- Email: [email protected]
Laryngeal adjustments or subglottal pressure? Studies on sentence stress production with excised human larynges
Journal of Phonelics (1988) 16, 349-353
Laryngeal adjustments or subglottal pressure? Studies on sentence stress production with excised human larynges 0. Aaltonen* Department of Phonetics, University of Turk u, SF-20500 Turku, Finlwzd
E. Vilkman The Phoniatric Department of the ENT Clinic of Tampere University Central Hospital, SF-33520 Tampere, Finland
and I. Raimo Department of Phonetics , University of Turku , SF-20500 Turku, Finland Received 20th October 1987, and in re visedform 15th M arch 1988
We used excised human larynges to produce F 0 changes perceived as stress in two ways: by manually rotating the cricothyroid articulation and by changing subglottal pressure. According to our results, it is possible to bring about a perceivable stress by either of these mechanisms. However, an increase in vocal fold tension also caused a marked rise in subglottal pressure, suggesting that in normal speech there is considerable interaction between these mechanisms. Rotation produced larger Fo changes than considerable pressure changes alone.
1. Introduction
The contribution of subglottal pressure and the underlying respiratory mechanism to stress production has been underlined by many researchers. Ladefoged (1961) has stated that change in F 0 is due to change in subglottal pressure. F 0 changes correlated directly with subglottal pressure changes in the recordings of the variations of subglottal pressure , rate of flow and F o . In voice physiology research, subglottal pressure variations have been observed to be a rather ineffective factor in F regulation. Ratios such as 2-5Hz: em HP (20- 50 Hz/kPa) have been reported in the chest register (Hixon, Klatt & Mead, 1971 ; Monsen, Engebretson & Vemula, 1978; Baer, 1979). Furthermore, a computer simulation of glottal vibrations has failed to show any significant dependancy between subglottal pressure and F (Titze & Talkin, 1979). Registrations of EMG activity in laryngeal musculature have given support to the opposite view, i.e. the priority of laryngeal adjustments instead of subglottal pressure in F 0 control (Hirano, Ohala & 0
0
*Author for correspondence. 0095- 4470/88/030349
+
05 $03.00/0
© !988 Academic Press Limited
0. Aaltonen et al.
350
Vennard, 1969; Collier, 1975; Atkinson, 1978). The inspection of laryngeal muscle activity and sub glottal pressure does not reveal unambiguously which of the two mechanisms is actually used by a speaker in stress production. For instance, we found a significant correlation between F 0 and subglottal pressure peaks in addition to a significant correlation between F o and peaks in the activity of the cricothyroid muscle (Vilkman, Aaltonen, Raimo, Ignatius & Komi, 1987). The present study examined F changes perceived as contrastive stress and produced by excised human larynges. In order to test the rival views independently, we simulated F 0 contours with varying stress patterns by means of vibrating excised larynges using either of these mechanisms. The use of excised larynges in voice physiological studies has a long tradition. Within certain limits, excised human larynges have been known to produce vibrations comparable to those produced by living subjects (Muller, 1837; van den Berg & Tan, 1959). 0
2. Methods
Two fresh larynges taken from autopsies were used. The examination of the specimens revealed no laryngeal diseases. Only male larynges were used because attaching considerably smaller female larynges to the apparatus is difficult. According to van den Berg & Tan (1959) there are no sex-related differences in the vibratory pattern of excised larynges. In the dissection, the extralaryngeal structures were removed. The epiglottis and ventricular folds were also dissected in order to get a better view of the vocal folds (cf. van den Berg & Tan, 1959; Lecluse, 1977). After the dissection the specimens were kept in slightly hypotonic saline (0.67%) and stored at a temperature of + 4 oc for not more than 72 h. According to van den Berg & Tan (1959) the excised larynges can be used for physiological studies for weeks if properly stored. For the experiment, the cricoid cartilage was fixed on an aery! plate just above the air-intake, using