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Prosodic structure and tongue twister errors1

Karen Croot

Claudia Au

Amy Harper

Word count (MS Word)

Abstract: 188 words

Manuscript excluding references: 8138 words, references 732 words, total 8870 words

Address for correspondence:

School of Psychology A18

University of Sydney

NSW AUSTRALIA 2006

Phone: +61 2 9869 4696

Fax: +61 2 9356 5223

Email:

karenc@psych.usyd.edu.au

claudia.au@gmail.com

ahar4375@mail.usyd.edu.au

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Abstract

Two experiments investigated whether segmental speech errors in tongue twister production were

associated with utterance-level prominence and prosodic phrase-initial position as described by an

autosegmental metrical model of intonational phonology. In Experiment One, 40 undergraduate

students produced words with confusable onset consonants (“tongue twister words”) in four-word

lists under prominence conditions that varied the number and location of emphatically-produced

words in the list. There was a three-way interaction between prominence condition, word position

in the list, and format (ABBA versus ABAB ordering of onset consonants). In Experiment Two, 38

undergraduate students produced tongue-twister words with and without narrow informational

focus in sentences. Onsets of tongue twister words with narrow focus were produced more

accurately than onsets in unfocussed tongue twister words. Neighbourhood density of tongue

twister words had no significant effect on error rate. Phrase-initial position yielded more errors

than non-initial position, but was confounded with format (all tongue twister words occurred in

ABBA format). Results suggest that information about utterance-level prominence and phrasing is

available at the time of segment-to-frame association, consistent with the prosody-first account of

phonological encoding proposed by Keating and Shattuck-Hufnagel (2002).

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1. Introduction

An account of the psycholinguistic processes that occur during speech production must

specify how the segmental content and prosodic structure of an utterance are integrated to produce

the acoustic-phonetic properties of the speech signal (Shattuck-Hufnagel and Turk 1996). For

several decades, researchers have analysed speech errors as a source of information about the

subprocesses occurring within word-form and phonological encoding (Dell 1986; Stemberger 1992;

Vitevitch 2002; Wilshire 1999). Speech error studies that systematically investigate prosodic

factors may therefore inform theory about the integration of segmental and prosodic information

during speech production. To date, however, only a small number of studies have investigated the

relationship between prosodic factors and segmental errors (Frisch 2000; Goldstein, Pouplier, Chen,

Saltzman, and Byrd in press; MacKay 1971; Shattuck-Hufnagel 1983, 1992). The present paper

reports two experiments in which the segmental speech errors elicited in a tongue twister task were

influenced by prosodic organization.

1.1 Investigations of phonological encoding using segmental speech errors

Corpora of speech errors have been collected from extensive samples of naturally-occurring

speech (Shattuck-Hufnagel and Klatt 1979; Vousden, Brown, and Harley 2000), and using

laboratory techniques including the SLIPs technique (Baars, Motley, and MacKay 1975; Pouplier in

press) and tongue twister tasks (e.g. Sevald and Dell 1994; Shattuck-Hufnagel 1992; Wilshire 1998,

1999). In SLIPs experiments, participants silently read pairs of words containing a repeating

sequence of onset consonants (e.g. case tick; can tim; Pouplier in press). At some unpredictable

time, a word pair appears in which the onsets occur in reverse order (e.g. tap kip; Pouplier in press).

Participants are cued to read aloud this latter pair, and typically make a high rate of errors. In

tongue twister experiments, participants repeat or read aloud word lists, phrases or sentences

containing combinations of words (“tongue-twister words”) that systematically manipulate factors

thought to be associated with high segmental error rates in speech production.

A number of effects reliably occur across speech error studies (Stemberger 1992). For

example, higher error rates are observed on word- or syllable-initial consonants than on other

segments in a word (Dell, Juliano, and Govindjee 1993). Shattuck-Hufnagel (1992) investigated

whether word-initialness or syllable-initialness was the critical factor in a tongue twister study,

finding higher error rates on word-initial than syllable-initial consonants matched on other factors.

In a study of naturally-occurring errors that incorporated a correction for chance error rate, Vousden

et al. (2000) found an effect of syllable-initial position after word-initial effects were accounted for.

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Many speech error effects, including initialness effects, can be described in terms of a

similarity relationship between units interacting to produce the observed error (Shattuck-Hufnagel,

1979). For example, comparatively high rates of error are observed between segments that share

phonetic features, position in a word or syllable or cluster, and/or adjacent segments (Dell 1986;

Frisch 2000; Pouplier in press; Shattuck-Hufnagel 1979). Shattuck-Hufnagel’s (1992) experiments

investigated the effect of shared lexical stress in tongue twister words, and whether there was an

interaction between initialness and stress. Sets of tongue twister words were constructed in an

orthogonal design that placed highly confusable segments (segments producing a high rate of errors

in the MIT-Arizona speech error corpus; Shattuck-Hufnagel and Klatt, 1979) at word- versus

syllable-initial position, and at the onset of stressed versus unstressed syllables. This created four

conditions: both word- and stress-position same, word-position same, stress-position same, and

neither word- nor stress-position same. The first experiment elicited the words as a list (e.g. parrot,

fad, foot peril), a second placed the words in grammatically well-formed but sometimes

semantically anomalous sentences (e.g. It’s a peril when fad takes a foot from the parrot.), a third

required participants to generate their own sentences containing the words, and a fourth replicated

the list experiment with a more powerful design.

In all Shattuck-Hufnagel’s (1992) experiments, participants made most segment interaction

errors (segments exchanging with, perseverating or anticipating a segment elsewhere in the list)

between segments that shared both word-position and stress-position. The next highest error rate

occurred between segments that were word-position same only; then between segments that were

stress-position same only. The fewest errors occurred between confusable segments that were

neither word- nor stress-position same. Shattuck-Hufnagel found no interaction between word- and

stress-position in two of the experiments (1, 1a), but a significant interaction in two other

experiments (1b, 2), one of which (Experiment 2) had a more powerful design than Experiments 1

and 1a. Frisch (2000) also found a strong interaction between word-position (word onset versus

syllable onset) and stress in tongue twisters constructed with four bisyllabic words in lists.

To account for the influence of both word-initial position and stressed-syllable-initial

position on segment interaction errors, Shattuck-Hufnagel (1992) revised her earlier “scan-copier

model” of segment-to-frame association, the process whereby segments are arranged in the correct

serial order during phonological encoding (Shattuck-Hufnagel 1979). According to the scan-copier

model, segments for words in an utterance are retrieved from long-term lexical memory and placed

into a short-term store until they can be allocated to their correct place in an organising frame that

specifies the prosodic structure of the utterance. This prosodic frame provides instructions to a

scanning mechanism about the criteria by which to select segments from the short-term store to fill

each location in the frame. Segment interaction errors occur because another segment that also

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meets the selection criteria is selected instead of the correct segment. Because the lexical stress

value of a syllable, as well as word-initial position, contributed to error rates, Shattuck-Hufnagel

(1992) hypothesised that the organising frame must specify prominent syllables at the phrasal level,

as well as word boundaries. Each prosodic unit in the frame was hypothesised to contain one or

more syllables that are prosodically prominent at the phrasal level (Shattuck-Hufnagel 1992). This

suggests that the units of the prosodic frame are phonological words (with one prominent syllable)

or intermediate phrases (with one or more prominent syllables).

