The Cricothyroid Muscle inVoicing Control*

Anders Lofqvist,t Thomas Baer, Nancy S. McGarr,tt andRobin Seider Storyttt

Initiation and maintenance ofvibrations of the vocal folds require suitable conditions ofadduction. longitudinal tension, and transglottal airflow. Thus, manipulation ofadduction/abduction. stiffening/slackening, or degree of transglottalflow may, in principle,be used to determine the voicing status ofa speech segment. This study explores the controlofvoicing and voicelessness in speech with particular reference to the role of changes in thelongitudinal tension of the vocalfolds. as indicated by cricothyroid (CT) muscle activity.Electromyographic recordings were madefrom the CT muscle in two speakers ofAmericanEnglish and one speaker ofDutch. The linguistic material consisted ofreiterant speech madeup ofCV syllables where the consonants were voiced and voiceless stops,fricatives. andaffricates. Comparison ofCT activity associated with the voiced and voiceless consonantsindicated a higher levelfor the voiceless consonants thanfor their voiced cognates.Measurements ofthefundamentalfrequency (FO) at the beginning ofa vowelfollowing theconsonant show the common pattern ofhigher FO after voiceless consonants. For onesubject, there was no difference in cricothyroid activity for voiced and voiceless affricates,' inthis case, the consonant-induced variations in the FO of the following vowel were also lessrobust. Consideration of timing relationships between the EMG curves for voiced andvoiceless consonants suggests that the differences most likely reflect control ofvocal10ldtensionfor maintenance or suppression ofphonatory vibrations. The same mechanism alsoseems to contribute to the well-known difference in FO at the beginning of vowels followingvoiced and voiceless consonants.

INTRODUCTION

For sustained vibrations of the vocal folds to occur. some physiological andaerodynamic conditions must be met (cf. Stevens. 1977: Tltze. 1980: van den Berg. 1958).In the larynx. the vocal folds must be adducted to a suitable degree and the longitudinaltension of the folds must be adjusted within an appropriate range. In addition. atransglottal air flow is reqUired. This flow is induced by a transglottal pressure dropand it in turn induces variations in pressure within the glottis. depending on the shapeand configuration of the vocal folds. When conditions for phonation are met, thesevariations in pressure and the movements of the vocal folds that they cause interactwith each other to produce sustained vibrations (cf. Titze, 1986).

Haskins Laboratories

Status Report on Speech Research

25

SR-97/98

1989

26

Manipulation of the conditions that are necessary for vibrations to occur can, inprinciple, also be used to control the cessation of voicing dUring periods ofvoicelessness in speecho Thus adduction/abduction and stiffening/slackening of thevocal folds and facilitation/inhibition of transglottal flow are all potentialmechanisms for determining the voicing status of a speech segment 0 The roles ofadduction/abduction and facilitation/inhibition of air-flow are relatively wellunderstood. However, there is no general agreement whether control of longitudinaltension of the folds is actually used to control voicing status. The present work is aimedat clarifying the role of this potential mechanism.

Vocal-fold abduction is the mechanism most often associated with voicelessness inspeech. The glottal abduction gesture is phased with respect to oral articulations toproduce contrasts of aspiration. and its amplitude and duration also vary withconsonant type (e.g., LOfqvist, 1980; LOfqvist & McGarr, 1987: LOfqvist & Yoshioka.1980. 1984)0 However, glottal abduction in itself may not always be suffiCient forpreventing laryngeal vibrations.

