palate into three different articulatory regions (F)mid-sagittal area indicated by - - - -.

Figure 4. (a–d) fast speech /n#k/ tokens showing varying amounts of tongue–palate contact in the alveolar region.


achieved even though there are gaps in tongue–palate contact in the alveolar region.Contact is less still in Fig. 4(c) where only the right-hand side of the tongue tip/blade appears to have made contact in the mid-sagittal area between frames 252 and259. This pattern, however, is open to different interpretations. For example, it maybe considered the result of the left-hand side of the tongue making contact with theteeth. This again is classed as a nonassimilation due to the presence of mid-sagittalcontact. Finally, Fig. 4(d) is a partial assimilation. There is no contact in thealveolar region at all although there is evidence of the remaining supporting tonguebody gesture, as indicated by the extended contact at the lateral margins of thepalate between frames 316 and 321. This extended contact, in this case, is furtherforward than lateral contact for the same speaker’s production of lexical /?/ in thesequence /?#k/, which is taken to be evidence of a residual tongue body gesturerather than a complete ‘‘disappearance’’ of the alveolar gesture.

2.2. Results

In the fast speech condition, a number of speaker-specific assimilatory strategieswere identified. Two subjects produced only nonassimilations, four subjects always


produced what appeared to be complete assimilations and the remaining foursubjects each produced forms of /n#k/ varying between classifications. In the carefulspeech condition there were four assimilations in total. Subject G produced 1 andsubject H produced 3. Both were subjects from the group of four who alwaysproduced what appeared to be complete assimilations in the fast speech conditionsuggesting that these subjects may have a predisposition to assimilation.

Fig. 5 shows the occurrence of assimilations for all speakers in fast speech. Allsuch tokens are categorized as either nonassimilations or assimilations. There wereonly two partial assimilations in the entire data set and these were produced bysubjects A and B. Because there were so few, these have been subsumed into theassimilation category for the purposes of Fig. 5. Speakers A–J are ranked from leftto right according to frequency of assimilations. Fast speech /n#k/ EPG patterns forthose speakers who produce habitual complete assimilations (G, H, I and J) werespatially indistinguishable from their fast speech lexical /?#k/ productions. For thesesubjects, a further comparison was made between the duration of the velar closure/?/ resulting from the assimilation of /n/ and the /?/ produced for the control velar–velar sequence. The duration of velar closure for both forms was measured as theinterval between onset of velar closure (EPG-defined) and the end of voicing duringthe stop closure (determined from the waveform). In order to factor out speech ratevariations that might occur between and within speakers’ tokens, this durationalvalue was then expressed as a percentage of a larger stretch of speechFbetweenonset of the first vowel /a/ and the end of /s/ in yban cutsy . Separate variance t-tests showed that percentage of velar closure for assimilated /n/ and for lexical /?/was not significantly different for subject H (p=0.9504) and subject I (p=0.7778).However, the difference between these two forms was highly significant for subjectsG (p=0.0044) and J (p=0.0093) in the direction of assimilated /n/ being the shorterof the forms of /?/.

Separate variance t-tests confirmed that all individual subjects’ fast speech tokenswere produced at a significantly faster rate of speech than their careful speechtokens (Shapiro–Wilk tests showed normal distributions for all experimentalsamples). The rate of speech for a token was measured as the duration betweenthe onset of /a/ in the word ban and the end of frication in /s/ in cuts for the

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assimilatedalveolarsnonassimilatedalveolars

Figure 5. Distribution of assimilations and nonassimilations for individualspeakers A–J in fast speech productions of /n#k/.


experimental sentence It’s hard to believe the ban cuts no ice. Measurement wastaken from the waveform. This particular section of speech was chosen because itincludes the consonantal sequence itself plus some contextualizing speech. Fig. 6shows a measure of speech rate for each production of the experimental sentencecontaining /n#k/ for all subjects in both the careful and the fast speech conditions.The horizontal line drawn at around 540ms separates all careful speech tokens fromfast speech tokens. Durational values for all fast speech tokens appear below thisline. There was no overlap between the duration for tokens from the differentconditions. Scrutiny of Fig. 6 allows us to determine whether the contrast inassimilation strategy for subjects E and F, who never assimilated in fast speech, andsubjects G, H, I and J, who always assimilated in a ‘‘complete’’ fashion, is due tocomparative magnitude of increase in speech rate from the careful to fast condition.We can see that the two samples above and below the dividing line for subject Fare, in fact, separated by the smallest margin of all the subjects, which may indicatethat speech rate was not sufficiently increased to motivate an assimilationstrategy for the experimental sequence. This subject’s mean value for the 10 carefultokens was 632.9ms while for the 10 fast tokens it was 488.5ms, a difference of144.4ms which was the smallest difference of all subjects. However, subject E hasone of the largest gaps between the careful and the fast samples, a difference inmean values of 325ms. Subject H, who by contrast implemented a habitual‘‘complete’’ assimilation strategy, had the largest mean speech rate separation of allat 370.9. However, overall, there is little evidence for a relationship betweenmagnitude of increase in speech rate from the careful to fast condition andfrequency of assimilation.

