Reaction time measurements of laryngeal closure

Journal of Phonetics (1980) 8, 305-315

Reaction time measurements of laryngeal closure Peter Roach Department of Linguistics and Phonetics, University of Leeds, England* Received 13th November 1978

Abstract:

In an experiment designed to investigate the articulatory dynamics of laryngeal closure, the time taken to effect a stop closure at different places of articulation was examined by measuring the time between presentation of a visual signal to a phonating subject and the moment when airflow ceased. The mean reaction time for laryngeal closure ("glottal stop") was significantly longer than for all oral stops. This difference appears unlikely to be a consequence of efferent neural transmission time or a language-specific trait, and the result is therefore taken to support the view that the laryngeal closure mechanism is articulatorily complex, involving supraglottal as well as glottal constriction.

Laryngeal closures The articulatory mechanism involved in the production of what is commonly called "glottal stop" is not well understood at present. In standard phonetics textbooks it is usual to describe the articulation simply in terms of closure of the vocal folds ( cf. Jones, 1967; Pike, 1943; Heffner, 1950; O'Connor, 1973), several writers adding that the closure is "abrupt", "sharp" or "sudden" (cf. Sweet, 1888; Zernlin, 1968; Gimson, 1970). On the other hand, medical literature contains many descriptions of (non-speech) laryngeal closures involving not only the true vocal folds themselves but also (in different ways according to dif(erent authors) the false vocal folds and a constriction formed with the aryepiglottic folds, the tubercle of the epiglottis and the tips of the arytenoid cartilages or the corniculate cartilages of Santorini (cf. Czermak, 1861; Stuart, 1892; Negus, 1949; Ardran, Kemp & Manen, 1953; Pressman , 1954; Fink, 1956.). Recognition by phoneticians of the fact that such closures may be used in speech has been very limited ; the most detailed theory is that of LindqvistGauffm (Undqvist , 1969 , 1972 a, b, c; Gauffin, 1977) in which two different laryngeal mechanisms are pro posed, one being vocal fold adduction/abduction and the other, independent at the motor command level, being laryngealization. This is laryngeal constriction above the level of the true _vocal folds, related to the protective gesture used _to prevent foreign bodies from entering the larynx, and according to the theory this gesture is the primary mechanism in glottal stop. Catford (1977) also recognizes the possibility of supraglottal laryngeal closures being employed in speech, but maintains that these are found only in special cases where adduction of the false vocal folds is produced simultaneously with true vocal fold closure to produce a distinctive type of stop ("ventricular plus glottal" stop) which contrasts with simple (true vocal fold) glottal stop in some Caucasian languages. He criticizes Undqvist -Gauffin for presenting fiberscope pictures purporting to show a typical glottal stop but in fact (according to Catford) showing an unusually high degree of upper larynx constrictio n. My own fibroscopic observations of laryngeal closures and constrictions produced by myself seem to support the Linqvist- Gauffin account ; it seems *The research reported here was carried out while the author was in the Department of Linguistic Science, Reading University. 0095 - 4470/80/030305+11 $02.00/0

© 1980 Academic Press Inc. (London) Ltd.