1.2 The prosodic organization of speech

In this paper we adopt the prosodic hierarchy described by Keating and Shattuck-Hufnagel

(2002) as our view of the prosodic constituents of speech. According to this view, linguistic units

are grouped into various prosodic constituents (utterances, Intonational Phrases, intermediate

phrases, phonological words, syllables) with a hierarchical structure, i.e. higher constituents (e.g.

utterances) encompass lower ones (Intonational Phrases). There are also different levels of

prosodic prominence within an utterance (Shattuck-Hufnagel and Turk 1996). A lexically-stressed

syllable is typically the most prominent syllable in a phonological word, and one of the stressed

syllables of an intermediate phrase will be accented within that phrase.

Shattuck-Hufnagel’s (1992) experiments demonstrated that prosodic information about

constituent boundaries and prominence at the level of the phonological word must be available at

the time of segment-to-frame association during phonological encoding. Questions about when

higher levels of prosodic structure become available relative to segment-to-frame allocation have

not yet been experimentally addressed. The following two experiments therefore aimed to

manipulate the prominence and initialness of confusable segments in tongue twister words in

prosodic units above the level of the phonological word. An influence of these factors on the rate

of segmental speech errors would provide evidence for the availability at the time of segment-to-

frame association of prosodic information represented above the level of the phonological word in a

prosodic hierarchy.

2. Experiment 1

To investigate experimentally the relationship between prosodic organisation and segmental

encoding, the tongue twister task is an obvious choice because it approximates connected speech

more closely than the SLIPs task (or other less frequently-reported laboratory tasks; Stemberger

1992), and provides opportunity for errors to be elicited under varying prosodic conditions.

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Previous tongue twister studies, however, have almost never considered the prosodic organization

of the lists, phrases or sentences elicited. Wilshire (1998) reported that participants in her tasks

produced four-word list tongue twisters with a list intonation, which suggests that lists were treated

as intonational phrases or utterances. Each lexical word in a list is most likely to be produced as a

phonological word within an intermediate phrase (Fougeron, personal communication, 16 January

2007).

Our first experiment investigated whether the number and location of prominent syllables

within tongue twister lists would influence the rate of segmental errors on each word-initial

consonant. We used one-syllable content words only, so that differences in prominence between

words could not bet attributed to differences in lexical stress; information proposed to be

represented in phonological word frames for segmental spell-out (Shattuck-Hufnagel, 1992). We

elicited four prominence conditions by asking participants to emphasise one or two of the words

within each list: the first word, the last word, the first and third words, or the second and fourth

words. We used tongue twister words with confusable segments in word-initial position only, to

ensure that errors would primarily occur on word-initial segments rather than at other word

positions. This would allow a comparison between error rates on word initial segments that were

list-initial versus non-initial, with list-initial position assumed to correspond to utterance- and/or

Intonational Phrase-initial position. A main effect of prominence or list-initial position, or an

interaction between the two, would suggest that prosodic information above the level of the

phonological word was available at the time of segment-to-frame association.

Previous tongue twister experiments using four-word lists with confusable segments at

word-initial position have ordered the alliterating segments in various ways (e.g. A1A2A3A4,

A1B1B2A2, A1B1A2B2, A1B1C1D1: Sevald and Dell 1994), where A1 is the first occurrence of the first

confusable word-initial segment and A2 is the second, B1 is the first occurrence of the second

confusable word-initial segment and so on. Few studies have analysed the effect of alliteration

format, however. Wilshire (1998, 1999) found no significant difference in error rates on lists with

ABBA versus ABAB formats, but words were not matched across format conditions on factors

such as word frequency, neighbourhood density, or segment confusability that influence segmental

error rates (Dell 1990; Vitevitch 2002). Sevald and Dell (1994) found slower speech rates in the

AAAA and ABBA conditions than the ABAB and ABCD conditions in tongue twister lists

containing four different words. Pouplier (in press) argued that switching from one initial segment

order (AB) to another (BA) across pairs of words (as in SLIPS tasks, but also in ABBA tongue

twisters) is more difficult than producing a regularly alternating sequence of initial segments (as in

ABAB tongue twisters). According to the coupled oscillator model of articulatory planning

(Goldstein, Byrd, and Saltzman 2006), ABBA tongue twisters require speakers to reverse the

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phasing of articulatory gestures for onsets across the first versus last pair of words in the list,

increasing the likelihood that the onsets will not be produced correctly. We therefore predicted

more errors on ABBA- than ABAB-format lists. The other question of interest, however, was

whether format would interact with prominence or initialness, a result that would provide further

evidence that post-lexical prosodic information is available at the time segmental errors occur

during phonological encoding.

2.1. Method

2.1.1. Participants

Forty-three native English speakers enrolled in an undergraduate psychology course

participated for optional course credit. They were screened for self-reported hearing, visual or

language impairments. One withdrew from the study, and two participants did not complete the

required number of repetitions per tongue twister when they made errors, leaving 40 participants.

2.1.2. Materials and Procedure

Tongue twisters were derived from materials created by Vitevitch (2002, Experiment 2;

Vitevitch personal communication 26 April 2005)2. Vitevitch (2002) created 20 tongue twister

lists consisting of four CVC (consonant-vowel-consonant) words in four-word lists with initial

segments selected because they were highly confusable in the MIT-Arizona corpus (Shattuck-

Hufnagel and Klatt 1979). Vitevitch’s tongue twisters manipulated neighbourhood density (10 low

and 10 high neighbourhood density lists), and contained initial consonants in an A1B1B2A2 format

(e.g. den ton tuck dial). Experiment 1 used only the 10 low neighbourhood density lists because

these yielded higher error rates in Vitevitch’s study, but we reversed the order of Words 3 and 4 in

each list to create an additional set of 10 ABAB format lists (e.g. den ton dial tuck), giving 20 lists

in total. These lists were spoken in each of four Prominence Conditions manipulating the number

and location of prominent words:

(i) first word emphasised (W1 w2 w3 w4)

(ii) fourth word emphasised (w1 w2 w3 W4)

(iii) first and third words emphasised (W1 w2 W3 w4), and

(iv) second and fourth words emphasised (w1 W2 w3 W4).

Participants were tested individually in a quiet room and took approximately 50 minutes to

complete the experiment. Written lists, metronome beat (160 beats per minute) and spoken

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examples were presented using SuperLab experiment presentation software (Cedrus 2004) on a

laptop PC. Prior to the experiment, participants completed demonstration and practice trials to

familiarise them with the task. Each trial commenced when the participant clicked the mouse. The

metronome beat began (and continued until the end of the trial), and numerals counting down 5 to 1

appeared on the screen, after which the four-word list appeared and remained on screen until the

end of the trial. The word(s) to be accented were presented in uppercase and bold; the others in

lowercase normal font, e.g. DEN ton DIAL tuck.