But the most notable result comes from the fast speech data for those fourspeakers who, as shown in Fig. 5, vary between nonassimilation and assimilation.Close attention to the EPG patterns reveals a fundamental contrast in assimilatorystrategy between subjects A and B on the one hand and subjects C and D on the

Subjects

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A B C D E F G H I J

Figure 6. Scatter-plot showing rate of speech for /n#k/ fast speech tokens(below the line) and for careful speech tokens (above the line). Rate of speechwas measured as time in ms between the onset of /a/ in ‘‘ban’’ and the end offrication in /s/ of ‘‘cuts’’. Alphabetic labels for subjects appear in lowercase.


other. The data for subjects A and B range from full alveolar stop closure throughintermediate partial assimilations to velar patterns indistinguishable from lexical/?#k/ controls. Fig. 7(a) shows all 10 (fast speech) repetitions of /n#k/ produced bysubject B. The patterns for subject A are very similar. Each line captures the

Figure 7. (a) All 10 fast speech /n#k/ repetitions produced by subject B rangedin order of amount of tongue–palate contact in alveolar region: repetitions atthe top are full alveolars and those at the bottom show apparent absence ofalveolar gestureFeach numbered line shows a single repetition capturing theonset of alveolar closure, or velar closure if assimilation has taken place, upto and including release of velar closure for /k/. (b) As for 7(a) above but forsubject C.

Figure 7. (continued ).


tongue–palate contact for a single realization of the target sequence. All realizationsare ordered to show a continuum from full alveolar stop closure at the top toapparently complete assimilation at the bottom. Repetition 3 on line 3 of Fig. 7(a)shows less contact in the alveolar region (rows 1–3) than repetitions 1 and 2. For


repetition 4, closure on the alveolar ridge (corresponding to contact in the first threerows of the palate) is only partial. Repetition 5 is a partial assimilation whereby aresidual tongue body gesture is evident as far forward as row 3, frames 316–320.The next five repetitions are classed as complete assimilations following aprogression of lessening side contact until repetition 10, which is the result ofeither a spirantized /k/ or a closure made too far back for the EPG palate tosample. These latter repetitions are thus classified due to the fact they areindistinguishable from the same speaker’s control data (see Section 2.1.4).

By contrast, Fig. 7(b) shows all fast speech repetitions of /n#k/ produced bysubject C, who, like subject D, adopted a segmental assimilation strategy whereassimilation appears to involve the wholesale substitution of an alveolar stop for avelar stop with no partial forms in between. For subject A, B, C or D no patternemerged when the length of velar closure for /?/ arising from complete assimilationwas compared with that for lexical /?/. A comparison of the duration of the twoforms of fast speech /?/ for all speakers taken together (i.e., those who assimilatedin the fast speech condition) was made and a t-test showed no significant difference.As before, t-tests were performed on velar closure expressed as a percentage of thesection of speech between onset of the first vowel /a/ and the end of /s/ in ybancutsyin order to normalize variations in speech rate.

The data from subjects C and D unexpectedly suggest the adoption of a binarysegmental assimilation strategy. That is, either full alveolar contact is achieved fortarget /n/ or an assimilation takes place whereby the place features for /n/ appear tohave been completely swapped for those of /?/. Analysis of speech rate for subjectA, B, C and D’s fast speech tokens shows that assimilation strategy was notattributable to this factor.