metal pins which were pushed through the cartilage. The pins were pressed against the aery! plate with metal sheets. The junction was sealed with a piece of soft foam rubber. In the second part of the experiment (pressure-induced stress) the thyroid cartilage was fixed to a U-shaped rod with blunt ended screws attached to the inferior cornua of the thyroid cartilage. The air stream used to elicit the vibrations of the vocal folds was warmed and humidified. The air stream passed through the aery! plate and under it there was a sampler for condensation water. On the side of the sampler there was an outlet for the measurement of the subglottal pressure. The air flow was monitored visually by means of a lead pearl flow meter attached to the air-intake (for further technical details, see Vilkman, 1987). In the first part of the experiment (laryngeallyinduced stress) the glottis was closed manually, and in the second part (pressure-induced stress) forceps were used. We simulated a three word sentence (/nalle meni ma:lle/) ("Teddy went to the country"), which had been used in our earlier studies on stress production with living subjects (e.g. Vilkman eta!., 1987). We tried to simulate the following sentence stress patterns: (1) no extra stress, (2) the first word stressed, (3) the second word stressed, (4) the third word stressed. The F 0 contours were produced as follows: (a) by manually changing the laryngeal adjustments only (constant air flow, approximately 300 ml/s). In this case the glottis was closed and opened manually and the intended stress was produced by rotating the thyroid cartilage. (b) by changing subglottal pressure by regulating the air flow to the subglottal space. The glottis was closed by using forceps.
Stress and excised larynges
351
The acoustic samples were tape-recorded (Tascam, Racal) by using a microphone (AKG C 567 E) attached to the frame of the apparatus. Electroglottographic signals were recorded by using an electroglottograph (Fmkjaer-Jensen EG 830) and coin-shaped electrodes attached symmetrically to the thyroid cartilage with a screw (cf. Lecluse, Brocaar & Verschuure, 1975). The electroglottographic signals were tape-recorded using both an ordinary and an instrumentation recorder (Racal). The subglottal pressure signals were obtained using a pressure meter (Fmkjaer-Jensen Manophone) connected to an outlet in the subglottal space. The pressure signal was recorded (Racal). The best 21 F 0 contours were chosen by the authors and recorded on a separate tape. The recordings were played to a panel of 20 naive listeners. Their task was to determine which words in the simulated sentences had extra stress. The instructions for the evaluation did not contain any information concerning the number of stressed "words". On the basis of this listening test, a set of four sentences, the best representatives of the four sentence types, were chosen for further analysis. The electroglottographic signal was used in the analysis of F 0 (Fmkjaer-Jensen Frequency Meter). Intensity was analysed from the acoustical signal by using an intensity meter (Fmkjaer- Jensen Intensity Meter). F 0 , intensity, and subglottal pressure curves were recorded on plotting paper (Mingograph). From the curves, the peak values of each variable were measured. The measuring points were the approximated site of the first stressed syllable of each word of the simulated three-word sentence. 3. Results Figure 1 shows an example of the F 0 , intensity, and subglottal pressure curves of one version of the sentence for both stress production strategies. The peak values of F 0 , relative intensity (maximum value = 0), and subglottal pressure for laryngeally caused stress and for stress caused by pressure changes are given in Table I. From both Fig. 1 and Table I it can be seen that the F 0 of the laryngeally-induced stress was lower than that of the flow-induced stress. This is due to a higher flow rate and consequently,
Funda.nental frequency
Hz
IZS IO(j
(b)
(a)
aa
6Q 40
zo 0
1;4
1,1 1•0
o.e
Subglottal
pre•aure kPa
-z o.z -4
1nten1it~
dB
0
-· -6
-10 -20 -30 0 Time
Figure 1. An example of fundamental frequencies, intensities and subglottal pressures for a stress pattern in which the second word has the extra stress. (a) laryngeally, (b) pressure-induced stress.
352
0 . Aaltonen et al.