Glottal abduction dUring speech production is usually combined with a decrease ofthe air flow through the glottis due to a supraglottal constriction. For voiceless stopconsonants. there is a build-up of oral air pressure behind the supraglottal occlusionand the pressure drop across the glottis is thus decreasedo The glottal vibrations cease asthe transglottal flow decreases and the glottal opening increases (cfo Hirose & Niimi,1987; Yoshioka. 1984). Venting the vocal tract dUring the period of stop closure. thusrestoring the transglottal flow, may sometimes result in glottal vibrations (Perkell,1976; Vencov, 1968). For voiceless fricatives. where the supraglottal constriction isincomplete. there is greater airflow than for stops, but the decrease in transglottalpressure drop in combination with the glottal abduction gesture may cause cessation ofglottal vibrationso On the other hand. for the laryngeal fricative /h/. an abductiongesture is often found but the vibrations may sUll continue; in this case. no supraglottalconstriction is made and the transglottal flow continues uninterruptedly. Thus. thelaryngeal abduction gesture may not in 'itself arrest glottal vibrations but may do soonly in combination with aerodynamic factors (cf. Yoshioka. 1981). The interactionbetween glottal abduction and aerodynamic factors results in a subtle differencebetween the voicing offsets of voiceless stops and fricatives. Vibrations generally end ata larger glottal opening for fricatives than for stops (Hirose & Niimi. 1987)0 This factcould be related to the different aerodynamic conditions prevailing at the onset of stopclosure and fricative constriction. respectivelyo In the former case. the vocal tract iscompletely sealed off. and the air flow through the glottis decreases abruptlyo Forfricatives, on the other hand. transglottal flow continues throughout the period of oralconstriction even though the oral pressure is increased behind the constrictiono EvendUr1ng stop occlusion. aerodynamic factors may be used to facilitate or inhibit glottalairflow. thus contributing to the voicing distinction. This is accomplished throughcontrol of supraglottal volume. Active or passive expansion of the vocal tract facilitatesvoicing while tensing it or actively constricting it inhibits voicing (cfo Bell-Berti, 1975;Mi.n.ifte. Abbs. Tarlow. & Kwaterski. 1974; Westbury. 1983)0

Muscular control of the abduction gesture for devoicing involves suppression ofactivity in the interazytenoid. lateral cricoarytenoid. and thyroarytenoid muscles. andactivation of the posterior cricoarytenoid. In comparison. for (unaspirated) voicedconsonants, there may be some suppression of the adductors. but the posteriorCricoarytenoid is not active.

While glottal abduction is the laryngeal gesture commonly associated with an arrestof glottal vibrations. its opposite. increased glottal adduction, can also be used for thesame purpose. In this case, a tight closure is produced. preventing transglottal flow.Such an adjustment occurs for glottal stops and also for voiceless ejective and

Lofqvist et ai.

27

ingressive consonants (cr. Fre Woldu. 1985; Hirose & Gay. 1973). Electromyographicrecordings of intrinsic laryngeal muscles suggest that the thyroarytenoid muscle isactive in this case. adducting the folds and increasing the tension of their bodies.

While changes in glottal abduction/adduction and transglottal flow are certainlyused for arresting glottal vibrations. it is more uncertain if the remaining possiblemechanism-increased longitudinal tension-is used in speech for the same purpose.This mechanism was suggested by Halle and Stevens (1971) on theoretical grounds. butthe evidence supporting it has been equivocal.

Physiological studies have shown that the cricothyroid muscle is most consistentlyassociated with changes in the longitudinal tension of the vocal folds. This musclecontrols the length and tension of the vocal folds by· rotating the cricoid and thyroidcartilages relative to each other (e.g.• Sonesson. 1982). Changes in vocal-fold tensionresult in changes of fundamental frequency dUring phonation. and thus cricothyroidmuscle activity is highly correlated with fundamental frequency.

Another muscle that could also be involved in the control of the tension of the vocalfolds is the vocalis muscle. EMG recordings from the vocalis indicate that it is active forboth adduction of the vocal folds and changes in tension (cf. L6fqvist, McGarr. & Honda.1984); in particular. it adjusts the relative tensions of the vocal fold body and cover.Although the experimental evidence is unclear. the vocalis most often shows reducedactivity for voiceless consonants.

While the experimental results are conflicting. some EMG studies have suggested thatthe activity of the cricothyrOid muscle is higher for voiceless than for voicedconsonants. Dixit and MacNeilage (1980) reported tilat the cricothyrOid was moreactive dUring voiceless than dUring voiced stops. fricatives. and affricates in Hindi.LOfqvist. et al. (1984) showed that the cricothyroid was more active just before thebeginning of the oral closure/constriction for voiceless as opposed to voicedconsonants in Swedish. Other studies reviewed by Sawashima and Hirose (1983) alsosuggest a similar difference. However. Hirose and Ushijima (1978) suggested that theseobserved differences in CT level could have been associated with differences in theintonation patterns for the utterances involved. Several other investigators report nodifference in the CT activity level associated with voiced-unvoiced pairs (e.g.. Collier.Lisker. Hirose. & Ushijima, 1979; Kagaya & Hirose. 1975). If such a relative increase incricothyroid activity for voiceless consonants occurs, it could be related to the well­known higher fundamental frequency of a vowel follOWing a voiceless consonant ascompared to a vowel follOwing a voiced consonant (e.g.• Hombert. Ohala. & Ewan. 1979;House & Fairbanks. 1953; Lehiste & Peterson, 1961; Ohde, 1984). This phenomenonserved as the impetus for hypothesizing a role for laryngeal tension in the voicingdistinction. However, the lack of experimental evidence shOWing a systematicdifference in CT activity has led investigators to seek alternate explanations for thisphenomenon. such as articulatory and aerodynamic explanations.