306

P. Roach

probable, judging from his description of its auditory quality, that Catford's "ventricular plus glottal" stop involves a considerable degree of constriction above both the true and false vocal folds . As a further factor in the articulatory description of "glottal stop", it has been noted by many phoneticians, for many languages, (e.g. Priestly, 1976, and references given therein) that in environments where glottal stop may be expected one frequently encounters instead something that might be better labelled "laryngealization", where vocal fold vibration does not actually cease but becomes irregular and creaky. Temporal measurement of laryngeal closures Whatever the mechanism or mechanisms involved, we lack temporal information on laryngeal closures, and the experimental study reported here was designed to provide such information in a way which permits comparison with supralaryngeal closures. Problems in measuring the time taken to produce a glottal closure include the following: (i) the movement may be from open glottis to phonation, from open glottis to glottal stop or (the movement of principal interest in this experiment) from phonation to glottal stop; (ii) it is difficult to decide from what point one measures the closing movement, and how one decides that the movement is complete. Lieberman (1967) shows a series of laryngoscopic pictures of the glottis printed from a fUm taken at 10 000 frames per s (p. 10). The sequence begins with the vocal folds open for inspiration; according to the caption to the pictures, the folds are nearly into phonation position 80 ms after this , and phonation started 54 ms later. This gives a total time from fully open to vibrating of 134 ms. Kim (1970), referring to cineradiographic data, writes: " ..... my own examination of the present film shows that it takes 100- 120 ms for my glottis to close , just before the beginning of each utterance .... from the respiratory position." (p.lll). Kim's well-known theory of aspiration depends partly on an assumption of a more or less constant rate of closing movement of the vocal folds, so that a lesser degree of initial aperture will result in a shorter time to the onset of phonation; he gives figures of 90-100 ms for heavily aspirated, 35 ms for moderately aspirated and 10 ms for "unaspirated" Korean plosives . Rothenberg (1968, 1972) makes measurements on a different basis; he postulates, as necessary concepts in the description of glottal behaviour in relation to consonant articulation, the cyclic glottal opening movement and the cyclic glottal closing movement; the former is a movement from the voiced glottal adjustment either to breathy-voiced or to open, then back to voiced, and the latter, which is more relevant to the present study, from voiced to tightly-voiced or stopped then back to voiced (the meaning of tightly-voiced is explained in Rothenberg, 1972 ; p. 387). His figures for these gestures when maximally fast are:

closing cycle stop: 140 ms tight voicing: II 0 ms

opening cycle open : 125 ms breathy voicing: I 00 ms

In a discussion appended to the above paper, C. Scully points out (p.387) that data obtained independently by her and by D. H. Klatt suggest an opening-closing time more in the region of 250 ms , and Rothenberg in reply agrees that his figures have been calculated in a way that will show the shortest times ; " Considerably slower movements can and do occur.. ..." (p.388). Rothenberg's figures may be compared with similar articulatory movements (i .e. of a cyclic nature) for lips, tongue tip and tongue back ( [p] , [t] , [k]) obtained by Hudgins & Stetson (1937) and discussed by Lehiste (1970). From average figures of 8-2 [t] articula-

Measurements of laryngeal closure

307

tions per s, 7 ·1 [k] articulations per s, and 6·7 [p] articulations per s we may calculate the following times for the complete open-closed-<>pen cycle: [t] : 122 ms

[k] : 141 ms [p] : 149 ms

These figures are quite close to Rothenberg's 140 ms for the glottal stop cycle.

Experitpent design and results Since no instrumental technique exists, other than high-speed cineradiography {which was considered unsafe for these purposes) which could be employed to make strictly comparable measurements of closing movements themselves, it was decided to measure the time from the subject's being instructed to perform a particular articulatory closing movement - an external and easily registered event - to the moment when airflow from the vocal tract was judged to have effectively ceased as a result of the closure being completed. The experimental design was arrived at after a series of small-scale trial experiments; a pilot experiment was then conducted with the experimenter as his own subject. A small computer (PDP-8/E) was used to present the stimulus and measure the time to closure. The subject was seated close to the computer in such a position as to be able to see the computer screen comfortably; the lower part of the face was covered by the mask of an airflow measuring device (Frcf>kjaer-Jensen Electro-Aerometer), which was not calibrated for this experiment. The Mouth Out ouput of the aerometer was connected to an analog input channel of the computer and during the first (initialization) stage of the program the airflow was displayed as on a normal oscilloscope display. The height of a horizontal line on the screen was controlled by a potentiometer on the computer front panel, and this was set to just above the airflow trace level displayed when there was no flow of air from the speakers' mouth. When this process was complete, the potentiometer setting was taken as the threshold above which airflow would be judged by the computer to be going on, and below which it would be judged not to be going on. The screen was then cleared. The threshold level was not changed between the different items of the experiment. The next stage of the experiment was to take one of a shuffled pack of 17 cards containing the test items to be pronounced . In all cases, the speaker was to begin producing an [a] vowel when a row of dots appeared on the computer screen {2 s after the start of the experiment), and to continue making this vowel until the line disappeared from the screen. This clearing of the screen took place at a random interval {to prevent the speaker from falling into a rhythm) between 2 and 4 s after the row of dots . When the screen was cleared the speaker had to produce the stop consonant indicated on the current card and this process was repeated up to the number of repetitions pre-set by the experimenter, with a 2-s "rest" between items. In the case of the pilot experiment being described here, each item was timed 50 times, so that the experiment required 17 x 50= 850 responses . For each item, immediately before clearing the screen the computer checked the airflow level to make sure that the airflow was over the threshold value ; if it was not, the program jumped back to continue the dot-row display for a further 2-4 s. Given a satisfactory airflow, the screen was cleared and the computer clock started , counting in ms. When the airflow dropped below the threshold, the clock was stopped and the elapsed time was recorded. The variables in the items for the experiment were as follows : {i) place of stop articulation : bilabial, alveolar, velar, glottai; 1 {ii) voiced or voiceless [a] vowel preceding stop consonant {i .e. vocal fold vibration or vocal fold separation before stop articulation); {iii) (for bilabial , alveolar and velar places of articulation) fortis or Ienis stop. The items [voiced vowel + glottal stop] and [voiceless vowel + glottal stop] were each included twice in the set of test items, partly because the glottal stop, the most important consonant , would otherwise be poorly represented in terms of number of occurrences , and partly in case one set should have been affected by position in the series. 1