TABLE I. Peak values of fundamental frequency (F relative intensity (I) and subglottal pressure (p,g) for laryngeally- and pressure-induced stress. The different versions of the simulated three-word sentence have been numbered from one to four according to their stress patterns. A letter (a = jnalle/, b = jmeni/, c = jma:lle/) is given for each "word" in the sentence. A capital letter is used when the corresponding "word" is stressed 0
),
Laryngeal
Pressure
F 0 (Hz)
I( dB)
Psg(kPa)
F 0 (Hz)
b c
67 67 67
-22 - 22 -24
0.60 0.57 0.55
115 115 115
-8 - 9 -9
1.55 1.40 1.45
2.A b c
113 57 57
- 13 -20 - 20
0.86 0.37 0.35
115 90 90
- 10 -20 - 18
1.56 0.78 0.80
3.a B c
59 77
-25 -16 -20
0.58 1.00 0.70
87 96 95
-16 -14 -15
0.80 1.00 0.95
4.a b
64 60
-22 -23 - 17
0.50 0.42 0.85
97 93 108
-14 -17 -10
1.04 0.80 1.34
I. a
c
59
77
I( dB)
Psg (kPa)
intensity, subglottal pressure and different setting the latter case. The higher flow rate was needed to produce a phonation which would resemble normal F 0 contours as much as possible in the sentences imitated. For instance, for getting an abrupt enough attack, flow values were required which exceeded the range of the flow meter (approximately 500 ml/s). However, in both cases the phonation type represented the chest register; the higher pressure values are also within physiological limits. Both for laryngeal- and pressure-induced stress, the correlations between the variables are statistically significant. However, they are only moderate in the former, but strong in the latter. The positive and negative ratios between the absolute differences of successive F 0 and subglottal pressure peaks within sentences were calculated on the basis of data presented in Table I. The changes in F o were larger in laryngeal stress than in flow-induced stress, even though the subglottal pressure was somewhat higher in flow-induced stress. For F o rise the ratios were approximately 64 Hz/kPa in laryngeally produced cases and 27 Hz/kPa in flow-induced cases. For F 0 decrease, the corresponding ratios were 42Hz/ kPa and 33 Hz/kPa, respectively. The listeners perceived stress almost unanimously , irrespective of whether it was produced by changing the laryngeal adjustments or by changing the subglottal pressure. 4. Discussion According to the results, F 0 peaks perceived as stress can be produced with excised larynges by means of both laryngeal and pressure adjustments. From the physiological point of view, the findings in the present study are well in line with earlier findings of physiological research. The increase in glottal resistance due to cricothyroid muscle twitch has been reported before in living subjects (Yanagihara & von Leden, 1968) and with excised larynges (e.g. Vilkman, 1987). The explanation to this is obviously the increased folding of the vocal folds when the cricothyroid muscle twitches and the
Stress and excised larynges
353
thyroid and cricoid cartilages approximate through rotation about a horizontally directed axis of the cricothyroid articulation (e.g. Fink & Demarest, 1978). The ratio of the Fa and subglottal pressure change which caused the change in Fa was also in the limits of earlier findings concerning chest voice phonation (e.g. Baer, 1979). Yet the origins of the effect of subglottal pressure changes on Fa are not obvious. They can be thought to be entirely the result of laryngeal changes (Titze & Talkin, 1979). It has been suggested that the result of studies on voice production should be interpreted in terms of systems physiology because there are considerable interactions between the contributing subcomponents (e.g. Muller, Abbs & Kennedy, 1981). An example of such interactions is that increased vocal fold tension causes a marked rise in subglottal pressure because of changes in glottal resistance. Consequently, the dichotomy between the physiological background mechanisms of stress is not justified. We prefer to interpret the regulation of Fa variations reflecting stress as a complex interactive system. References Atkinson, J. (!978) Correlation analysis of the physiological factors controlling fundamental voice frequency, Journal of the Acoustical Society of America. 63, 211 - 222. Baer, T. (1979) Reflex activation of laryngeal muscles by sudden induced subglottal pressure changes, Journal of the Acoustical Society of America. 