There is thus conflicting eVidence concerning the role of variations in thelongitudinal tension of the vocal folds in the control of voicelessness. The present studywas therefore designed to investigate further the actMty of the cricothyroid muscle invoicing control using electromyographic techniques.

I. METHODS

Electromyographic recordings were obtained from the cricothyroid muscle in threesubjects. 1\vo ofthe subjects. one male (TE) and one female (NSM). were native speakersofAmerican English; the third subject (LB). was a male native speaker of Dutch.

The Cricothyroid Muscle in Voicing Control

28

The linguistic material consisted of reiterant speech modeled after the sentence "Theman went to market." (The underline indicates main lexical stress). Different CVsyllables. drawn from 16 consonants and 3 vowels, were reiterated to form 48 differentutterance types. The vowels used for the reiterant syllables were Ii a ul and theconsonants were voiced and voiceless stops (lp b t d k g/), fricatives (If v e 0 s z I 3/),and affricates (ItI d3/). Only stops and fricatives were used for the Dutch subject. For theanalysis of voicing contrasts, we used the consonant occurring in the syllable carryingthe sentence stress in the utterance, that is, the second syllable in the sentence. Five toten repetitions of each utterance type were obtained.

Hooked-wire electrodes were inserted percutaneously under topical anesthesia of theskin. Verification of electrode position was made by having the subject perform selectedspeech and nonspeech gestures, such as pitch changes, vocal attacks, jaw movements,and swallowing. Placement into the CT was verified by showing activity correlated withFO but not with adduction, abduction, or jaw opening, and a characteristic pattern ofactivation and suppression during swallow. After amplification and high-pass filteringat 80 Hz. the Signals were recorded on an FM tape recorder together with the speechsignal. For averaging, the Signals were full-wave rectified. integrated over a 5-mswindow. then sampled and digitized. resulting in a computer sampling rate of 200 Hz. Inthe averaging process. the signals were aligned with reference to a predetermined,acoustically defined line-up point. and also further smoothed using a 35-ms triangularwindow. The line-up point for fricatives and affricates was the apparent end offrication. The line-up point for stops was the release burst. This point was chosenbecause it clearly marks the end of the period in which differing activity would beexpected. if it were associated with the voicing distinction. Alternatively, we could havechosen the point of stop closure or onset of frication. However. in the present study, wehave chosen to concentrate more on the CV rather than the VC junction.

In order to obtain a single numerical value for the level of cricothyroid activity, thearea under the EMG curve for each token was calculated dUring a short time window.This window was chosen so as to cover the period of consonant occlusion and part of thepreceding vowel: its end always coincided with the line-up point. The duration of thewindow was chosen individually for each subject and consonant type based on acousticmeasurements. Typical values were 125 ms for stops, 175 ms for fricatives. and 150 msfor affricates. The position of the Window was based on theoretical and empiricalconsiderations. If an increase in the tension of the vocal folds is used to suppress glottalvibrations, the neuromuscular events ought to occur within this Window. They would beexpected to begin near the transition from the voiced vowel to the voiceless consonant.This is also the point at which Lofqvist et al. (1984) found an increase in cricothyroidactivity for voiceless consonants.

Measurements were also made of the fundamental frequency at the beginning of thevowel follOWing the consonant. For this, the duration of the first 10 pitch periods ofeach token were measured interactively on a computer. To keep the measurement taskmanageable. these measurements were only made for utterances containing the vowella/.

II. RESULTS

A. Electromyography

The averaged electromyographic curves for each of the three subjects are shown inFigures 1-3: in these averages, the utterances containing different vowels have beenpooled. since there were no systematic differences across vowels. For the present

Uifqvist et al.

