Characterization of features observed on the derivative of electroglottographic signal by the use of high-speed cinematography

International audienceThe electroglottography (EGG) is a well-known and commonly used non-invasive method for<br />measuring vocal folds contact area. However, few studies have paid attention to its derivative,<br />although the differentiated EGG signal (DEGG) helps to enhance sudden changes in the vocal<br />folds contact during the opening and closing phases. During the production of voiced sounds,<br />the DEGG signal presents strong peaks, which can accurately be related to the glottal closing<br />instants defined as the instants of termination of glottal area variation, and weak peaks of<br />opposite sign, which can be related to the glottal opening instants defined as the instants of<br />initialization of glottal area variation (Childers et al., 1990). As glottal closing is usually<br />abrupt, the closing peaks are often very strong and precise, whereas the opening peaks are<br />often weaker and less precise, a fact that leads to some reservations on their usefulness for<br />glottal opening instant detection (Baken, 1992). The DEGG peaks can be either single, or<br />double, or imprecise (Henrich et al., 2004). In particular, the double peak feature is not<br />uncommon, for opening as well as for closing, and it may be found in specific cases: for a<br />given singer, it can be consistently associated with either soft or loud production, and/or with<br />either low or high pitches (Henrich et al., 2004). These peaks may offer visual clues to some<br />characteristic features of the vocal folds vibratory movement. As an example, Henrich et al.<br />observed that the double peak feature could occur during a laryngeal mechanism transition,<br />indicating that there may be some slower adjustments in the vocal fold contact process, even<br />when the transition appears to be sudden when looking at the EGG amplitude change.<br />These observations call for research combining electroglottography with some kind of<br />visualization. The purpose of the present study is to characterize some DEGG features using<br />high-speed cinematography. The questions addressed by the study are the following ones: is<br />the peak doubling feature related to a typical vibratory movement ? Can the imprecision<br />sometimes observed in the case of DEGG opening peaks be explained in terms of glottal<br />abduction over the length of the vocal folds ? What is the relation between the glottal<br />vibratory movement and the amplitude of the DEGG signal ?<br />So as to answer these questions, high-speed images and EGG signals have been recorded<br />simultaneously in the case of various voiced production. The recording session took place in<br />the Universitätsklinikum Hamburg-Eppendorf (Phoniatrie und Pädaudiologie, Pr. Dr. Hess).<br />Two French male subjects participated in the study : a male speaker (26 years old) and a<br />professional counter tenor singer (40 years old). Both subjects were asked to produce<br />sustained sounds with different voice qualities (modal, creaky, breathy, tensed, harsh). Prior<br />to each production, a sound example was played to the subject and the pitch was kept constant<br />when possible. In addition, the professional singer was asked to sing sustained voiced sound<br />on various pitches and using different voice registers (modal, produced in laryngeal<br />mechanism M1; falsetto, produced in laryngeal mechanism M2; « voix mixte », produced<br />either in M1 or M2), and glissandos with or without a noticeable voice break. The high-speed<br />images and the EGG signal (Laryngograph Ltd London) were recorded simultaneously on a<br />computer, using the high-speed camera unit Wolf HS Endocom 5560. The high-speed images<br />of size 265*128 pixels were sampled at 4000 Hz and stored in binary files. The EGG signal<br />was sampled at 44170 Hz, coded on 8 bits and stored in wav files. The data were processed<br />using Matlab. The EGG signal was differentiated, and both EGG and DEGG signals were<br />synchronized with the high-speed images. The observed oscillation patterns were<br />characterized with the help of multiline kymograms. Measurements of glottal area, anteroposterior<br />and transversal distances were obtained by applying an image processing algorithm<br />based on the detection of contrast (light energy threshold).<br />The relation between the DEGG closing and opening peaks and the instants of initialization<br />and termination of glottal area variation is explored to assess the validity of the previous<br />findings on this extended database. Especially, much attention is given to the opening peaks,<br />which seem to be dependent on the horizontal localization of initial vocal fold opening<br />(anterior, midmembranous, or posterior). The midmembranous-to-anterior part of the glottis<br />seems to play an important role in the precision of these peaks, together with the degree of<br />interarytenoid contact. The relation between the closing peak amplitude and the glottal<br />vibratory amplitude is also explored