DISCUSSION OF T H E PRECEDING PAPER

Paul Moore Department of Speech, University of Florida

Gainesville, Fla.

It is a privilege to be invited to discuss Dr. Lieberman’s paper, and I should like to call your attention initially to the excellence of the motion picture film that he presented. Those of you who have worked with high-speed photography recognize the technical problems that have been overcome in these color photo- graphs of the larynx in which exposure speeds of approximately 10,000 pictures a second have been used. Even though the film does not always reproduce the tissue color precisely, this type of film reveals many details of motion that are not observable in black and white film.

The following comments are of two general types. The first contains informa- tion from our own films that pertains directly to some of the comments made by Dr. Lieberman. The second presents related concepts of a somewhat theoretical nature that are related to some of the hypotheses presented in the original paper.

The initiation of phonation, as seen in our own films, does not always begin with a medial motion, as suggested by Dr. Lieberman. Of course, if the glottis is closed prior to vibration, the initial movements must be lateral. However, I am not referring to that type of phonation, but rather to vibration that begins when the vocal folds are abducted. The amplitude of the beginning vibratory motions is extremely small. Movement usually starts during the adduction of the folds. Consequently, the combination of the inward sweep of the gross structures and the simultaneous, minute disturbances of the glottal border make it extremely difficult to recognize and measure the direction of motion. However, some of our film leaves little doubt that the initial vibratory thrust under certain circumstances is in a lateral direction.

A second point to which reference should be made pertains to the sensory and motor regulation of phonation. Our film reveals that when phonation begins, amplitude increases rapidly in successive cycles up to a relatively stable maximum. The total period needed for this transition varies from 3 or 4 cycles to as many as 20, depending upon the nature of sound initiation. I f an individual initiates phona- tion at the pitch of a tone that he is hearing, the rate of motion from the first cycle onward will be approximately at the proper frequency. When phonation occurs under these conditions, it is apparent that the factors of elasticity, mass, etc., that determine frequency in the vocal cords have been established before vlbratlon begins.

There can be little doubt that the pressure changes heard as vocal sound are produced by pulsatile variations in the flow of air through the glottis. It is de- sirable and necessary to use an idealized model of the t y p presented by Dr. Lieberman for the construction of an hypothesis of vocal cord functlon. However, unless such models are used cautiously, their analogies may be taken to0 IIterallY and laryngeal behavior may be assumed to be simpler than it appears to be. I believe this tendency to simplify leads to incomplete theorizing. which produces Some of the inconsistencies that appear in theoretical presentations.

For balance in the discussion, I should like to stress the fact that variation in the glottal opening is the result of the motion of two somewhat independent vibratofs. For present purposes, please postulate a condition in which vibration occurs while the arytenoid cartilages are separated enough to allow the vocal folds to osclllate

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40 Annals New York Academy of Sciences without creating a closed phase in the vibratory cycle. In the normal larynx, when the vocal folds do not contact, they tend to move synchronously, because their internal conditions (elasticity, compliance, mass, and length) are comparable and because the force acting upon them exerts pressure equally on both. The signif- icance of this condition becomes apparent when it is contrasted with a larynx in which similar gross adjustments are present, but in which the elasticity or mass of one fold differs from that of the other. It is apparent that the vibrator with less resistance will be disturbed first by the air pressure, and it is also evident that the amplitude of one vibrator will be greater than that of the other.

If a second type of vibrator is assumed, in which the rest positions of the two elements places them in contact with each other, there will be a closed phase in the vibratory cycle. If the internal conditions of elasticity, mass, length, and com- pliance are comparable in the two vibrators, they will move synchronously and in phase. Furthermore, while they are in contact with each other, they are forced to function as a unit, which contributes to their synchronous motion. However, if the basic condition of one of the folds is varied as in the preceding postulate, the affected member can cause such changes in the vibratory pattern as quicker movement of one vocal fold from its position of rest, greater amplitude of one fold, asymmetrical glottal closure (i.e., contact may occur to one side or the other of the median sagittal plane, which may be accompanied by a shift of the closed glottis from one side to the other), one vibrator actually oscillating with a different frequency from the other, and the periodicity and the amplitude varying randomly from cycle to cycle. (Dr. Lieberman and others have referred to this factor in the literature.) If there is a protruding tissue mass on one of the vibra- tors, it may interfere with the motion of the opposite fold and cause both transient and regular vibrations in limited sections of the folds.

The term “compliance” has been used several times in the preceding remarks. Compliance means here the difference in elasticity between various parts of the vibrator and can be thought of as compressibility of the vocal folds. For example, the vibrators become more compressible at lower frequencies when the vocal cords are shortened or when there is generalized edema of the vocal folds. Under these conditions, the length of the closed phase is substantially increased.

The purpose of these latter comments is not to divert attention from the con- cept that it is the pulsatile train of pressure changes emanating from the larynx that create the sound. Rather, it is to encourage consideration of the fact that the variation in vocal sound may be related to the complex behavior of the larynx that results from the fact that two vibrating elements compose the sound generator. The vibration of the vocal cords, even though unpredictably variable at times. cannot be considered to he capricious. There is a reason for the be- havior. It may be related to the fact that the two vocal cords can have different vibrational characteristics that may interfere with each other and thereby create a phonatory product that reflects this interference.

DISCUSSION

p. LIEBERMAN: The later21 movements that you describe may perhaps be generated by means of a force vector component perpendicular to the long axis of the vocal cords by means of the Bernouilli effect. This could come about because the Bemmilli effect is not uniform along the entire edge of the glottis. For ex- ample. on many high-speed movies, one can see that the initial closing movement m x r s at the widest Part Of the glottis. The velocity of the air flowing through the

Moore: Discussion 41

glottis is greatest where the opening is widest because of friction effects. The Bernouilli effect, which is a function of the air velocity, thus is at its maximum at the widest part of the glottis. This means that the Bernouilli force is not uniform along the length of the vocal cords and that it therefore could generate lateral movements of the vocal cords.

J. L. FLANAGAN (Bel l Telephone Laboratorie.r, Murray Hill, N . J . ) : Can you comment on the relative magnitude of the two lateral restoring forces, i.e., the tissue force and the Bernouilli force?

DR. LIEBERMAN: In the chest register, the Bernouilli force seems to be the dom- inant one, as judged from lsshiki's studies of airflow and of driving pressure versus impedance. As subglottic pressure increases, the opening quotient appears to decrease, or, the vocal cords close more rapidly as subglottic pressure is increased. This more rapid closing can only be due to the Bernouilli force. In the falsetto register, the reverse seems to be true.

DR. FLANAGAN: Is there any evidence that laryngeal tension is analogous to that in a simple spring. or is there, in addition, an adjustment of vocal cord mass? The mechanism could go into relaxation oscillation without any tissue-restoring force.

DR. LIEBERMAN: If you stretch the cords, you change the mass constant. Both mechanisms probably are involved, as suggested by many detailed studies of vocal cord thickness and fundamental frequency of phonation. DR. FLANAGAN: This is a nice area for further experimentation.