29

analysis and display purposes, data have also been pooled across place of articulationfor the stops and fricatives. Average audio amplitude envelopes are also shown inFigures 1-3. The audio envelope curves provide a rough indication of consonant/vowelboundaries, though it must be remembered that the tokens contributing to theseaverages displayed a range of temporal characteristics so that alignment of segmentboundaries is imprecise except near the line-up point.

English (TB)StopsfLV .-- ......- ~

300

EMG

o

Audio

Voiced

Voicelesso

FricativesfLV ...- .-- ~

300

EMG

Audio

AffricatesfLV300 r---------.,r---------.,

Audio

O\---'----'--I----'-_....L-_

EMG

Figure 1. Average EMG and audio signals for subject TB, (n =47 for the stops and fricatives, and 15 forthe affricates.)

The plots of cr activity shown in Figures 1-3 all share a common feature. In the lefthalf of each figure, covering a period associated with the preceding vowel, thetransition, and the consonant occlusion, the curves for voiced and voicelessconsonants begin at the same level, diverge from each other with the voiceless oneshOWing higher activity, and then' reconverge in the vicinity of the line-up point.Qualitatively, where the curves have separated, they are clearly at visually distinctlevels in all cases, except, perhaps, for the affricates of subject TB t shown in Figure 1.The divergence of the two curves is due mostly to an increase of activity for the voicelessconsonants; activity for the voiced consonants remains roughly constant, increasing

The Cricothyroid Muscle in Voicing Control

30

later. so that when the curves reconverge at the consonant release, they are at a higherlevel than for the preceding vowel, in accordance with the stress pattern of theutterance. The cuxves also appear to diverge in the right halves of each figure, for thefollowing consonant, although the tlIiling is more variable.

English (NSM)IJ.V ,... -r-__S__to~p_s

300

EMG

o

Audio

Voiceless

Voiced

FricativesIJ.V300 ,.-----...-------,

Audio

EMG

o

Affricates',LV300 ,------,r-------,

Audio

o

EMG

Figure 2. Average EMG and audio signals for subject NSM. (n = 89 for the stops, 120 for the fricatives,and 28 for the affricates.)

It is useful to consider the timing relationships in somewhat greater detail Forexample, we consider the timing demonstrated in the top panel of Figure 1, which :showsthe data for stops for subject TB. The audio envelope curves indicate that timing wassimilar for the voiced and voiceless consonants. since the curves follow about the samecourse except dUring the period associated with aspiration for the voiceless consonants.The EMG levels are the same dUring the nucleus of the preceding vowel. but the curvesbegin to diverge shortly before the rapid descent of the audio envelope. which indicatesthe beginning of the transition period. The difference between the two EMG curvesreaches its maximum about when the audio envelope cuxves approach the baseline. thepoint at which stop closure has been achieved for almost all tokens. The curves thenapproach each other again. meeting shortly after the line-up point; they are at

Ltifqvist et aI.

31

essentially the same level through the following vowel. diverging again as the audioenvelope begins its descent for the following consonant.

For fricatives, shown in the middle panel,. a similar pattern is obselVed, although thepattern is a little less clear because the voiceless fricatives have longer durations thantheir voiced counterparts. Here the EMG CUlVes begin at the same level, dUring thepreceding vowel. The CUlVe for the voiceless consonant begins to increase almostimmediately, however, shortly before the corresponding audio envelope begins its rapiddescent. The maximum difference occurs when the audio envelope for the voicelessfricative nears its minimum, and the CUlVes converge again, this time somewhat beforethe lineup point rather than after it as in the upper panel. The CUlVes diverge again fromeach other in the right half of the figure, at about the same place within the cycle for thevoiceless consonants (Le., at about the lead before the rapid descent of the audioenvelope).

Dutch (LB)

IlV Stops300 ,.......----r-----'-----,

EMG

Audio

Voiceless

Voiced

FricativesIlV300,...--------,r-------,

EMG

Audio

100 ms

Figure 3. Average EMG and audio signals for subject LB. (n = 4S for the stops and 29 for the fricatives.)