65, 1271-5. van den Berg, J. & Tan, T. (!959) Results of experiments with human larynges, Practica Oto-rhinolaryngologica. 21, 425-450. Collier, R. (1975) Physiological correlates of intonation patterns, Journal of the Acoustical Society of America, 58, 249-255. Fink, B. & Demarest, R. (!978) Laryngeal biomechanics. Cambridge: Harvard University Press. Hirano, M. , Ohala, J. & Vennard, W. (1969) The function of the laryngeal muscles in regulating fundamental frequency and intensity of phonation, Journal of Speech and Hearing Research, 12, 616-628. Hixon, T. , Klatt, D. & Mead, J. (1971) Influence of forced transglottal pressure on fundamental frequency, Journal of the Acoustical Society of America, 49 (Suppl.), 105. Ladefoged, P. (1962) Subglottal activity during speech. In Proceedings of the fourth international congress of phonetic sciences (A. Sovijarvi & P. Aalto, Eds), pp. 73-99. The Hague: Mouton. Lecluse, F. (1977) Electroglottografie. Unpublished dissertation. Erasmus University , Rotterdam. Lecluse, F., Brocaar, M. & Verschuure, J. (!975) The electrography and its relation to glottal activity, Folia Phoniatrica, 27, 215-224. Monsen, R., Engebretson, A. & Vemula, N. (1978) Indirect assessment of the contribution of the subglottal pressure and vocal-fold tension to changes of fundamental frequency in English, Journal of the Acoustical Society of America, 64, 65-80. Miiller, J. (1837) Handbuch der Physiologie des Menschen. Teill. Koblenz: Hiilsner. Miiller, E., Abbs, J. & Kennedy, J. (!981) Some systems physiology considerations for vocal control. In Vocal fold physiology (K. Stevens & M. Hirano, Eds), pp. 209-227. Tokyo: University of Tokyo Press. Titze, I. & Talkin, D. (1979) A theoretical study of the effects of various laryngeal configurations on the acoustics of phonation, Journal of the Acoustical Society of America, 66, 60-74. Vilkman, E. , Aaltonen, 0., Raimo, 1., Ignatius, J. & Komi , P. (1987) On stress production in whispered Finnish, Journal of Phonetics, 15, 157-168. Vilkman, E. (!987) An apparatus for studying the role of the cricothyroid articulation in voice production of human excised larynges, Folia Phoniatrica, 39, 169-77. Yanagihara, N. & von Leden, H. (!968) The cricothyroid muscle during phonation. Annals of Otology, Rhinology and Laryngology, 75, 987- 1007.
Laryngeal adjustments or subglottal pressure? Studies on sentence stress production with excised human larynges 0. Aaltonen* Department of Phonetics, University of Turk u, SF-20500 Turku, Finlwzd
E. Vilkman The Phoniatric Department of the ENT Clinic of Tampere University Central Hospital, SF-33520 Tampere, Finland
and I. Raimo Department of Phonetics , University of Turku , SF-20500 Turku, Finland Received 20th October 1987, and in re visedform 15th M arch 1988
We used excised human larynges to produce F 0 changes perceived as stress in two ways: by manually rotating the cricothyroid articulation and by changing subglottal pressure. According to our results, it is possible to bring about a perceivable stress by either of these mechanisms. However, an increase in vocal fold tension also caused a marked rise in subglottal pressure, suggesting that in normal speech there is considerable interaction between these mechanisms. Rotation produced larger Fo changes than considerable pressure changes alone.
1. Introduction
The contribution of subglottal pressure and the underlying respiratory mechanism to stress production has been underlined by many researchers. Ladefoged (1961) has stated that change in F 0 is due to change in subglottal pressure. F 0 changes correlated directly with subglottal pressure changes in the recordings of the variations of subglottal pressure , rate of flow and F o . In voice physiology research, subglottal pressure variations have been observed to be a rather ineffective factor in F regulation. Ratios such as 2-5Hz: em HP (20- 50 Hz/kPa) have been reported in the chest register (Hixon, Klatt & Mead, 1971 ; Monsen, Engebretson & Vemula, 1978; Baer, 1979). Furthermore, a computer simulation of glottal vibrations has failed to show any significant dependancy between subglottal pressure and F (Titze & Talkin, 1979). Registrations of EMG activity in laryngeal musculature have given support to the opposite view, i.e. the priority of laryngeal adjustments instead of subglottal pressure in F 0 control (Hirano, Ohala & 0