The familiar term "glottal" will be used instead of the possibly more suitable " laryngeal".

P. Roach

308

The 17 cards thus read (before being shuffled): (with voiced vowel)

ap

at

ak

ab

ad

ag

a?

a?

(with voiceless vowel, [b, d, g] to be pronounced as whispered)

hp

ht

hk

hb

hd

hg

h?

h?

0

0

plus one item where the speaker had to produce a voiceless vowel upon appearance of the dot-row signal and change to a voiced [a] vowel when the screen was cleared , threshold being set above the normal phonation flow-rate. This was written on the card as ha. The distinction between the [p,t,k] and [b, d, g] in the voiceless set is, of course , very 0 0 contrived and artificial, and was not used in the tests that followed. The item [ha] was also dropped, principally because one could produce a reaction time measurement simply by reducing pulmonic pressure. These items were, however, felt to be interesting enough to be worth including in a pilot experiment. The results, with mean reaction times and standard deviations, are set out in Table I. Table I

s.d .

s.d.

ap

at

ak

ab

ad

ag

a?

a?

198 31

251 56

282

193 22

204 37

289 35

296 38

351 79

hp

ht

hk

hb0

hd0

hg

h?

h?

197 34

237 47

247 70

207 36

204 44

295 36

310 60

383 38

72

ha S.d.

227 45 For each item, N=50, times are in ms .

It is immediately obvious that the glottal closure times are markedly longer than the other times. An analysis of variance was carried out on the data , and established that while the differences between [p t k b d g ?] are highly significant, the difference between voiced [a] preceding and voiceless [h] is not significant. This was in spite of the fact that in the case of the voiced vowels the distance to be travelled by the vocal folds was virtually nil , while closing the folds from a "voiceless vowel" position involves a distinct inward movement of the folds. The difference between fortis and Ienis consonants was not examined in the analysis of variance, but it is clear from an examination of the figures that there is no systematic difference . The mean times for all "fortis" consonants in [ap at ak hp ht hk] was 235 -3 ms, while the mean for all "Ienis" consonants in [ab ad ag hb hd hg] was 232 ms .- We may therefore give the results in condensed form, putting all items with a particular place of articulation together, in Table II. The column headed " % diff." gives the difference between the supraglottal stop time and the glottal stop time , expressed as a

309

Measurements of laryngeal closure

percentage of the supraglottal stop time; thus, the glottal stop mean time is 68% longer than the bilabial stop time. Table II

alveolar

bilabial

velar

% diff.

glottal

68

199 (N=200)

49

224 (N=200)

21

276 (N=200) 334 (N=400)

Clearly , the results of the above experiment needed to be confirmed by testing other subjects. Various problems arose in doing this . The aerometer mask leaked badly on subjects with small faces , and using it on female subjects was found not to be a practical possibility (a different sort of mask is now being worked on to overcome this problem). Subjects' behaviour was somewhat unpredictable , some regularly scoring reaction times of only 1 or 2 ms by producing anticipatory closures, guessing that the signal was about to given, while others recorded occasional reaction times of several seconds during moments of inattention or confusion. To avoid the distortion of the results caused by such factors the program was modified to ignore (without informing the subject) any time value below 100 ms or above 500 ms. It was found very difficult to keep a subject's attention for a full range of consonants in a single experimental session, so each subject was tested with just one of [p t k] and also [?] , all after voiced [a] . The item to be tested first was assigned randomly. All subjects had had some phonetic training . Table III below shows results from English speakers (male). The column headed P shows probabilities, calculated by t-test for correlated samples (two-tail) as set out in Robson (1973). Table III

Subj .

