i 7' 4 



II WDHOuk Ml- PHYSIOLOGY 



NLl'KOI'IIYSIol.OGY III 



respiration also appears to depend in sonic measure 

 on the rate of speech, with a predominance of ab- 

 dominal movements at the slower rates (l22, 123). 

 Recent studies of breathing activity in conversational 

 speech suggest that both the rate of respiration during 

 speech and the output of speech per expiration are 

 relatively consistent in the same subject. Both indices 

 are however markedly affected by emotion (41, 42). 



/ he I "i "I Cords 



The appearance of the vocal cords in action has 

 been studied bv inspection (direct and stereoscopic), 

 by high-speed motion photography and by the use of 

 various stroboscopic procedures. The earlier results 

 have been reviewed by Kerridge (67); the more 

 recent, by Fletcher (37). Broadly, the results may be 

 said to show that laryngeal movements in speech are 

 decidedly complex and by no means wholly limited 

 to the vocal cords themselves. Thus the false vocal 

 cords are more active than was formerly supposed. 

 The vibration of the cords has both periodic and 

 aperiodic elements, and both cords do not necessarily 

 vibrate at the same rate. Some aspects of vocal cord 

 activity have also been clarified by the use of models 



(67)- 



In recenl years, the use of high-speed photography 

 has permitted more detailed analysis of vocal cord 

 activitv 111, >fi, 37). Thus, in a study by Bracked ( 1 1 I, 

 the action of the cords was analyzed by Studying the 

 glottal openings during the cycle of vibration, the 

 phase" relationship to the cycle, the adjustment of the 

 superior laryngeal structures and the lengths of the 

 glottis on successive frames. With a subject phonating 

 at 256 cps, the cycle of cord vibration showed a 

 phase relationship of one-third closed, one-third 

 opening and one-third closing. There was also 

 evidence that glottal length for a given subject 

 appeared to vat) at comparable frequencies and 

 intensities oi voice. In general, the results indicate 

 that the movement of the cords is most complex at 

 low frequencies, becoming simpler as the tone is 

 raised until at extremel) high frequencies only the 

 edges ol the cord adjacent to the glottis are seen to 

 vibrate (37). I here is also evidence that the fraction 

 oi the cycle time during which the cords remain 

 closed decreases with increasing pitch and thai 



closure of the cords is firmer and more prolonged 



with increasing iniensiiv of voice Differences in 

 glottal length, glottal area and in proportion of time 

 allocated to the different phases oi the cycle appeal to 



be related to type of voice production and to effects 

 of voice training. 



Although the results of these studies have in general 

 supported the traditional theory of vocal cord action, 

 the work of Husson & Dijan (62) might suggest that 

 this tlieorv stands in need of revision. These workers 

 claim that the resonating structures superior to the 

 larynx act essentially by providing an impedance to 

 vocal cord vibration, and they adduce radiological 

 evidence in support of this view. The findings are 

 brought into relation with Husson's theory of direct 

 neurological control of phonation. 



Neurophysiology oj Phonation 



Until recently, it has been generally supposed that 

 the frequency of vocal cord vibration is determined 

 by purely mechanical factors operating at the level of 

 the glottis, e.g. the width of the rima glottidis and the 

 tension and elasticity of the cords. Work by Husson 

 and his associates (40, 58-62, 73, 95, 108, 135) has 

 however suggested that it may prove more correct to 

 regard the larynx as an integrated neuromuscular 

 effector system. The argument is based principally on 

 an experiment by Laget (73) in which vocal cord 

 vibration was successfully induced in the dog by 

 electrical stimulation of the recurrent laryngeal nerve 

 independently of the passage of air through the 

 pharynx, i.e. in the absence of phonation as conven- 

 tionally defined. It is reported that the stimulated 

 cord approximated suddenly to iis fellow, irrespective 

 of its prior position. With stimulus frequencies up to 

 400 per sec, the rhythm of vocal cord response was 

 found to follow th.it of the stimulus, although its 

 amplitude varied in a complex fashion with both 

 frequency and intensity. With frequencies above 400 

 per sec, vocal cord responses were vrrv small and 

 occurred onlv with high-stimulus intensities. At the 

 same time, Husson and his associates suggesl that 

 higher rales ol vocal cord vibration may be effected by 



inc. uis of a diphasic (or even triphasic) response 

 mechanism comparable 10 that postulated bv Stevens 



& Davis in the case of the cochlear nerve 1 u v 



Application til ihesc findings to human vocal per- 

 formance has been attempted (40, 99 



Although the idea ol the larynx as .1 neuroeffector 

 svsicin integrated .11 bulbar, diencephalic and conical 



levels is .m interesting one, ii cannot be said at 

 present 10 repose on secure foundations. Repetition ol 



the basic experiments willi improved recording 



technique and more' adequate controls is necessar) 



before linn conclusions can be drawn. 