All the CUlVes for the other two subjects. shown in Figures 2 and 3. exhibitqualitatively the same type of behavior as Figure 1. The EMG CUlVes diverge roughlywhere the audio envelope for the voiceless consonant begins its rapid descent. Themaximum difference between the two EMG CUlVes occurs around occlusion for thevoiceless consonant (although not so closely for the fricatives and affricates for subjectNSM in Figure 2), and the CUlVes converge again in the vicinity of the lineup point.Subject NSM also differs somewhat from the other subjects in the right halves of theplots, in that the CUlVes for voiced and voiceless consonants separate much earlier,

The Cricothyroid Muscle in Voicing Control

32

dUring the vowel in the preceding (stressed) syllable. The results for subject LB in Figure3 show the same tlmtng of CT activity as those for subject TB in Figure l.

The difference in the EMG CUlVes was tested quantitatively using the integrated "areaunder the culVe" measure, described in Methods. Figure 4 shows the integrated value ofcricothyroid activity averaged over all tokens for each condition. Standard deviationsare also indicated. In all cases except one, the affricates for subject TB, the differencebetween the voiced and voiceless consonants is highly significant (p < 0.001). For theaffricates of subject TB. the difference was not statistically significant (~9 = 0.43.p> 0.5).

Fricative

FricatN. AHricate

• Voiced

o Voiceless

Subject TB

Subject NSM

Subject LB

Fricative Affricate

u;- 20

""c8QlII)

l!l

~ 10.!.l.§..Qj

~-'

0SlOP

30

u;-

""c~II) 20

~eu

I 10Qj>Ql

...J

0Slop

20

iii'

""§II)

l!l'0

10~l:l

Il..J

0SlOp

Figure 4. Mean and standard deviation of cricothyroid activity level. ..1M = P < 0.001.

The audio envelopes in Figures 1~3 suggest that the vocal folds were vibrating for thevoiced consonants produced by subjects NSM and LB. On the other hand, the voicedstops for subject TB in Figure 1 would seem to have been produced at least partlydevoiced. This can be deduced from the amplitude of the audio envelope dUring stopclosure, which is almost similar for the voiced and voiceless cognates. The vibrationsseem to stop dUring the later part of the oral closure for the voiced stops.

Uifqvist et al.

33

B. Fundamental Frequency

The measurements of fundamental frequency at the beginning of the vowel follOwingthe consonant are shown in Figures 5-7 for the stops and fricatives for the threesubjects and in Figure 8 for the affricates. As expected, the FO at this point of the vowelis higher after a voiceless consonant. In this voiceless context, the fundamentalfrequency generally starts at a high level and drops dUring the first periods. In thevoiced context, FO at the onset of the vowel is lower and may remain at the same level,rise, or even fall slightly. The difference in FO following voiced and voicelessconsonants was in most cases highly Significant for all the ten periods measured. Theonly exception to this general trend consisted of the affricates for subject TB in Figure 8.Here, the difference disappeared at pitch period number 7 (~B =1.26, P > 0.1) and also forthe preceding pitch periods the difference was less robust. Recall that for this speakerthe difference in Cricothyroid activity between the voiced and voiceless affricates wasnot significant, cr. Figures 1 and 4.

... Voiced__ Voiceless

... Voiced

-- Voiceless

SUbject TB

Fricatives170

N;:s 160~c:CD::::l 150C"

~

~140

CD

~ 130'0c:::::lu..

120

2 4 6 8 10

Pitch period

Stops170

N~ 160~c:CD::::l

150go.::]i

140c:CDEctl'0 130c:::::lu..

120

2 4 6 8 10

Pitch period

Figure 5. Mean and standard deviation of FO during the first ten pitch periods of the vowel followingstops and fricatives for subject TB. (n = 15.)

The Cricothyroid Muscle in Voicing Control

Figure 6. Mean and standard deviation of FO during the first ten pitch periods of the vowel followingstops and fricatives for subject NSM. (n = 15.)

Subject LB

Figure '1. Mean and standard deviation of FO during the first ten pitch periods of the vowel followingstops and fricatives for subject LB. (n = 15 and 10.)

Lofqvist et al.

35

Overall, the consonant induced variations in FO appear to be less robust followingaffricates, cf. Figure 8. As already noted, the difference disappeared at pitch periodnumber 7 for subject TB. For subject NSM, the difference was highly Significant for thefirst 6 pitch periods and significant for periods 7 through 10. It is evident, however. thatfor this subject, NSM, the absolute difference in Hz at the tenth period after theaffricates is much smaller than at the same period following the stops and fricatives inFigure 6. Furthermore, given the higher FO of subject NSM, her tenth pitch periodoccurs approximately at the same point in time after vowel onset at pitch periodnumber 7 of subject TB.