[p]

[t]

[k]

[?]

N

p

% diff.

DG

243

299

33


+23

MG

264

335

40


+27

RM

284

334

40


+18

GN

229

292

40

<0·001

+28

IP

269

347

40

< 0·001

+29 - 22

AS

414

325

40

c
DH

254

275

40

NS

+8

Mean reaction times in ms.

While these results mostly confirm the results of_ the first experiments, two · of the subjects' results need comment. In the case of AS, the subject remarked before starting his [k] test that he normally produced this sound, when in final position, with a glottal stop; he

310

P. Roach

tried hard during the test to avoid doing this, though he had not been asked to do so. The result is highly significant, but in the opposite direction from the others. The airflow data from each subject was recorded during the experiment and later averaged by computer, using the moment of downward crossing of the airflow threshold as the "line-up point" for the averaging. Figure I shows the averaged airflow traces for AS's 40 repetitions of [ak]

MO

Vo•celess velar stop

GM

0 ·5 s

Figure 1 (upper pair .of traces) and [a?] (lower pair), labelled MO. The large peak before the [k] closure constitutes .. strong preaspiration of the consonant in the subject's effort to avoid glottalization of_ the .consonant. The traces labelled GM are a record of trans-laryngeal electrical impedance (inverted) taken from the "gross movements" output of a Laryngograph (Fourcin and West, 1968). Figure 2 shows, for comparison, MG's [ap) (upper) and

Measurements of laryngeal closure

311

MO

Voiceless bilobnJI stop

GM

0 ·5

5

MO

Glo llat s top

GM

Figure 2 [a?j (lower); again, these traces represent the average of 40 repetitions. Subject DH's figures fail to reach a standard significance level (t= 1·6 with 39 di.) but are in general agreement with the other subjects' results. Two male subjects with languages other than English said that glottal stops did not occur in their languages, subject JS's mother tongue being Spanish and YM's being Kikongo. Their results are given in Table IV, and appear to agree with those of the English-speaking subjects. Table IV

Subj.

[p]

[t1

JS

YM

285

Mean reaction times in ms.

p

[k]

[7]

N

372

407

40

<0-01

401

40

<0-001

% diff. +9 +41

312

P. Roach

Finally, two male speakers of a language (Sudanese colloquial Arabic) in which syllablefmal glottal stops are common underwent the same test. The results are set out in Table V. mean reaction time figures are given in ms. Table V Subject

[t]

N

p

% diff.

HA

317

321

30

NS

+ 1·3

SM

323

352

40

NS

+9

Mean reaction time in ms.

Discussion The time measurements given indicate strong support for a conclusion that the "glottal stop" is not produced in a shorter time than other voiceless plosives, and in fact usually takes rather more time than other plosives. (It is perhaps worth noting, though the figure must have very little validity, that the average of all the figures given above for percentage differences between supraglottal and glottal stops is over 25%.) It is necessary to consider how this may be explained. We can consider five temporal components that make up the time values. Transmission of information from eye to brain The time for this is not known, though it could probably be fairly easily calculated by reference to work on the neurology of vision. The figure is not, however, of much concern to us here, since it is taken to be constant for each subject. "Central reaction time" The term is adopted from experiments by Fry (1971, 197 5); his study, though interesting to compare with the above, is different in several important ways. Nevertheless, it is worth quoting some of his measurements and conclusions for the sake of comparison. Fry's experiment tested each subject with two different auditory stimuli, one speech-like- an [a] vowel and one non-speech - a click, and two different responses, one speech-like- an [a] vowel and one non-speech - key-pressing. Insofar as there is any resemblance between any of Fry's conditions and mine, the situation of my subjects is closest to Fry's "PV" - non· speech stimulus, speech response, for which Fry's subjects' mean response time was 257 ms. This is longer than the time for his other conditions, but is in the same area as my results, particularly those for [p t k] . Fry calculates that afferent neural transmission (auditory in his case) will not occupy more than 10 ms, efferent neural transmission may take up to 15 ms and muscle action, in the case of a key-pressing response, about 35 ms. Fry then states: " ....the total is unlikely to be markedly different for key and voice responses." (p.179), though he does not give his grounds for making this statement. He suggests a central reaction time for PV (non-speech stimulus, speech response) of about 200 ms, and concludes: "It appears from these results that, allowing about 60 ms for nerve transmission and muscle action, the central reaction time for speech is of the order of 200 ms when starting up and about 115 ms in immediate response to incoming speech sounds." (p.180). However, in further experiments reported in his 1975 paper, Fry fmds that non-speech stimuli of the same duration as the speech stimulus give reaction times on this very simple task only slightly different from reaction times to the speech stimulus. The voice response takes longer, but there is no interaction between stimulus and response mode. Fry's latest estimate (pers. comm.) is that, allowing 60 ms for neural transmission, etc. the minimum central processing time is 120 ms.