The design of six practice blocks was determined by piloting various means and amounts of

practice to elicit the target prominence conditions and rate. No lists in Practice Block 1 were

tongue twisters, one list in each of Practice Blocks 2—5 was a tongue twister, and all lists in

Practice Block 6 were tongue twisters, so the blocks gradually became more difficult. Participants

were told the metronome was to indicate approximately how fast they were to speak, but they were

not required to keep in time with the metronome. In Practice Block 1, all words of each list were

presented in lowercase and no prominence condition was specified. The experimenter demonstrated

the first practice list by clicking the mouse, waiting for the countdown to finish and the list to

appear on screen, then reading the four words aloud three times in a row, at the required rate. The

participant then did the same for two practice lists. In Practice Block 2, the prominent word

appeared in uppercase and the experimenter demonstrated the procedure for three repetitions of the

first list in this block, demonstrating Prominence Condition W1 w2 w3 w4. The participant did the

same for two further lists with this prominence pattern. Participants were given feedback about rate

or required prominence at the end of a trial if either were not produced as required and were able to

correct this on subsequent productions. In Practice Blocks 3—5, the above procedure was repeated

with Prominence Conditions (ii), (iii) and (iv) respectively. In Practice Block 6, the procedure was

modified to be the identical to the experimental trials, with the spoken model presented as a sound

file. Participants were instructed to repeat the list six times at the offset of the spoken example,

imitating the prominence pattern in the model and at approximately the rate of the metronome. The

numerals 1 through 6 appeared at the bottom of the screen with each repetition to help participants

keep count. After training, participants appeared to have little difficulty producing the required

prominence conditions.

Items were presented in eight blocks of 10 tongue twister lists, with the ABBA and ABAB

lists spoken in each of the four prominence conditions by each participant, and format and

prominence condition blocked. Participants were randomly allocated to one of eight groups, with

order of prominence condition and format condition fully counterbalanced across groups. List

order within block was randomised for each participant. The experimenter monitored the prosody

and rate throughout to verify that participants were producing the tongue twisters as required, and

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reminded participants of the required rate and prominence on the few occasions where this was

necessary. Participants took a short break at the end of each block and a longer break at the

halfway point.

2.1.3. Scoring and Analysis

Speech errors were noted in situ then transcribed in full from the recordings until all six

repetitions in a trial were heard with no further changes to the transcription (Shattuck-Hufnagel

1992). Words were scored as correct if they were produced correctly and in the correct order.

Addition, deletion or substitution of at least one of the segments, dysfluencies (e.g. “p-palm”) and

corrected segments (e.g. “p-calm”) were all scored as incorrect (Shattuck-Hufnagel 1992; Wilshire

1999). Unintelligible responses were omitted from analysis. Errors were classified as contextual if

the segment participating in the error was the confusable segment from the initial position in the

other tongue twister words.

A 4 (prominence condition) x 4 (position in list: Words 1—4) x 2 (format: ABBA, ABAB)

analysis of variance by subjects with repeated measures on all factors was carried out using SPSS

with planned contrasts performed in Excel. Alpha was set at an overall level of .05 and reduced

using the Bonferroni method to control for Type One errors on the contrasts. Degrees of freedom

were adjusted using the Greenhouse-Geisser procedure when the data did not pass a test for

sphericity. All errors were included in the analysis.

2.2. Results

Participants made a total of 6410 errors (8.4%). The majority of errors involved word

onsets and contextual errors (Table 1). Percentage of total errors on ABAB lists was 40.3%; on

ABBA lists 59.7%.

Insert Table 1 near here

2.2.1. Effects of prominence condition, position in list and format

All main effects were significant: prominence condition (F = 5.54, d.f. = 3, 117, p = 001),

position in list (F = 64.70, d.f. = 2.02, 78.61, p < .000), and format (F = 32.67, d.f. = 1, 39, p <

.000).

All two-way interactions were significant: prominence condition X position in list (F = 6.40, d.f. =

6.64, 258.77, p < .000), prominence condition X format (F = 20.93, d.f. = 2.47, 96.35, p < .000),

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and position in list X format (F = 11.34, d.f. = 2.46, 95.99, p < .000). Mean error rates and 95%

confidence intervals for the main effects and two-way interactions are provided in Table 2. All

these effects were overshadowed by the significant three-way interaction between prominence

condition, position in list and format (Figure 1; F = 3.31, d.f. = 6.01, 234.54, p = .004), illustrated in

Figure 1.

The following patterns are evident from visual inspection of Figure 1. Participants made

fewer errors to Word 1 in ABAB than ABBA lists, with accented Word 1 produced more accurately

in Prominence Conditions (i) and (iii), than unaccented Word 1 in Prominence Conditions (ii) and

(iv) in both formats. On Word 2, overall error rates were similar in the ABAB and ABBA lists.

Unaccented Word 2 was most error prone in Prominence Condition (ii) in both formats. Accented

Word 2 was less vulnerable to error in Prominence Condition (iv) in the ABAB lists than the

ABBA lists. Word 3 yielded the highest error rate of any position in the list in both formats, and

was more error-prone in ABBA than ABAB lists. Compared with the error rates on unaccented

Word 3 in Prominence Conditions (i) and (ii), errors were reduced on accented Word 3 in

Prominence Condition (iii) in ABAB lists. By contrast, error rates on unaccented Word 3 in

Prominence Condition (iv) were strikingly high in the ABBA lists. Finally, Word 4 was error-

prone compared with Words 1 and 2 but produced more accurately than Word 3 in both formats.

There were fewer errors on unaccented Word 4 in Prominence Condition (iii) compared to the other

prominence conditions in the ABAB lists but not the ABBA lists. By contrast, accented Word 4

was comparatively error-prone in Prominence Condition (iv) in the ABBA lists.

Insert Table 2 near here

Insert Figure 1 near here

The ABBA format yielded more errors than ABAB format, consistent with Pouplier’s (in

press) hypothesis that switching the order of onsets between the first two words of a four-word list

(AB) and the final two words (BA) involves a change in phase of articulatory gestures that makes

ABBA format more error prone than ABAB format where this reversal is not required. Rather than

pooling errors obtained as a result of potentially different articulatory requirements, post-hoc

analyses to examine effects of list-initial position were carried out for ABAB-format lists only.

2.2.2. Errors at list-initial position

Error rates to list-initial versus list-medial words matched on prominence were compared in

3 different analyses: unaccented Word 1 versus unaccented Words 2 and 3 in Prominence Condition

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(ii), accented Word 1 versus accented Word 3 in Prominence Condition (iii), and unaccented Word

1 versus unaccented Word 3 in Prominence Condition (iv). Segment identity at the different list-

positions was also matched in the latter two comparisons. In all three comparisons, there were

fewer errors on the list-initial word than on the other word(s), and these differences were

statistically reliable (Prominence Condition (ii): F = 20.91, d.f. = 1.68, 65.61, p < .000; Prominence

Condition (iii): t = 4.01, d.f. = 39, p < .000; Prominence Condition (iv): t = 4.22, d.f. = 39, p <

.000). There was no significant difference between error rates on Word 2 and Word 3 in the first

comparison. The same error patterns were present in the first repetition of each list only, and in

repetitions 2—6 combined, thus were not due to low error rates on the first word per six repetitions

of each list alone.