0
*Author for correspondence. 0095- 4470/88/030349
+
05 $03.00/0
© !988 Academic Press Limited
0. Aaltonen et al.
350
Vennard, 1969; Collier, 1975; Atkinson, 1978). The inspection of laryngeal muscle activity and sub glottal pressure does not reveal unambiguously which of the two mechanisms is actually used by a speaker in stress production. For instance, we found a significant correlation between F 0 and subglottal pressure peaks in addition to a significant correlation between F o and peaks in the activity of the cricothyroid muscle (Vilkman, Aaltonen, Raimo, Ignatius & Komi, 1987). The present study examined F changes perceived as contrastive stress and produced by excised human larynges. In order to test the rival views independently, we simulated F 0 contours with varying stress patterns by means of vibrating excised larynges using either of these mechanisms. The use of excised larynges in voice physiological studies has a long tradition. Within certain limits, excised human larynges have been known to produce vibrations comparable to those produced by living subjects (Muller, 1837; van den Berg & Tan, 1959). 0
2. Methods
Two fresh larynges taken from autopsies were used. The examination of the specimens revealed no laryngeal diseases. Only male larynges were used because attaching considerably smaller female larynges to the apparatus is difficult. According to van den Berg & Tan (1959) there are no sex-related differences in the vibratory pattern of excised larynges. In the dissection, the extralaryngeal structures were removed. The epiglottis and ventricular folds were also dissected in order to get a better view of the vocal folds (cf. van den Berg & Tan, 1959; Lecluse, 1977). After the dissection the specimens were kept in slightly hypotonic saline (0.67%) and stored at a temperature of + 4 oc for not more than 72 h. According to van den Berg & Tan (1959) the excised larynges can be used for physiological studies for weeks if properly stored. For the experiment, the cricoid cartilage was fixed on an aery! plate just above the air-intake, using metal pins which were pushed through the cartilage. The pins were pressed against the aery! plate with metal sheets. The junction was sealed with a piece of soft foam rubber. In the second part of the experiment (pressure-induced stress) the thyroid cartilage was fixed to a U-shaped rod with blunt ended screws attached to the inferior cornua of the thyroid cartilage. The air stream used to elicit the vibrations of the vocal folds was warmed and humidified. The air stream passed through the aery! plate and under it there was a sampler for condensation water. On the side of the sampler there was an outlet for the measurement of the subglottal pressure. The air flow was monitored visually by means of a lead pearl flow meter attached to the air-intake (for further technical details, see Vilkman, 1987). In the first part of the experiment (laryngeallyinduced stress) the glottis was closed manually, and in the second part (pressure-induced stress) forceps were used. We simulated a three word sentence (/nalle meni ma:lle/) ("Teddy went to the country"), which had been used in our earlier studies on stress production with living subjects (e.g. Vilkman eta!., 1987). We tried to simulate the following sentence stress patterns: (1) no extra stress, (2) the first word stressed, (3) the second word stressed, (4) the third word stressed. The F 0 contours were produced as follows: (a) by manually changing the laryngeal adjustments only (constant air flow, approximately 300 ml/s). In this case the glottis was closed and opened manually and the intended stress was produced by rotating the thyroid cartilage. (b) by changing subglottal pressure by regulating the air flow to the subglottal space. The glottis was closed by using forceps.