Measurements of laryngeal closure

313

Can we account for the length of laryngeal closure time in terms of central reaction time? It may be argued that the glottal stop response is a less familiar one to English speakers than

that for [p t k ] ; if it is true that glottal stop is common in syllable fmal position in conjunction with /p t k tf I in the speech of many English speakers (Roach, 1973), it may be objected nevertheless that few English speakers are conscious of this fact. However, all the subjects in this experiment were phonetically trained; in addition, though the difference between times for [t] and [?] in the case of the Arabic speakers was less marked, [?] still came' ~ut as lopger. It seems, therefore, that central reaction time, as one of the variable components, is unlikely to be a crucial factor in explaining the long larynx closure time. Neural transmission from brain to muscles If it could be shown that neural transmission time from the motor cortex to the larynx muscles was considerably greater than that for transmission time to the supralaryngeal articulators we would probably have a reasonable explanation for the long larynx closure time. There has been speculation and calculation on this question by several writers. The time necessary for transmission of a neural impulse along a particular nerve fibre (the latency) can be said to be determined by the length of the nerve fibre, the type of sheath and its diameter. Krmpotic (1959) devised an index for latency of different fibres, the formula for which is: 1 I=d

where I = index, I = length of nerve fibre and d = diameter of the nerve fibre in question, and measured various nerves involved in speech production. In terms of this index, the recurrent laryngeal nerves have an unusually high value. As Lenneberg (1967) puts it: "It is interesting that the longest nerve, the recurrent, has statistically the smallest fibres which thus aggravates the timing problem .... .." (p. 96). Krmpotic's index for the recurrent laryngeal nerves is 5·83, compared with, for example, 0·77 for the posterior digastric, 1·02 for the trigeminal, 1-29 for the accessory, 1-35 for the vagus, 1-69 for the hypoglossal and 2-55 for the superior labial. Lenneberg concluded from this that "the anatomy of the nerves suggests that innervation time for intrinsic laryngeal muscles may easily be up to 30 ms longer than innervation time for muscles in and around the oral cavity." (p.96) - a figure whi.ch, if correct, would be of considerable significance for the question under discussion. This figure is, however, challenged by several writers. MacNeilage (1972) writes: "Even if one makes rather generous assumptions about conduction velocity of neurons and the distance to be travelled to the larynx and the upper articulators - namely 50 m per s in conduction velocity and 0·5 m difference between the locations, the difference in time of arrival of impulses between the two sites is only 10 ms "(p. 343). Ohala (1972) revises calculations made in an earlier work (Ohala 1970) in the light of new information. He writes: " .. ... .1 estimated that impulses sent out to the larynx and velum would arrive at the larynx some 5·5 - 9· 1 ms later at the larynx than at the velum. Subsequently I learned that Flisberg & lindholm (1970) made direct measurements of nerve impulse velocity in the human recurrent nerve and their data permits the derivation of the truer value for the delay, 5 ·3 ms ......... " (p. 203). Lenneberg's figure thus appears to be a considerable over-estimate. Muscle contraction and articulator movement Daniloff (1973) writes: "Articulator speed is largely a function of neural inervation, articulator mass, and articulator structure. For example, the tongue tip probably moves a little faster than the back of the tongue because of the tip's smaller mass; the velum rises faster than it falls because the muscles which close it are more powerful and faster acting than those which act to open it." (p.185). In the case oflaryngeal closure there seems to be