2.3. Discussion

The number and location of emphatically-produced words in these tongue twister lists

influenced the number of errors made to word-initial confusable segments. Prominence condition

was a significant main effect in the full analysis of variance, and interacted with position in the list

and with format in the two- and three-way interactions. Post-hoc analyses showed fewer errors at

list-initial position on words matched for prominence and for segment identity in the ABAB lists.

We assumed that the four-word lists were typically produced as a series of four intermediate

phrases within an Intonational Phrase, and that Intonational Phrases usually also corresponded to

utterances, although it was possible that two or more lists were produced as a single utterance. The

effects of prominence condition and list-initial position would therefore suggest that prosodic

information at higher levels of prosodic organisation than the phonological word or intermediate

phrase is available at the time of segment-to-frame association during phonological encoding, when

segmental speech errors are hypothesised to arise.

The target prominence patterns across words were elicited by asking participants to read

uppercase words aloud more emphatically than lower case words. Participants practised the task

extensively before beginning, and the experimenter monitored participants’ productions and

provided feedback that was readily used to modify the emphasis after infrequent errors. It was,

however, beyond the scope of this study to analyse the prosodic phrasing of each production at the

time of transcribing the segmental speech errors, therefore we cannot be certain of the precise

prosodic organization used by participants. For example, lists in the two-accent prominence

conditions (iii) and (iv) may have been realised as two Intonational Phrases rather than one. If this

was so, the list-initial effect in the ABAB lists would suggest that information about utterance-

initial position only was available at the time of segment-to-frame association, because Word 1

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yielded fewer error than Word 3; see Figure 1. Our results do not therefore permit the conclusion

that a full specification of prosodic information above the level of the phonological word or

intermediate phrase is available at the time of segment-to-frame association, only that at least some

higher-level prosodic information is available. A closer prosodic analysis of each rendition of each

tongue twister in a future study (feasible with a smaller corpora of speech data) would establish

precisely whether observed errors were associated with utterance- or Intonational Phrase-initial

position.

2.3.1 Effects of prominence condition, position in list and format

The significant main effect of format and the interaction between format and word position

support an account in which reversing the order of onsets across successive pairs of tongue twister

words is more difficult than repeating the same order across pairs because the former requires a

reversal of phasing of the articulatory gestures for the onsets (Pouplier, in press). The significant

three-way interaction further suggests that the number and location of prominent syllables may also

influence the stability of the articulatory coupling in a coupled oscillator model of articulatory

planning (Goldstein et al. 2006). In our results, for example, unaccented Word 3 was highly

vulnerable to error in the ABBA format lists in Prominence Condition (iv) w1 W2 w3 W4, but less

so when there was only one accented word (Prominence Conditions (i) and (ii)). By contrast,

accented Word 3 in the ABAB lists in Prominence Condition (iii) W1 w2 W3 w4 was highly

resistant to error.

As noted earlier, few experimentally controlled studies have considered the effect of

phrasal prominence on speech errors. MacKay’s (1971) participants produced four-word list tongue

twisters consisting of CV words with different onset consonants (i.e. ABCD format) and identical

codas under five prominence conditions: Word 1, Word 2, Word 3 and Word 4 emphasised

respectively, and no instruction about prominence. Unaccented onsets were more vulnerable to

error when they preceded an accented word than would be predicted by chance. In our experiment,

we similarly found that within the initial or final pair of words in the two-prominence lists there

were higher error rates to unaccented words preceding an accented word within a pair, an effect

amplified in the ABBA lists compared to the ABAB lists. Goldstein et al. (in press) elicited two-

word tongue twister lists with AB format onset consonants. Lists were repeated multiple times by

six participants and the researchers analysed the effect of first versus second word emphasised in

the list. They observed more errors on Word 1 when Word 2 was emphasised than when Word 1

was emphasised, but the main effect of prominence condition and the interaction between

prominence and position in the list were not significant. To compare our data with these, we pooled

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errors on Words 1 and 3 and on Words 2 and 4 in the two-prominence conditions on the ABAB lists

only. We found a similar pattern of errors, but a comparatively higher error rate to the unstressed

word in the initial position in the pair (Mean error rates per word, pair-initial and accented: 5.5,

pair-initial and unaccented: 8.9, pair-final and accented: 7.1, pair-final and unaccented: 6.2). By

contrast with the materials of Goldstein et al. (in press), our lists did not control for segmental (and

thus gestural) content of the codas across items or formats, however we used a powerful design

with 40 speakers repeating 20 tongue twisters six times each per prominence condition. Further

evaluation of phrasal prominence effects on speech errors will require a sufficiently powerful

experimental design and materials controlled for segmental and gestural content.

2.3.2 Effects of list-initial position

Whereas word-initial position has been associated with high rates of segment interaction

errors in both observational and experimental studies (Dell 1986; Stemberger 1992), list-initial

position seems to have been protective against error in the ABAB quadruples in this experiment.

Initialness effects may therefore differ depending on the level of prosodic organization at which a

segment is initial. A crucial factor in explaining word-initial errors is similarity: segments interact

erroneously with other segments that are similar in one or more ways (e.g. position in word,

adjacent vowel, natural class etc.) (Dell 1986; Frisch 2000; Pouplier in press; Shattuck-Hufnagel

1979). If positional similarity is defined more narrowly, in terms of similarity of location within a

prosodic hierarchy, and there are typically fewer phrase-initial than phonological word-initial

positions within an utterance, there will be limited opportunities for phrase-initial segments to

interact with other phrase-initial segments to participate in errors. In Experiment 1, we assumed

that participants typically produced four-word lists as utterances, giving one list-initial position per

utterance – and we found fewest errors at this position.

Can the list-initial effect be explained in terms of Shattuck-Hufnagel’s (1992) revised scan

copier model? If the prosodic frame to which segments are associated specifies each segment’s

position within an utterance, Intonational and intermediate phrase, as well as phonological word,

and if segments for an entire utterance are available in the short-term store before being associated

to the frame (Keating and Shattuck-Hufnagel 2002), then there may only be one (or at most a few)

segment(s) tagged for utterance- or phrase-initial position per utterance, making these segments less

susceptible to error. There would probably, however, be several segments tagged as phonological

word-initial, accounting for higher error rates to these segments. This account would imply that

constituency within phonological words and higher level constituency have both been computed at

the time segments are allocated to prosodic structures, but the latter reduces the error-susceptibility

14

of an onset consonant whereas the former increases it, as a function of the number of competing

segments available in the short-term store.

Experiment 1 elicited speech errors using four-word tongue-twister lists read aloud

according to instructions to emphasise certain words and not others. This task differs substantially

from natural speech the meaningfulness of the utterances and the motivation for accenting words,

thus may not tell us much about the availability of prosodic information during phonological

encoding of normal connected speech. Experiment 2 was therefore designed to investigate speech

error rates on tongue twister words in sentences produced in a more naturalistic speaking task.