Stress and excised larynges
351
The acoustic samples were tape-recorded (Tascam, Racal) by using a microphone (AKG C 567 E) attached to the frame of the apparatus. Electroglottographic signals were recorded by using an electroglottograph (Fmkjaer-Jensen EG 830) and coin-shaped electrodes attached symmetrically to the thyroid cartilage with a screw (cf. Lecluse, Brocaar & Verschuure, 1975). The electroglottographic signals were tape-recorded using both an ordinary and an instrumentation recorder (Racal). The subglottal pressure signals were obtained using a pressure meter (Fmkjaer-Jensen Manophone) connected to an outlet in the subglottal space. The pressure signal was recorded (Racal). The best 21 F 0 contours were chosen by the authors and recorded on a separate tape. The recordings were played to a panel of 20 naive listeners. Their task was to determine which words in the simulated sentences had extra stress. The instructions for the evaluation did not contain any information concerning the number of stressed "words". On the basis of this listening test, a set of four sentences, the best representatives of the four sentence types, were chosen for further analysis. The electroglottographic signal was used in the analysis of F 0 (Fmkjaer-Jensen Frequency Meter). Intensity was analysed from the acoustical signal by using an intensity meter (Fmkjaer- Jensen Intensity Meter). F 0 , intensity, and subglottal pressure curves were recorded on plotting paper (Mingograph). From the curves, the peak values of each variable were measured. The measuring points were the approximated site of the first stressed syllable of each word of the simulated three-word sentence. 3. Results Figure 1 shows an example of the F 0 , intensity, and subglottal pressure curves of one version of the sentence for both stress production strategies. The peak values of F 0 , relative intensity (maximum value = 0), and subglottal pressure for laryngeally caused stress and for stress caused by pressure changes are given in Table I. From both Fig. 1 and Table I it can be seen that the F 0 of the laryngeally-induced stress was lower than that of the flow-induced stress. This is due to a higher flow rate and consequently,
Funda.nental frequency
Hz
IZS IO(j
(b)
(a)
aa
6Q 40
zo 0
1;4
1,1 1•0
o.e
Subglottal
pre•aure kPa
-z o.z -4
1nten1it~
dB
0
-· -6
-10 -20 -30 0 Time
Figure 1. An example of fundamental frequencies, intensities and subglottal pressures for a stress pattern in which the second word has the extra stress. (a) laryngeally, (b) pressure-induced stress.
352
0 . Aaltonen et al.
TABLE I. Peak values of fundamental frequency (F relative intensity (I) and subglottal pressure (p,g) for laryngeally- and pressure-induced stress. The different versions of the simulated three-word sentence have been numbered from one to four according to their stress patterns. A letter (a = jnalle/, b = jmeni/, c = jma:lle/) is given for each "word" in the sentence. A capital letter is used when the corresponding "word" is stressed 0
),
Laryngeal
Pressure
F 0 (Hz)
I( dB)
Psg(kPa)
F 0 (Hz)
b c
67 67 67
-22 - 22 -24
0.60 0.57 0.55
115 115 115
-8 - 9 -9
1.55 1.40 1.45
2.A b c
113 57 57
- 13 -20 - 20
0.86 0.37 0.35
115 90 90
- 10 -20 - 18
1.56 0.78 0.80
3.a B c
59 77
-25 -16 -20
0.58 1.00 0.70
87 96 95
-16 -14 -15
0.80 1.00 0.95
4.a b
64 60
-22 -23 - 17
0.50 0.42 0.85
97 93 108
-14 -17 -10
1.04 0.80 1.34
I. a
c
59
77
I( dB)
Psg (kPa)
intensity, subglottal pressure and different setting the latter case. The higher flow rate was needed to produce a phonation which would resemble normal F 0 contours as much as possible in the sentences imitated. For instance, for getting an abrupt enough attack, flow values were required which exceeded the range of the flow meter (approximately 500 ml/s). However, in both cases the phonation type represented the chest register; the higher pressure values are also within physiological limits. Both for laryngeal- and pressure-induced stress, the correlations between the variables are statistically significant. However, they are only moderate in the former, but strong in the latter. The positive and negative ratios between the absolute differences of successive F 0 and subglottal pressure peaks within sentences were calculated on the basis of data presented in Table I. The changes in F o were larger in laryngeal stress than in flow-induced stress, even though the subglottal pressure was somewhat higher in flow-induced stress. For F o rise the ratios were approximately 64 Hz/kPa in laryngeally produced cases and 27 Hz/kPa in flow-induced cases. For F 0 decrease, the corresponding ratios were 42Hz/ kPa and 33 Hz/kPa, respectively. The listeners perceived stress almost unanimously , irrespective of whether it was produced by changing the laryngeal adjustments or by changing the subglottal pressure. 