314

P. Roach

clear evidence (cf. M~rtensson & Skoglund, 1964; M~rtensson, 1968; Hirose, Ushijima, Kobyashi & Sawashima, 1969; Ohala , 1972; Sawashima, 1974) that the intrinsic laryngeal muscles have fast contraction properties; the mass of the vocal folds is not known, but it seems reasonable to suppose that it is rather smaller than that of the lips and the tongue tip, and much smaller than the tongue body. In addition, the distance to be travelled before closure to terminate an [a] vowel must be very much smaller in the case of the vocal folds than in the other articulators. All the articulatory factors , then, dispose one to expect a laryngeal closure to be accomplished considerably more rapidly than [p t k], unless some very large time delay were introduced at the second or third stage, for which there does not appear to be substantial evidence.

Time from airflow dropping below threshold to the stopping of the computer clock In this experiment, this time has been taken to be constant. If the results were to have been the result of some artefact of the measuring technique this would most probably have been the result of the airflow mask responding to cessation of airflow in different ways for different points in the vocal tract (different distances from the flow-measuring valve). It is not felt likely that this did happen, but the possibility should, of course , never be ruled out. Some very small fluctuations in time must have occurred between the moment of the aerometer output dropping below threshold and the operation of stopping the computer clock prior to reading the count of elapsed ms, due to the physical time taken to complete an analog-to-digital conversion ; these fluctuations are unlikely to have amounted to more than a small fraction of a millisecond in any particular measurement. If the measurements are not due to an artefact , the most likely explanation for the long larynx closure time is based on the supposition [supported by the views of, for example, Negus (1949), Pressman (1954) and Fink (1956)] that the vocal folds do not by themselves constitute an efficient outlet valve for preventing air-flow. If the lindqvist-Gauffin view of glottal stop articulation is correct and supraglottal constriction is used as well, the false vocal folds would be closed, preventing the passage of air much more effectively than the true folds could. Effecting this supraglottal reinforcing stricture would take time - visual observation by laryngoscopy shows it to be slow ; if contraction of the true vocal fold adductor muscles were to be inhibited until such a stricture were completely or nearly effected there could be well be a delay commensurate with the time difference being examined in this experiment. If this explanation is correct it would be strong evidence in favour of the Lindqvist-Gauffin theory. References Ardran, G. M. Kemp, F . H. & Manen, L. (195 3). Closure of the larynx. British Journal of Radiology 26,497-509. Catford, J . C. (1977). Fundamental Problems in Phonetics. Edinburgh: Edinburgh University Press. Czermak, J . N. (1861). On the Laryngoscope, and its employment in physiology and medecine. The New Sydenham Society, 11. Daniloff, R. G. (1973). Normal articulation processes. Normal Aspects of Speech, Hearing and Language, Minifie, F. D. Hixon & Williams, Ed .) Prentioe·Hall. Fink, B. R. (1956). The mechanism of closure of the human larynx. Transactions of the American of - Academy of Ophthalmology and Otolaryngology, 60, 117-127 . Flisberg, K. & Lindholm, T. (1970). Electrical stimulation of the human recurrent laryngeal nerve during thyroid operation. Acta Otolaryngologica 263, 63- 67 . Fourcin, A. J. & West, J. (1968). Larynx movement detector. Progress R eport. University College , London. Fry; D. B. (1971). Time constants in speech. Form and Substance. (Hammerich, L. L. et al. Eds) . Akademisk Forlag. Fry, D. B. (1975) . Simple reaction-times to speech and non-speech stimuli. Cortex XI, 355-360. Gauffin, J. (1977). Mechanisms of larynx tube constriction. Phonetica, 34, 307-309.