3. Experiment 2

Experiment 2 aimed to elicit tongue twister words in spoken sentences in a near-replication

of Shattuck-Hufnagel’s (1992) Experiment 1(a), but manipulating the location of accented words

and the initialness of segments in Intonational Phrases. Tongue twister words with highly

confusable initial segments were located in the following syntactic positions in sentences: sentence-

initial, medial in the first clause, clause-initial in the second clause, and sentence-final. These four

sentence-positions were intended to manipulate initialness within Intonational Phrases, with tongue

twister words 1 and 3 at phrase-initial positions, and tongue twister words 2 and 4 at non-phrase-

initial positions. The prominence of the tongue twister words was manipulated by asking

participants to respond to questions that would elicit a narrow informational focus on each of the

tongue twister words in turn.

We hypothesised that if higher error rates on initial segments in lexically-stressed syllables

in Shattuck-Hufnagel’s (1992) study occurred because initial segments in prosodically prominent

syllables are vulnerable to error, then we would see more errors on focussed words than unfocussed

words. Second, if initialness in prosodic constituents makes segments vulnerable to error, then we

would see higher error rates for initial compared with non-initial segments in prosodic phrases.

However, an opposite set of hypotheses can be formulated if a scan-copying procedure selects

segments on the basis of prominence or positional criteria including information about their

representation at upper levels of a prosodic hierarchy, and segmental mis-selection depends on the

similarity of the mis-selected segment to the target. If the latter account is correct, there would be

fewer errors to onsets of focussed words and Intonational Phrase-initial segments because for any

utterance there may be few or no competing segments with a similar prosodic specification to those

segments.

3.1 Method

15

3.1.1. Participants

Forty native English speakers from the same participant pool as in Experiment 1 (mean age

19.8 years; range 15—54) participated in the experiment. Two were excluded on the basis of self-

reported hearing, visual or language impairments, leaving a total of 38 speakers who completed the

study.

3.1.2. Materials and Procedure

Sentence stimuli3 were constructed using the words from all twenty of the four-word list

tongue twisters created by Vitevitch (2002, Experiment 2; Vitevitch personal communication 26

April 2005). Half the lists contained words from high density and half from low density

phonological neighbourhoods, with density conditions matched on initial segments, word

frequency, neighbourhood frequency and familiarity (Vitevitch 2002). In this experiment we used

both low and high neighbourhood density items to elicit a wider range of tongue twister words than

Experiment 1. Sentences were meaningful to allow participants to produce them naturally, and we

minimised as far as possible (while maintaining meaningfulness) the number of non-tongue-twister

words within each sentence that began with the confusable initial segments of the tongue twister

words in that sentence.

Each sentence consisted of two clauses4 presented over two lines of text in sentence case on

a PC screen, one clause per line, with a comma at the end of the first clause, the second clause

indented, and a full stop at the end of the sentence. Each clause contained two tongue twister words

separated by two syllables. Many of the tongue twister words were used as proper nouns even

when typically not in use as names (e.g. “Den” and “Tuck” in the examples below) due to the

difficulty of creating meaningful sentences with the required syntactic, segmental and syllabic

structure using the tongue twister words.

Participants read each sentence aloud to answer each of four written questions designed to

elicit a narrow informational focus on the tongue twister word that was shown in large font and

surrounded by a box, for example,

(i) Who weighs a ton or more? Den weighs a ton or more,

Tuck broke the dial.

(ii) How much does Den weigh? Den weighs a ton or more,

Tuck broke the dial.

16

(iii) Who broke the dial on the scale? Den weighs a ton or more,

Tuck broke the dial.

(iv) What did Tuck break on the scale? Den weighs a ton or more,

Tuck broke the dial.

Numbers (i) to (iv) in the above examples indicate Prominence Conditions (i) – (iv). Participants

repeated each sentence six times in succession in one of two randomised sentence orders. Error

rates did not differ across presentation order, and presentation order did not interact with

neighbourhood density, prominence condition or sentence position, so further analyses pooled

across presentation order.

Elicitation questions and tongue twister sentences were presented on a laptop PC using

SuperLab (Cedrus 2004). Responses were recorded for later transcription of errors. On each trial,

the question was displayed for 4000 ms, disappeared for 1000 ms then reappeared with the target

sentence below it. The sentence remained in view while the participant repeated it six times, then

the participant moved on to the next trial by clicking a mouse. A digital metronome (Enable

Software 2005) set at 160 beats per minute was used to standardise speech rate across participants.

Participants completed a block of 10 sentences then were encouraged to take a short break. There

were 20 sentences, each spoken in four prominence conditions, thus 80 items per participant

presented in eight blocks of 10.

Prior to the main experiment, participants worked through six demonstration items in which

a recorded voice modelled the required response and speech rate, then they completed six practice

items. The experimenter ensured they were maintaining speech rate and emphasising the required

word before they were able to proceed. At the end of each break between blocks, a further two

demonstration items were given to remind participants about the required rate and how to answer

the questions using the sentences. Participants were tested individually, taking approximately 50

minutes to complete the experiment.

3.1.3. Scoring and Analysis

Error transcription and scoring occurred as in Experiment 1, although this time dysfluencies

(e.g. “r-rice”) were scored as correct because the target segment was not replaced with the onset

used in the other tongue twister words. Errors on two sentences were removed from the corpus, one

because participants commented that they were making semantically-motivated word substitutions,

and another because they frequently pronounced the word “Gill” as “Jill” when the target segment

17

was [g]. Errors on matched sentences from opposing neighbourhood density conditions were also

removed, leaving errors from 16 sentences in the error corpus.

A 2 (prominence condition: accented, unaccented) x 4 (sentence position: Clause 1 initial,

Clause 1 medial, Clause 2 initial, Clause 2 final) x 2 (neighbourhood density: high, low) analysis of

variance by subjects with repeated measures on all factors was carried out using SPSS with planned

contrasts in Excel and control for Type I error as described for Experiment 1.

3.2. Results

Participants made 3106 errors (8.5%). Most errors involved word onsets substituted by

word onset segments elsewhere in the clause (Table 3). Error rates for the two prominence

conditions at the four sentence positions are illustrated in Figure 2.

Insert Table 3 near here

Insert Figure 2 near here

The main effect of prominence condition was significant, with fewer word-initial errors on

accented than unaccented tongue twister words (F = 31.61, d.f. = 1, 37, p < .001). The main effect

of sentence position was also significant (F = 14.73, d.f. = 2.03, 75.26, p < .001). There were more

errors on clause-initial than clause-medial position in Clause 1 (F = 14.35, d.f. = 1, 37, p = .01), and

more errors on clause-initial than clause-final position in Clause 2 (F = 20.29, d.f. = 1,37, p < .001).

