from the inferior portions of the vocal fold cover to the
superior (a summary of vocal fold observation techniques
can be found in Moore [4]). This wave is typically referred
to as the mucosal wave or vertical phase difference and is
most clearly observed in the coronal plane. Vocal fold
motion in this plane (along the axis of airflow) was first
viewed with X-ray techniques [5] and more recently with
observations of excised hemilaryngeal vibration using
videostroboscopy [6] and high-speed video [7]. It is also
noteworthy that Chiba and Kajiyama [8] used a stroboscopic
technique to obtain motion pictures of the vocal
folds during vibration. They did not specifically mention
the mucosal wave but perhaps allude to it in their
observation on page 24 that,‘‘ it seems that every small
portion constituting the vocal chords is set in elliptical
motion in vertical cross-section In this way,the vocal
chords as a whole come to be set in wave motion.’’
Figure 4 shows an idealized cycle of vocal fold
vibration in the coronal plane. In the first frame (Fig. 4a)
the vocal folds on the left and right sides are initially in
contact and the airway is closed. The next frame (Fig. 4b)
indicates a lateral movement of the inferior portion of
cover surface which continues until the left and right sides
separate (Fig. 4c) thereby opening the airway. Once the
maximum lateral displacement is achieved (Fig. 4d),the
lower portion of the vocal folds begins to move medially
with the upper portion following (Fig. 4e). Eventually,the
lower portions on the left and right sides collide with each
other and again close the airway (Fig. 4f). Medial
displacement continues as the upper portions also collide
(Fig. 4g). The entire process then repeats itself cyclically at
the fundamental frequency of vibration (F0).
Note that the lateral displacement of the upper (superior)
portion of each vocal fold is not in phase with the
lower portion. That is,the lower part of the vocal fold leads
Fig.