2.3. Discussion

According to our classification, only two partial alveolar realizations were yieldedfrom the entire data set and these were produced by subjects A and B. Althoughthese subjects only produced one each, their patterns clearly show a continuum ofreduction in the alveolar gesture. Their patterns are of the type generally taken todemonstrate that assimilation is a gradient and phonetic phenomenon. The very lownumber of partials over the entire data set is very surprising in the face of evidencefrom previous studies that assimilation is a gradual process. Instead, it appears fromthe data that this applies only to some subjects while the strategy others adoptsuggests an all-or-nothing segmental choice between nonassimilation and completeassimilation. These assimilations are indistinguishable from the same subjects’production of lexical velar–velar sequences. Subjects C and D produced eithernonassimilations or complete assimilations and subjects G, H, I, and J producedcomplete assimilations exclusively. The finding that some speakers tolerate fewerallophonic variants (as the result of assimilation) compared to others has come tolight from the observation of individual speakers’ patterns of assimilation, anapproach lacking in some previous EPG studies (Kerswill, 1985; Wright & Kerswill,1989; Hardcastle, 1994). In the case of place assimilation, it appears that thescrutiny of individual variability yields some surprising results. If we consider at thispoint the relationship between the accent of individual speakers and their fast speechassimilatory behavior, we find that accent could well be a factor in the patterns we


found. Three of the four speakers who had no variation in their output, in that theyalways assimilated in an apparently categorical fashion (subjects G, H, I, and J),had accents that could be described as Standard Southern British English (SSBE).Another result that perhaps supports the notion of assimilatory patterns as dialect/accent specific is that the subject from NE England did not assimilate at all in fastspeech. Kerswill (1987) had previously noted an absence or near absence ofassimilation in this part of the U.K. Further, the two subjects who showed acontinuum of patterns were both from the West of Scotland. The remaining subjectsshowed no clear pattern with respect to accent and assimilatory pattern. Evidence isgrowing that assimilation is dialect/accent or language-specific (Farnetani & Busa,(1994) for Italian, and Barry (1992) for Russian) and not universal. This means thataccounts of gradient data based on gestural weakening or inherent inertial propertiesof the articulators look less likely. Nolan (1992, p. 278) comments that inevitably:‘‘it is a phenomenon over which speakers have control. This will provide furtherevidence that a greater amount of phonetic detail is specified in the speaker’sphonetic representation or phonetic plan than is often assumed’’. In a similar veinBarry (1992, p. 399) comments that evidence of gradient assimilation means that‘‘phonological theory, in its widest interpretation as that part of a theory oflanguage which accounts for the speaker’s knowledge of the sounds patterns of hisor her language, may need to set its bounds much closer to the fine detail ofarticulatory activity than has generally been acknowledged’’.

3. Experiment 2Fcombined EPG/EMA

3.1. Introduction

EPG data gives information on tongue–palate contact only. Details of lingualmovements which do not involve contact with the hard palate are not revealed withthis technique. This information is important in the light of speculation thatapparently complete assimilated /n#k/ sequences (of the type produced in fast speechby all subjects at least some of the time in this study apart from E and F) may beaccompanied by some residual movement in the form of vertical tongue-tip raising(Wright & Kerswill, 1989). Kuhnert (1993, 1996) found that identical EPG patternsarising from assimilated alveolar–velar sequences and from velar–velar controlsequences may sometimes show different tongue tip/blade trajectories in the EMAdisplay. The finding that they also may not, undermines the view that differences inlexical form will always result in distinct phonetic output.

Two subjects D and H who, when they produced assimilations in fast speech didso in an apparently categorical fashion, were selected to be recorded using combinedEPG/EMA to look again for any gradience. In the EPG-only experiment, subject Dappeared to apply a binary segmental strategy (complete assimilation vs.nonassimilation) while subject H appeared to apply a habitual complete assimilationstrategy. Velar to velar sequences /?#k/ acted as controls assuming that theseinvolve minimal or no tongue-tip raising.

Subject A was originally selected for re-recording with the combined method butthe data could not be analyzed due to the fact that the transmitter coils housed inthe helmet (see following section) were, correctly, not positioned on the midline


during the recording. Subject A was one of two speakers who produced the fullrange of assimilatory variants and was considered an interesting candidate from thepoint of view of looking more closely at whether the complete assimilationsproduced at one end of the reduction continuum were, in fact, complete and showedno residual tongue-tip raising.