There was no difference between error rates at initial positions in Clause 1 versus Clause 2, or

between error rates at clause-medial and clause final positions. There was a marginally significant

interaction between prominence condition and sentence position (F = 2.98, d.f. = 2.43, 91.52, p =

.045). There was a greater disparity between error rates on accented versus nonaccented words in

initial versus final position within Clause 2 (F = 5.44, d.f. = 1, 37, p = .025) but no disparate effect

of accent across the two sentence positions within Clause 1 (F = .96, d.f. = 1, 37, n.s.). The main

effect of neighbourhood density was not significant, and there were no significant interactions

between neighbourhood density and prominence, neighbourhood density and sentence position, or

neighbourhood density, prominence and sentence position.

3.3. Discussion

Experiment 2 investigated whether focussed tongue twister words would show more errors

than unfocussed ones. In Shattuck-Hufnagel’s (1992) experiment, syllable-initial segments in

18

lexically-stressed syllables participated in more errors than matched initial segments in unstressed

syllables. We found a significant main effect of prominence condition, but in the opposite

direction: words with narrow focus were subject to fewer segment-initial errors at all four sentence

positions in the tongue twister sentences. Thus word-initial segments at the phrasally-prominent

locations investigated in Experiment 2 did not behave the same way as the initial segments in

lexically-stressed syllables in Shattuck-Hufnagel’s (1992) study because prominence in higher

constituents was protective against segmental error. Prominence relations over larger constituents

than the phonological word must, therefore, have been computed by the time segments were ready

for association to prosodic structures to give this protective effect. If the prosodic frame specifies

accented syllables in addition to lexical stress (Keating and Shattuck-Hufnagel, 2002), then a scan-

copier would be able to narrow the search to segments marked for accent. There would be few (if

any) other onset segments of accented syllables competing for selection, whereas there would likely

be several segments tagged as initial in lexically-stressed syllables within phonological words,

leading to more segment interaction errors between stressed-syllable-initial segments than accented-

syllable-initial segments within an utterance.

Experiment 2 also found that participants made more errors to tongue twister words at

phrase-initial than -medial or -final positions. Interpretation of this finding is limited, however, by

the confound between initialness within clauses and format in this experiment, as all tongue twister

words were presented in an ABBA format (e.g. den, ton, tuck, dial). The high error rates at both

clause-initial positions may therefore have been due to the reversal of gestural phasing required at

these positions by the ABBA format over subsequent repetitions of the sentence. Many tongue

twister experiments (Frisch 2000; Goldrick and Blumstein 2006; Shattuck-Hufnagel 1992;

Vitevitch 2002) have used an ABBA format because it reliably elicits high rates of errors, however

this confound in Experiment 2, and the main and interaction effects of format in Experiment 1 urge

careful consideration of the potential consequences of different formats when designing materials

for future tongue twister studies. Our Experiment 2 should be repeated using ABAB order.

Neighbourhood density did not influence error rates in this experiment, in contrast with the

finding of Vitevitch (2002) in a tongue twister task, SLIPs task and two picture-naming tasks. We

did, however, remove four sentences from the analysis as described above, and although two were

of high density and two of low density, this may have compromised listwise differences in density

in our experiment, or listwise matching of high and low density words on frequency,

neighbourhood frequency or familiarity. Alternatively, uncontrolled effects of prosodic structure in

Vitevitch’s (1992) Experiment 2, or item difficulty due to the required phasing of gestures for all

words across the two neighbourhood density conditions in Vitevitch’s materials or ours may have

confounded the investigation of neighbourhood density in one or both studies.

19

4. General Discussion

4.1 When does prosodic structure become available during speech production?

Experiments 1 and 2 indicated that emphatic production of tongue twister words in four-

word lists, narrow informational focus in sentences, and list-initial position influence the rate of

speech errors in tongue twister production. These results provide evidence that at least some

information represented at upper levels of a prosodic hierarchy is available at the time of segment-

to-frame association during phonological encoding. Most current models of phonological encoding

have little to say, however, about how segmental information and higher-level prosodic information

are integrated during speech planning. The proposals of Levelt (1989), Levelt, Roelofs, and Meyer

(1999) and Keating and Shattuck-Hufnagel (2002) are an exception, thus they are summarised

below.

Levelt (1989: 365) acknowledged the rudimentary nature of his “rough sketch of a possible

architecture” underlying the prosodic formulation of connected speech. The key component of this

architecture is the Prosody Generator, a processing component that works incrementally from the

beginning of an utterance forward, with limited lookahead. The Prosody Generator combines

surface structure, number of syllables and lexical stress information as they become available to

build a prosodic structure which details a pattern of accented versus unaccented syllables,

Intonational Phrases and a smaller phrase he called a phonological phrase. Next the speaker’s

intonational meanings (not captured in the grammar) are consulted to determine which phrase

accents and boundary tones will apply to each phrase, and which syllables will receive prominence.

Finally, the Prosody Generator computes how the segments associated with individual words will

be combined into phonological words and structured into syllables. The outputs from the Prosody

Generator are a prosodic frame for the whole utterance that details durations, loudness, F0 and

pausing, and a string of segments tagged with their syllable position ready to be allocated to

phonological words (Keating and Shattuck-Hufnagel 2002; Levelt 1989). These are combined

during the process of phonetic encoding that specifies the articulatory gestures required for speech

and their relative timing.

In a newer, and in many ways more powerful theory, Levelt, Roelofs, and Meyer (1999)

propose that the number of syllables and location of lexical stress are only stored and retrieved in

English for the approximately 10% of words that have an atypical stress pattern (i.e., stress on other

than the first syllable). For all other words, phonological word structure, including number of

syllables and location of lexical stress, is generated incrementally and online from the segments as

20

they become available from long-term memory, using the syllabification rules of the language. A

“binding-by-checking” procedure ensures that segments are combined into words that correspond to

the target words required for the utterance, and segmental speech errors are attributed to occasional

failures in the checking procedure. Prosodic phrases are constructed according to a process

described by Levelt (1989) that depends on information about the number of syllables and location

of lexical stress that is contained in the phonological word structures. The newer theory therefore

requires that phrase-level prosodic information (prosodic phrase boundaries and locations of pitch

accents) cannot be available before phonological words are generated from segments.

Keating and Shattuck-Hufnagel (2002) argue that the theory of Levelt, Roelofs and Meyer

(1999) is unable to explain many features of connected speech. They propose instead that a

preliminary default prosodic structure for a whole utterance is initially computed on the basis of the

words in the utterance and their surface structure. This preliminary structure is progressively

modified as various other types of information required for the utterance (lexical, syntactic,

morphological, phonological and phonetic) are retrieved from long-term memory. Before any

segments are retrieved, the evolving prosodic structure contains correct Intonational and

intermediate phrase boundaries, the pitch accents and boundary tones associated with these phrases,

and the locations of phonological word boundaries and pitch accents on phonological words, taking

into account the speaker’s intonational meanings. According to Keating and Shattuck-Hufnagel,

only after all this prosodic information is available do speakers retrieve segments for all words in

the utterance and map them to this near-complete prosodic structure – at which point segmental

speech errors may occur.