Subject NSM • affricates

290

'N 280;; 270~c: 260Ql:::l

250g.l::

240mc: 230Ql

~ 220"0c:

210:::lu..

200

_ Voiced

-0- Voiceless

2 468

Pitch period

10

200

'N 190;;

180goQl 170:::l

I 160

m 150&i~

140

c: 130:::lLl.

120

Subject TS - affricates

l/~ .....

-- ....- ......._ Voiced_ Voiceless

2 488

Pitch period

10

Figure 8. Mean and standard deviation of FO during the first ten pitch periods of the vowel folloWingaffricates for subjects NSM and TB. (n = 10 and 15, respectively.)

The relationship between the activity of the cricothyroid muscle during theconsonant and the fundamental frequency at the beginning of the following vowel isillustrated in Figure 9. This figure plots the average levels of the cricothyroid (cf. Figure4) against the FO at the onset of the following vowel; the value of FO represents the meanof the first three pitch periods. A positive relationship is found with Pearson productmoment correlation coefficients of .64, .85, and .92 for subjects TB, NSM, and LB.respectively. .

The Cricothyroid Muscle in Voicing Control

36

-N::J: 300-(/)"0.Q +'-Q) + +c.

.s::

.~ + - TBc. + +Q)

200Q) NSM'- +.s::-- LB~ x - x;;:::- • - .-00 • •LL.. x •c::~ 100Q)

~ 0 10 20

CT level (JlV)

Figure 9. Plot of cricothyroid activity level versus FO at the beginning of the following vowel.

III. DISCUSSION

The results of the present study show that the Cricothyroid muscle increases itsactivity for voiceless consonants for all three subjects. ThiS activity would act toincrease the 'longitudinal tension of the cover of the vocal folds and should contributeto the inhibition of glottal vibrations. This was found for speakers of two languageswith different stop systems. In both American English and Dutch, the stops are eithervoiced or voiceless. However, the Dutch voiceless stops are unaspirated with a VOT ofabout 10~15 ms in the present study while the American English voiceless stops areaspirated with a VOT of 60-80 ms (cf. Lisker & Abramson, 1964).

The present measurements of fundamental frequency at the beginning of the vowelfollowing the voiced and voiceless consonants are in good agreement with publishedresults (cf. Ohde, 1984; Silverman, 1986). That is, FO is higher after a voiceless thanafter a voiced consonant irrespective of the manner of production, that is, stop,fricative. or affricate. This is true also for the Dutch data in Figure 7 where the voicelessstops are unaspirated; this has also been shown for French stops where the voicelessseries is unaspirated (Fischer-Jorgensen. 1972). Similarly. Ohde (1984) found that theFO of a vowel was similar if the preceding consonant was a voiceless aspirated orunaspirated stop. Thus. it appears that the conditioning factor for the FO difference isvoicing and not aspiration. This conclu~ion is further strengthened by the fact thatstudies of FO after voiceless aspirated and unaspirated stops have shown conflictingresults where interspeaker and interlanguage differences seem to occur to a great extent(cf. Gandour, 1974; Hombert & Ladefoged, 1977; Jeel, 1975: Kagaya, 1974; Kagaya &Hirose. 1975: Zee, 1980). Presumably, aerodynamic factors are also involved. However.the results presented by Hutters (1984, 1985) suggest that the cricothyroid has a higherlevel of activity in Danish aspirated than unaspirated stops.

Lofqvist et al.

37

If there is thus a distinct difference in the level of FO after voiced and voicelessconsonants. the pattern of pitch change at the beginning of the vowel may differ. Aftervoiceless consonants. FO generally drops. although this is not clear for the vowelsfollOWing stop consonants for subject NSM. cf. Figure 6. or for those follOWingaffricates for subject TB. cf. Figure 8. After voiced consonants. the fundamentalfrequency may either rise. fall. or stay level; all these three patterns can be found inFigures 5-8. The FO change at the beginning of the vowel thus depends not only on thepreceding consonant but also on the overall intonation pattern of the utterance (cf.Silvennan. 1986).