3.2. Method

Subjects D and H were re-recorded using EPG in combination with the CarstensAG100 Electromagnetic Articulograph (EMA). EMA is a transduction device whichtracks x–y movement of coils attached to the articulators in the mid-sagittal plane.The EMA signal was synchronized with the EPG signal by the use of serial portcommunication. Rouco & Recasens (1996) evaluated the effects of EMA coils onEPG contact patterns where EMA and EPG data are acquired simultaneously andfound that for /l/, there may be some loss of EPG contact on the midline in thealveolar region. They suggest that recovery of affected electrodes can be successfullyachieved by comparing EPG data collected without tongue coils on the tonguesurface with EPG data collected with coils attached to the tongue: ‘‘Inspection ofthe two linguopalatal contact patterns for the same sequences should provide preciseinformation about the areas of the palate where overlap between coils and palateoccurs’’. This check was easily carried out for the present study since identicalmaterials were used for the EPG-only study reported in Section 2 and for thefollow-up combined EPG/EMA study.

A total number of eight coils were attached to the articulators on the midline.These were: upper incisor (reference coil); lower incisor ( jaw movement); upper lipand lower lip; tongue tip; tongue middle; tongue dorsum and bridge of nose(reference coil). Some coils were used as reference points for the purposes ofpostprocessing such as head movement correction (upper incisor and bridge of nose)and rotation to allow data normalization across subjects and between tokens fromthe same recording session. All postprocessing was carried out using the ‘‘Tailor’’software from Carstens. Data was processed for head movement correction in themid-sagittal plane to ensure that the two reference coils register at the samegeometrical coordinates. This compensates for any head movement backwards,forwards, up and down relative to the helmet (see Hoole (1993) for an EMAdata reliability study). The method of rotation used here was to use a T-bar torecord the occlusal plane of subjects. At the end of the recording session, two sensorcoils were attached to a flexible plastic T-bar at either end. The subject then placesthis in the mouth until it reaches the rear molars and bites down on it while a briefrecording is made. All data for that particular session was then rotated so that thisocclusal plane is horizontal. The coils of experimental interest for this study weretongue-tip and tongue dorsum coils. The stimuli and data elicitation proceduresfor this experiment were identical to that of the EPG-only study described inSection 2.1.1.

3.2.1. Data analysis

A set of Matlab programs called EMA tools created for the analysis of EPG/EMAdata was used (Hoole & Nguyen, 1992; Nguyen, 2000). Measurement and display of


the data took two forms. Firstly, the x and y articulatory coordinates for the tonguetip coil and the tongue dorsum coil at the onset of the articulatory beginning for thesequence were measured. The onset corresponds to the moment of maximumtongue-tip displacement for all target /n#k/ sequences and the moment of maximumtongue dorsum displacement for all control /?#k/ sequences. Minimum tangentialvelocity was used to identify maximum displacement of the coils in both dimensions.If tongue-tip position for any EPG-defined assimilated /n#k/ token at this momentwas higher than it was for any /?#k/ tokens, then this was identified as a residualalveolar gesture and thus complete assimilation has not taken place. The secondtype of analysis of the data was for the purpose of a visual reference to show thespatial trajectory described by all three tongue coils over a particular time span. Thebeginning point of this time span was the middle of the preexperimental sequence/a/ and the end point was the middle of the post-experimental sequence /e/(for y.ban cutsy), capturing the production of either target /n#k/ or /?#k/ for thecontrol sequence.

3.3. Results and discussion

On analysis of the EPG data, it was found that subjects D and H replicated theassimilation strategy they each used in the EPG-only study. On analysis of alltokens, there was no evidence of gradience in the form of tongue-tip displacementfor assimilated /n#k/ sequences of a greater magnitude than that displayed for aneutral velar control articulation. The articulatory positions of tongue-tip andtongue dorsum coils for fast speech /n#k/ and /?#k/ tokens at the beginning of theconsonant cluster for subjects D and H are shown in Figs 8(a) and (b).x- and y-axis position is shown in mm. The left-hand cluster on each graph showstongue-tip positions and the right-hand cluster shows tongue dorsum positions.Subject D’s articulatory positions are plotted for nonassimilated alveolar sequences(numbering 5), apparently assimilated alveolar sequences (numbering 5) and all velarcontrol sequences (numbering 10). For subject H, articulatory positions are plottedfor apparently assimilated alveolar sequences (10) and velar control sequences (10)only. Subject H produced only ‘‘complete’’ assimilations.