The results of the present study are most consistent with the proposals of Keating and

Shattuck-Hufnagel (2002), in suggesting that information about pitch accents that realise narrow

informational focus, and prosodic constituency at upper levels of a prosodic hierarchy (e.g. the

location of utterance and/or Intonational-Phrase initial boundaries), has been computed prior to the

association of segments to the prosodic frame during phonological encoding. Our results are not

compatible with the claim that phrase-level prosodic information is generated after phonological

words are generated from segments (Levelt et al. 1999).

4.2 Why do segments at prosodically strong positions resist error?

Keating (2006) discussed the possibility raised by Frisch (2000) and Dell (2000) that initial

segments of words are highly vulnerable to error because they are less predictable than subsequent

segments, being less (or even un-) constrained by context (Fougeron and Keating 1997). The

higher a constituent is in the prosodic hierarchy, the less predictable its initial segment may be.

21

Keating suggested that the unpredictability of constituent-initial segments, and the high information

load carried by word-initial segments in word recognition, might also motivate articulatory

strengthening. In articulatory strengthening the articulators become stronger and, in the case of

consonants, form a more extreme constriction, on the initial segments of prosodic constituents.

There is also evidence that the higher the constituent the greater the strengthening (Fougeron and

Keating 1997). Articulatory strengthening also occurs on accented words (Beckman, Edwards, and

Fletcher 1992), which typically introduce new information and are less predictable than deaccented

words.

Keating (2006) hypothesised that if the unpredictability of constituent-initial segments leads

to both articulatory strengthening and segmental errors, and if initial positions of higher constituents

show greater strengthening, then constituent-initial and accented positions may also be more

vulnerable to error. Our findings, that segmental errors were reduced on accented words in

sentences, and on utterance-initial segments in several post-hoc comparisons in the ABAB lists, did

not support this prediction. We speculate that the unpredictability, and its converse, information

load, noted by Keating (2006) and Fougeron and Keating (1997) may yield the opposite effect:

because unpredictable words carry the greatest information load they may by some mechanism be

less vulnerable to error. The relatively high rate of errors on phonological word-initial segments

would still be explained with reference to the high number of positionally similar candidates

available for selection for phonological word-initial slots in the prosodic frame for an utterance.

According to the revised scan-copier model, prosodic information about lexical boundaries

and lexical stress is available along with the segments as part of the lexical representation

(Shattuck-Hufnagel 1992). How might segments in the short-term store come to be tagged for

initial position at the Intonational Phrase- and/or utterance-levels of the prosodic hierarchy, or for

accent? A phonological competition model (e.g., Sevald and Dell 1994) may be able to simulate

such effects if there was a mechanism whereby segments at informationally strong locations

received increased activation relative to other segments, reducing the likelihood of mis-selecting

those segments. Higher levels of activation cascading to the phonetic features associated with these

segments (Goldrick and Blumstein 2006), or to the motor units that articulate them (MacKay,

1971), might then further account for articulatory strengthening effects (Beckman, Edwards, and

Fletcher 1992; Fougeron and Keating 1997).

4.6 Concluding comments

These two experiments demonstrate the potential for tongue twister experiments to inform

theory about how segmental content and prosodic structure of an utterance are integrated during

22

speech planning. The significant effects of ABBA versus ABAB format in Experiment 1 urge that

future work should consider the relationship between prominence and speech errors within the

theoretical framework of a coupled oscillator model of articulatory planning. This will require

collection of kinematic data, rather than phonetic transcriptions of errors, and careful control of the

composition of entire stimuli, not just target words or segments. Finally, although previous tongue

twister studies have not always manipulated or controlled prosodic factors, especially at higher

levels of the prosodic hierarchy, it will clearly be informative to do so in future.

23

Notes

1. Many thanks to Cécile Fougeron, Patricia Keating and Marianne Pouplier for their constructive

feedback on an earlier version of this manuscript, and to Amanda White for assistance in preparing

the final version of the manuscript.

2. A copy of the tongue twister words used in this study is available on request to,

Micheal S Vitevitch, Department of Psychology, 1415 Jayhawk Blvd., University of Kansas,

Lawrence, KS 66045, USA. Email: mvitevit@ku.edu

3. The sentence materials are available from the first author, after tongue twister words have been

obtained as described in Note 2. Contact, Karen Croot, School of Psychology, University of

Sydney, NSW AUSTRALIA 2006. Email: karenc@psych.usyd.edu.au

4. With one exception, Lung wore a robe of silk, rice and gold leaf.

24

References

Baars, Bernard J., Michael T. Motley and Donald G. MacKay 1975 Output editing for lexical status from artificially elicited slips of the tongue. Journal of Verbal Learning and Verbal Behaviour 14: 382—391. Beckman, Mary E., Jan Edwards and Janet Fletcher 1992 Prosodic structure and tempo in a sonority model of articulatory dynamics. In: Gerard J. Docherty and Robert Ladd (eds.), Gesture, Segment, Prosody, 68—86. (Papers in Laboratory Phonology II.) Cambridge: Cambridge University Press. Cedrus 2004 SuperLab 2.0.4. Dell, Gary S. 1986 A spreading-activation theory of retrieval in sentence production. Psychological Review 93: 283—321. Dell, Gary S 2000 Counting, connectionism and lexical representation. In Michael B. Broe and Janet B. Pierrehumbert (eds.), Acquisition and the Lexicon, 335—348. (Papers in Laboratory Phonology V.) Cambridge: Cambridge University Press. Dell, Gary S. 1990 Effects of frequency and vocabulary type on phonological speech errors. Language and Cognitive Processes 5: 313—349. Dell, Gary S., Cornell Juliano and Anita Govindjee 1993 Structure and content in language production. Cognitive Science 17: 149—195. Enable Software 2005 Metronome. Retrieved 2005, from http://enableencore.com/encore/metronome.htm Frisch, Stefan 2000 Temporally organised lexical representations as phonological units. In Michael B. Brow and Janet B. Pierrehumbert (eds.), Acquisition and the Lexicon, 283—298. (Papers in Laboratory Phonology V.) Cambridge: Cambridge University Press. Fougeron, Cécile and Patricia A. Keating 1997 Articulatory strengthening at the edges of prosodic domains. Journal of the Acoustical Society of America 101: 3728—3740. Goldrick, Matthew and Sheila E. Blumstein 2006 Cascading activation from phonological planning to articulatory processes: Evidence from tongue twisters. Language and Cognitive Processes 21(6): 649—683. Goldstein, Louis, Dani Byrd and Elliot Saltzman 2006 The role of vocal tract gestural action units in understanding the evolution of phonology. In Michael A. Arbib (ed.), Action to Language via the Neuron System, 215—249. Cambridge: Cambridge University Press. Goldstein, Louis, Marianne Pouplier, Larissa Chen, Elliot Saltzman, and Dani Byrd In press Dynamic action units slip in speech production errors. Cognition. Keating, Patricia A 2006 Phonetic encoding of prosodic structure. In Jonathan Harrington and Marija Tabain (eds.) Speech production: Models, phonetic processes, and techniques, 167—186. (Macquarie Monographs in Cognitive Science) New York: Psychology Press.