Measurements of laryngeal closure

315

Gimson, A. C. (1970). An Introduction to the Pronunciation of English. 2nd. edition. London: Edward Arnold. Heffner, R-M. S. (1950). General Phonetics. Madison; University of Wisconsin Press. Hirose, H., Ushijima, T., Kobayashi, T. & Sawashima, M. (1969). An experimental study of the contraction properties of the laryngeal muscles in the cat. Annals of Otology, Rhinology and Laryngology 78, 297-306. Hudgins, C. V. & Stetson, R. H. (1937). Relative speeds of articulatory movements. Archives ' · Ne'erlandaises de Phonetique Experimentale 13, 85-94. Jones, D. (1967). An Outline of English Phonetics. 9th edition. Cambridge: Heffer. Kim,C - W. (1970). A theory ofaspiration.Phonetica 21,107-116. Krmpotic, J. (1959) . Donnees anatomiques et histologiques relatives aux effecteurs laryngo-pharyngobuccaux. Revue de Laryngologie, d'Otologie et de Rhinologie, 80, 829-848. Lenneberg, E. H. (1967). Biological Foundations of Language. New York: Wiley. Lehiste, I. (1970) Suprasegmentals. M.I.T. Lieberman, P. (1967) . Intonation, Perception and Language. M.I.T. Lindqvist, J. (1969). Laryngeal mechanisms in speech. S. T.L. Q.P.S.R. Stockholm. 2-3, 1969, 26-32. Lindqvist, J . (1972a). A descriptive model of laryngeal articulation in speech. S. T. L. Q.P. S. R. Stockholm, 2-3, 1972, 1-9. Lindqvist, J. (1972b). Laryngeal articulations studied on Swedish subjects. S. T. L. Q.P.S.R. Stockholm, 2-3,1972,10-27. Lindqvist, J . (1972c). Laryngeal articulation in Swedish. Proceedings of the Vllth International Congress of Phonetical Sciences, (Rigault, A. & Charbonneau, R., Eds.) MacNeilage, P. F . (1972). Discussion of paper by C-W. Kim (Directionality of voicing and aspiration in initial position). Proceedings of Vllth Internatioal Congress on Phonetical Sciences, (Rigault A. & Charbonneau, R., Eds.) M~rtensson, A. (1968). The functional organization of the intrinsic laryngeal muscles. Annals of the New York Academy of Sciences 155, 91-97. M~rtensson, A. & Skoglund, C. R. (1964). Contraction properties of intrinsic laryngeal muscles. Acta Physiologica, Scandinavica, 60, 318-336. Negus, V. E. (1949). The Comparative Anatomy and Physiology of the Larynx. New York: Grune & Stratton. O'Connor, J.D. (1973). Phonetics. Harmondsworth: Pelican. Ohala, J . J. (1970) . Aspects of the control and production of speech. Working Papers in Phonetics 15. U.C.L.A. Ohala , J . J. (1972) . Some comments on the paper 'Timing Control in Speech Production 'by George Allen. Proceedings of Symposium on Temporal Aspects of Speech Production. University of Essex Department of Language and Linguistics Occasional Papers 13, 202-203. Pike, K. L. (1943). Phonetics. Michagan' Ann Arbor. Pressman, J. J. (1954). Sphincters of the larynx. A.M.A. Arch. Otolar. 59, 221-236. Priestly, T . M.S. (1976). A note on the glottal stop. Phonetica 33;268-274. Roach , P .J. (1973). Glottalization of English /p/, /t/, /k/ and /tf/; J.I.P.A . 3.1, 10-21. Robson, C. (1973). Experiment, Design and Statistics in Psychology. Harrnondsworth:Penguin . Rothenberg, M. (1968). The Breath-Stream Dynamics of Simple-Released Plosive Production. Basel: S. Karger. Rothenberg , M. (1972) . The glottal volume velocity waveform during loose and tight voiced glottal adjustments. Proceedings of Vllth International Congress of Phonetical. Sciences. (Rigault A. & Charbonneau R., Eds.) Sawashima, M. (1974). Laryngeal Research in Experimental Phonetics. Cu"ent Trends in Linguistics 12. T . Sebeok. Stuart, T . P. A. (1892). On the mechanisms of the closure of the larynx. Proceedings of the Royal Society London . 50, 323 - 329. Sweet, H. (1888). A History of English Sounds. (Extract reprinted in Henderson, E. J. A.: The Indispensable Foundation.) Zemlin, W. R. (1968). Speech and Hearing Science. Prentice-Hall.