In Fig. 8(a), maximum vertical displacement for the tongue tip is defined by fullalveolar closure for the five nonassimilations produced by subject D. For thisspeaker, however, it is clear that the tongue-tip cluster for the ‘‘complete’’ /n/assimilations overlaps with the cluster for the underlying velars. This means thatthere is no vertical displacement for any assimilated /n#k/ sequence beyond thatwhich accompanies a neutral velar control sequence and thus no intermediate partialassimilation stage in between full alveolar stop closure and complete assimilation.

For subject H, a similar picture emerges with overlap of tongue-tip positions forassimilated /n/ and lexical /?/. Since this subject produced no full alveolar stopclosures for /n#k/ no maximum tongue-tip raising is defined, but it seems that theraising movement for underlying velars is surprisingly advanced for some tokens.Once again, the height range for underlying forms is greater than that forassimilated /n/ forms suggesting a constraint on movement variability for the latter.These results support the interpretation that subjects D and H have a binarysegmental choice between nonassimilation and complete assimilation when theopportunity to assimilate arises.

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/n/ non-assimilations /n/ 'complete' assimilations underlying velars

/n/ 'complete' assimilations underlying velars

Figure 8. (a) Subject D: articulatory positions (mm) for tongue tip (left-handcluster) and tongue dorsum (right-hand cluster) at the moment of maximumtongue tip displacement for all nonassimilated /n/ tokens (***), assimilated/n/ tokens (***) and all lexical /?/ tokens (� � � ). (b) Subject H:articulatory positions (mm) for tongue tip and tongue dorsum at the momentof maximum tongue tip displacement for all assimilated /n/ tokens (***) andall underlying /?/ tokens (� � � ).

"

Figure 9. (a) Coil trajectory displays for individual fast speech /n#k/ tokensproduced by subject D. The nonassimilations are grouped as tokens 1–5, thisdoes not reflect the order in which they were produced. Movement paths ofthe tongue-tip, tongue body and tongue dorsum coils are shown from the mid-point of preexperimental sequence /a/ up to the mid-point of the following /e/.The solid line is an interpolation of tongue configuration either at maximumtongue-tip displacement (for nonassimilations, here numbers 1–5) or atmaximum tongue dorsum displacement (for assimilations numbers 6–10).(b) Coil trajectory displays for individual fast speech control /?#k/ tokensproduced by subject D. Movement paths of the tongue-tip, tongue body andtongue dorsum coils are shown from the mid-point of preexperimentalsequence /a/ up to the mid-point of the following /e/. The solid line is aninterpolation of tongue configuration at maximum tongue dorsumdisplacement.


Figure 9. (continued ).



Fig. 9(a) and (b) show displays of tongue movement trajectories for all 10 of thefast speech /n#k/ and control /?#k/ sequences, respectively, produced by subject D.Each pictogram shows the trajectory of the coils from the middle of the vowel /a/up to the middle of the posttarget sequence vowel /e/ in fast speechyban cutsy orybang comesy . A solid line linking all three coil trajectories for each tokenrepresents an interpolated tongue configuration either at maximum tongue-tip ortongue dorsum displacement. For subject D who sometimes assimilated theexperimental sequence and sometimes did not, displays for nonassimilated /n#k/tokens (numbering 5) show tongue configuration at maximum articulatorydisplacement of the tongue-tip coil which corresponds to the formation of full stopclosure for /n/, i.e., the beginning of the consonant sequence. All other displays forthis subject including those of control sequences, show tongue configuration at themoment of maximum articulatory displacement for the tongue dorsum coil. This isso because it has already been established from the articulatory position results inFig. 8(a) that the apparent completely assimilated /n#k/ tokens do not, in fact,reveal residual alveolar gestures. It is therefore assumed in these cases that theobserved trajectory of alveolar movement is due to biomechanical movement ‘‘insympathy’’ with the prevailing tongue dorsum movement for velar closure. Thecontrol sequences, of course, have no alveolar specification and so the onset ofthe sequence is velar anyway. Therefore, the onset of velar closure is taken to be thebeginning of the sequence. It must be noted here that the tongue-tip coil in thedisplays for subject D’s /n#k/ tokens where a full alveolar closure is made (displays1–5), never actually appears to reach the alveolar ridge shown as part of thedisplayed palate trace. This is probably because the position of that particular coilon the surface of subject D’s tongue was just outside the area of the tongue tip/blade which actually made contact with the ridge. It must also be pointed out thatthe trajectory display 5 for subject D /?#k/ in Fig. 9(b) is very unusual. Themovement pathway for the tongue dorsum is very high and would seem to indicatethat the dorsum coil had been loosened from the tongue surface. Neither theprevious nor the following repetition produced by subject D appear to be similarlyaffected. This accounts for the outlier in the articulatory position graph in Fig. 8(a)with a value of 180mm in the y-axis. There appears to be no consistent difference intongue configuration when subject D’s assimilated /n#k/ displays are compared withthe lexical /?#k/ displays. Similar displays for subject H also failed to showconsistent differences between assimilated and control sequences.