4. Discussion According to the results, F 0 peaks perceived as stress can be produced with excised larynges by means of both laryngeal and pressure adjustments. From the physiological point of view, the findings in the present study are well in line with earlier findings of physiological research. The increase in glottal resistance due to cricothyroid muscle twitch has been reported before in living subjects (Yanagihara & von Leden, 1968) and with excised larynges (e.g. Vilkman, 1987). The explanation to this is obviously the increased folding of the vocal folds when the cricothyroid muscle twitches and the
Stress and excised larynges
353
thyroid and cricoid cartilages approximate through rotation about a horizontally directed axis of the cricothyroid articulation (e.g. Fink & Demarest, 1978). The ratio of the Fa and subglottal pressure change which caused the change in Fa was also in the limits of earlier findings concerning chest voice phonation (e.g. Baer, 1979). Yet the origins of the effect of subglottal pressure changes on Fa are not obvious. They can be thought to be entirely the result of laryngeal changes (Titze & Talkin, 1979). It has been suggested that the result of studies on voice production should be interpreted in terms of systems physiology because there are considerable interactions between the contributing subcomponents (e.g. Muller, Abbs & Kennedy, 1981). An example of such interactions is that increased vocal fold tension causes a marked rise in subglottal pressure because of changes in glottal resistance. Consequently, the dichotomy between the physiological background mechanisms of stress is not justified. We prefer to interpret the regulation of Fa variations reflecting stress as a complex interactive system. References Atkinson, J. (!978) Correlation analysis of the physiological factors controlling fundamental voice frequency, Journal of the Acoustical Society of America. 63, 211 - 222. Baer, T. (1979) Reflex activation of laryngeal muscles by sudden induced subglottal pressure changes, Journal of the Acoustical Society of America. 65, 1271-5. van den Berg, J. & Tan, T. (!959) Results of experiments with human larynges, Practica Oto-rhinolaryngologica. 21, 425-450. Collier, R. (1975) Physiological correlates of intonation patterns, Journal of the Acoustical Society of America, 58, 249-255. Fink, B. & Demarest, R. (!978) Laryngeal biomechanics. Cambridge: Harvard University Press. Hirano, M. , Ohala, J. & Vennard, W. (1969) The function of the laryngeal muscles in regulating fundamental frequency and intensity of phonation, Journal of Speech and Hearing Research, 12, 616-628. Hixon, T. , Klatt, D. & Mead, J. (1971) Influence of forced transglottal pressure on fundamental frequency, Journal of the Acoustical Society of America, 49 (Suppl.), 105. Ladefoged, P. (1962) Subglottal activity during speech. In Proceedings of the fourth international congress of phonetic sciences (A. Sovijarvi & P. Aalto, Eds), pp. 73-99. The Hague: Mouton. Lecluse, F. (1977) Electroglottografie. Unpublished dissertation. Erasmus University , Rotterdam. Lecluse, F., Brocaar, M. & Verschuure, J. (!975) The electrography and its relation to glottal activity, Folia Phoniatrica, 27, 215-224. Monsen, R., Engebretson, A. & Vemula, N. (1978) Indirect assessment of the contribution of the subglottal pressure and vocal-fold tension to changes of fundamental frequency in English, Journal of the Acoustical Society of America, 64, 65-80. Miiller, J. (1837) Handbuch der Physiologie des Menschen. Teill. Koblenz: Hiilsner. Miiller, E., Abbs, J. & Kennedy, J. (!981) Some systems physiology considerations for vocal control. In Vocal fold physiology (K. Stevens & M. Hirano, Eds), pp. 209-227. Tokyo: University of Tokyo Press. Titze, I. & Talkin, D. (1979) A theoretical study of the effects of various laryngeal configurations on the acoustics of phonation, Journal of the Acoustical Society of America, 66, 60-74. Vilkman, E. , Aaltonen, 0., Raimo, 1., Ignatius, J. & Komi , P. (1987) On stress production in whispered Finnish, Journal of Phonetics, 15, 157-168. Vilkman, E. (!987) An apparatus for studying the role of the cricothyroid articulation in voice production of human excised larynges, Folia Phoniatrica, 39, 169-77. Yanagihara, N. & von Leden, H. (!968) The cricothyroid muscle during phonation. Annals of Otology, Rhinology and Laryngology, 75, 987- 1007.