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Keating, Patricia and Stephanie Shattuck-Hufnagel 2002 A prosodic view of word form encoding for speech production. University of California Los Angeles Working Papers in Phonetics 101: 112—156. Levelt, Willem J. M. 1989 Speaking: From Intention to Articulation. Cambridge, Massachusetts: Massachusetts Institute of Technology Press. Levelt, Willem J. M., Ardi Roelofs and Antje S. Meyer 1999 A theory of lexical access in speech production. Behavioural and Brain Sciences, 22(1): 1—75. MacKay, Donald G. 1971 Stress pre-entry in motor systems. American Journal of Psychology, 84(1): 35—51. Pouplier, Marianne In press Tongue kinematics during utterances elicited with the SLIP technique. Language and Speech. Sevald, Christine A. and Gary S. Dell 1994 The sequential cuing effect in speech production. Cognition 53: 91—127. Shattuck-Hufnagel, Stephanie 1979 Speech errors as evidence for a serial-ordering mechanism in sentence production. In William E. Cooper and Edward C.T. Walker (eds.), Sentence Processing: Psycholinguistic Studies Presented to Merrill Garrett, 295—342. Hillsdale, New Jersey: Lawrence Erlbaum. Shattuck-Hufnagel, Stephanie 1983 Sublexical units and suprasegmental structure in speech production planning. In Peter F. MacNeilage (ed.), The Production of Speech, 109—136. New York: Springer-Verlag. Shattuck-Hufnagel, Stephanie 1992 The role of word structure in segmental serial ordering. Cognition 42: 213—259. Shattuck-Hufnagel, Stephanie and Dennis H. Klatt 1979 The limited use of distinctive features and markedness in speech production: Evidence fro speech error data. Journal of Verbal Learning and Verbal Behavior, 18:41—55. Shattuck-Hufnagel, Stefanie and Alice E. Turk 1996 A prosody tutorial for investigators of auditory sentence processing. Journal of Psycholinguistic Research, 25(2): 193—247. Stemberger, Joseph P. 1992 The reliability and replicability of naturalistic speech error data: A comparison with experimentally induced errors. In Bernard J. Baars (ed.), Experimental Slips and Human Error: Exploring the Architecture of Volition, 195—215. New York: Plenum Press. Vitevitch, Michael S. 2002 The influence of phonological similarity neighborhoods on speech production. Journal of Experimental Psychology: Learning, Memory and Cognition 28(4): 735—747. Vousden, Janet I., Gordon D.A. Brown and Trevor A. Harley 2000 Serial control of phonology in speech production: A hierarchical model. Cognitive Psychology, 41: 101—175. Wilshire, Carolyn E. 1998 Serial order in phonological encoding: An exploration of the ‘word onset effect’ using laboratory-induced errors. Cognition 68: 143—166.

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Wilshire, Carolyn E. 1999 The ‘tongue twister’ paradigm as a technique for studying phonological encoding. Language and Speech 42(1): 57—82.

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Table 1: Percentages of different error types produced in Experiment 1. Contextual Non-contextual Total

Onset 83.3 1.2 84.5

Coda 2.8 3.7 6.5

Whole word 1.6 7.4 9.0

Total 87.8 12.2 100

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Table 3: Percentages of different error types produced in Experiment 2. Contextual Non-contextual Total

Onset 76 9 85

Coda 11 1 12

Whole word 3 N/A 3

Total 90 10 100

29

FIGURE CAPTIONS

Figure 1. Mean number of total errors per word at each position in the a) ABAB format and b)

ABBA format tongue twisters in Experiment 1. Participants repeated four-word lists 6 times each

under each of four prominence conditions. Bars show standard errors.

KEY: Prom Cond = Prominence Condition. W1 w2 w3 w4 = first word emphasised. w1 w2 w3 W4

= fourth word emphasised. W1 w2 W3 w4 = first and third words emphasised. w1 W2 w3 W4 =

second and fourth words emphasised.

Figure 2. Mean number of contextual errors per tongue twister word produced in sentences repeated

six times each in Experiment 2. Tongue twister words occurred at clause-initial versus non-initial

position and were accented (given narrow focus) versus unaccented.

30

Figure 1.

31

Figure 2.

Table 2. Mean number of total errors per word and 95% confidence intervals for means by prominence condition, format and position in list,

and for two-way interactions between these variables in Experiment 1. KEY: CI = Confidence Interval, W1 w2 w3 w4 = first word emphasised.

w1 w2 w3 W4 = fourth word emphasised. W1 w2 W3 w4 = first and third words emphasised. w1 W2 w3 W4 = second and fourth words

emphasised.

Prominence Condition x Position in List

Prominence Condition (i) W1 w2 w3 w4 (ii) w1 w2 w3 W4 (iii) W1 w2 W3 w4 (iv) w1 W2 w3 W4 Total

Mean CI Mean CI Mean CI Mean CI Mean CI Position in List Word 1 2.8 2.2, 3.5 4.1 3.3, 4.9 3.2 2.5, 3.9 4.6 3.9, 5.4 3.7 3.1, 4.2

Word 2 3.2 2.5, 3.9 4.9 4.0, 5.8 3.4 2.7, 4.2 3.4 2.8, 4.1 3.7 3.1, 4.3

Word 3 7.1 5.7, 8.4 7.6 6.3, 8.9 6.0 5.0, 7.0 8.9 7.5, 10.3 7.4 6.5, 8.3

Word 4 5.5 4.2, 6.8 4.8 3.7, 5.9 4.5 3.4, 5.7 6.1 4.8, 7.3 5.2 4.3, 6.2

Total 4.7 3.8, 5.6 5.3 4.4, 6.2 4.3 3.5, 5.0 5.8 4.9, 6.6 5.0 4.3, 5.7

Prominence Condition x Format Prominence Condition (i) W1 w2 w3 w4 (ii) w1 w2 w3 W4 (iii) W1 w2 W3 w4 (iv) w1 W2 w3 W4 Total

Mean CI Mean CI Mean CI Mean CI Mean CI

Format ABAB 4.4 3.6, 5.2 4.9 4.0, 5.7 2.9 2.4, 3.4 4.0 3.3, 4.7 4.0 3.5, 4.6

ABBA 4.9 3.8, 6.0 5.8 4.7, 6.9 5.7 4.6, 6.8 7.5 6.3, 8.7 6.0 5.1, 6.9

Total 4.7 3.8, 5.6 5.3 4.4, 6.2 4.3 3.5, 5.0 5.8 4.9, 6.6 5.0 4.3, 5.7

Position in List x Format

Position in List Word 1 Word 2 Word 3 Word 4 Total

Mean CI Mean CI Mean CI Mean CI Mean CI Format ABAB 2.4 2.0, 2.8 3.6 3.0, 4.2 5.7 4.8, 6.6 4.4 3.5, 5.4 4.0 3.5, 4.6

ABBA 5.0 4.1, 5.9 3.9 3.0, 4.7 9.1 7.8, 10.3 6.0 4.8, 7.2 6.0 5.1, 6.9

Total 3.7 3.1, 4.2 3.7 3.1, 4.3 7.4 6.5, 8.3 5.2 4.3, 6.2 5.0 4.3, 5.7