4. Discussion and conclusions

This study has found that inter- and intra-speaker assimilation differences areconsiderable. In fast speech, some speakers assimilate more than others, but themost significant result came from the data for those subjects who showed variationwithin their /n#k/ productions: one group assimilated gradually while assimilationfor the other group appeared to involve a wholesale segmental substitution of placeof articulation. It would appear that the data for the former group, subjects A andB appears to bear out the more recent instrumentation-based analysis which seeks toattribute assimilatory effects to general (motor control) principles of gestural overlapand reduction. The data for the latter group, subjects C and D, bear out the


phonological analysis which locates assimilation at the abstract planning stage ofspeech production. For the four subjects who always assimilated in an apparentlycomplete fashion in fast speech, durational analysis was carried out (Section 2.2)comparing the duration of velar stop closure /?/ resulting from apparently completeassimilation of /n/ and duration of lexical velar stop closure. No significantdifference was found for subjects H and I. The lack of a measurable difference lendssupport to the identity of these forms and therefore the categorical nature of theassimilations. However, significant differences were found for subjects G and J in thedirection of shorter velar closure for the assimilated form of /n/. This provides somecounter-evidence to the claim that assimilatory forms of the complete variety andunderlying forms are likely to be indistinguishable. No claims are made in this paperabout differences in cognitive representations regarding assimilation, at the level ofthe speaker (some previous studies have attributed some assimilatory patterns tolow-level phonetic effects and other patterns, from the same data set, to theimplementation of a cognitive assimilation rule, e.g., Holst & Nolan (1995) asdescribed in Section 1).

The very few partial assimilations found overall (numbering two) stronglyconflicts not only with the general current approach to assimilation, which assumesit is gradual, but with the results of previous studies. For instance, Kuhnert (1993,1996) proposed that all speakers produce partial assimilations at least some of thetime.

Articulatory phonology predicts the gradient assimilatory effects found in some ofthe data presented in this study, but cases of complete disappearance of the alveolargesture are treated as the result of gesture magnitude blending. This is anunsatisfying ad hoc account that edges away from the notion that all connectedspeech phenomena can be accounted for by the principle of overlapping gestures. Amore serious problem is that all gestures are treated as formally identical entities,leaving no opportunity to express coronals as phonologically special.

Aside from theoretical issues, this study has highlighted an importantmethodological issue. The lack of reporting of individual-speaker assimilatoryvariation in some previous EPG studies is a serious flaw. The analysis of intra-speaker variation undertaken in the present study has shown that assimilation maynot spring from a single mechanism common to all speakers of a languageFinsteadspeakers may select from a choice of more than one strategy. If, however,assimilatory forms were itemized from the results of the present study across allsubjects, a group continuum of forms would emerge and hence the interpretationmade that assimilation is gradual, as was the case in previous studies.

The present study has noted clear differences across a relatively large subjectgroup. Other accounts of individual differences can be found for instance in Byrd &Tan (1996) and Byrd (1992) with regard to the timing involved in the production ofthe front–back and back–front sequences /d#g/ and /g#d/. Differences are noted indegree of overlap in the onset of stop closures, amount of tongue–palate contactand intra-speaker variability of such tongue–palate contact. Studies looking at otheraspects of speech production such as the interplay between movement velocity andmovement amplitude have also found considerable variation in individuals (Kuehn& Moll, 1976; Ostry & Munhall, 1985). Ostry & Munhall (1985) for instance, foundthat as speakers increased their speech rate some individuals did so by increasing thevelocity but not the amplitude of movements, while other speakers decreased the


amplitude but not the velocity of movements. Individual speaker strategy andvariability in speech production seems to be a fruitful area of research activity thatdeserves further attention.

The authors would like to express their gratitude to Phil Hoole, Anders Lofqvist, Barbara Kuhnert andJames Scobbie for their very helpful comments on earlier drafts of this manuscript.

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