We thus argue that an increased longitudinal tension of the vocal folds is associatedwith voicelessness. The dUIerence in tension between voiced and voiceless consonantsis still manifest at the beginning of a follOWing vowel thus accounting for thecommonly found consonant-induced variations in FO. cf. Figure 9 that shows a positivecorrelation between the cricothyroid activity dUring the consonant and the FO level atthe beginning of the following vowel. According to the present results. there is no cleardifference in cricothyrOid activity during the vowel follOWing the consonant. Ifvariations in cricothyroid activity are associated with the voicing distinction. theyshould occur at the transition between the vowel and the consonant. given the fact thatthe electrophySiological events lead the resulting mechanical effects with the latencydepending on the contraction and relaxation time of the muscle. If CT activity weremeant only to control FO and not to contribute to devoicing. it might be expected to beinitiated later relative to stop closure and fricative constriction. Although somewhatconflicting. studies by Baer (1981) and by Larson and Kempster (1985) of Single motorunits in the human cricothyroid muscle and by Atkinson (1978) of the EMGinterference pattern dUring utterances with various intonation patterns indicate thatthe effective contraction time (actually the latency between EMG and the onset of pitchincrease) is in the range of 20-80 ms. and relaxation times are somewhat longer thancontraction times. The present results would also rationalize why the voicing status ofa consonant appears to affect the fundamental frequency of a preceding vowel.independently from the prosodic structure of the utterance. to a lesser extent than itaffects the following vowel.

While we would thus suggest that the well-attested FO difference between vowelsfollOWing voiced and unvoiced consonants is due to a change in vocal fold tensioncaused by the cricothyroid muscle. it is obvious that other factors may also be involved.As reviewed by Hombert et al. (1979) these factors may include aerodynamiC effectsdUring stop closure and release and changes in the vertical position of the larynx thatcan affect the tension of vocal-fold tissues. While we have concentrated on thecricothyroid muscle. our reading of the literature suggests that the other adductormuscles of the larynx are suppressed for voiceless consonants; thus the body of the foldswould be relaxed as the activity of the vocalis muscle decreases for voicelessconsonants.

Kingston (1986) has suggested an alternate explanation for the consonant-voicingpitch effect. Analysis in different languages suggests that the pitch effect consistentlyaccompanies the phonological voicing distinction. regardless of whether or not thevocal folds are actually vibrating. He suggests. then. that the pitch in the neighborhoodof obstruents may be controlled independently as a cue to phonological voicing. ratherthan being a by-product of the gestures that control vibrations of the vocal folds andpresence of aspiration. Although the present experiment did not specifically addressthis hypothesis. we would argue that the timing of increased CT activity for thevoiceless consonants suggests that it is related to devoicing rather than to pitch control;if CT activity was only related to FO control. the increase in activity should occur closerto the onset of the vowel following the consonant. Furthennore. the perceptual

The Cricothyroid Muscle in Voicing Control

38

effectiveness of these FO variations in cueing consonant voicing appears to be debatable(cf. Abramson & Lisker, 1985: Silvennan, 1986).

Perhaps the most compelling evidence in favor of the line of reasoning that we havepresented here is the finding for the affricates of subject 1'8. In this case, there is nosignificant difference in cricothyroid activity between the voiced and unvoicedcognates. Nor is the FO difference in the following vowel c1ear$cut and robust in thecontext of the affricates. Taken together, these results, together with a review of theliterature, suggest that control of laryngeal tension by means of the cricothyroidmuscle is a mechanism that may be used to supplement abduction and reduction oftransglottal flow as devoicing mechanisms. Though it is evidently not an essentialcomponent of the mechanism, there is converging evidence that it is used frequently, atleast in prestressed position. The EMG curves suggest that the CT is also used tocontribute to consonant devoiCing in unstressed syllables, but further analyses arerequired to detennine whether the timing relationships are similar to those forstressed syllables.

ACKNOWLEDGMENT

This work was supported by NINCDS Grants NS-13617 and NS$13870 and NIHBiomedical Research Support Grant RR-05596 to Haskins LaboratOries.

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FOOTNOTES

"Journal of the Acoustical Society of America, in press.

tAlso Department of Logopedics and Phoniatrics, Lund University, Lund, Sweden.

ttAlso Center for Research in Speech and Hearing Sciences, Graduate School and University Center,The City University of New York.

tttAlso Department of Speech and Hearing Sciences, University of Connecticut, Storrs.

Lofqvist et al.