Electroglottography - An Update.

Electroglottography (EGG) is a low-cost, noninvasive technology for measuring changes of relative vocal fold contact area during laryngeal voice production. EGG was introduced about 60 years ago and has gone through a "golden era" of increased scientific attention in the late 1980s and early 90s. During that period, four eminent review papers were written. Here, an update to these reviews is given, recapitulating some earlier landmark contributions and documenting noteworthy developments during the past 25 years. After presenting an algorithmic bibliographic analysis, some methodological aspects pertaining to measurement technology, qualitative and quantitative analysis, and respective interpretation are discussed. In particular, the interpretation of landmarks in the (first derivative of the) EGG waveform is critically examined. It is argued that because of inferior-superior and anterior-posterior phase differences of vocal fold vibration, vocal fold (de)contacting does not occur instantaneously, but over an interval of time. For this reason, instants of vocal fold closing and opening cannot be resolved exactly from the EGG signal. Consequently, any quantitative analysis parameter relying on the determination of (de)contacting events (such as the EGG contact quotient) should be interpreted with care. Finally, recent developments are reviewed for the various fields of application of EGG, including basic voice science and voice production physiology, speech signal processing and classification, clinical practice including swallowing, phonetics, hearing sciences, psychology, singing, trumpet playing, and mammalian and avian bioacoustics. Overall, EGG has over the past six decades developed into a mature technology with a wide range of applications. However, due to current limitations, the full potential of the methodology has as yet not been fully exploited. Future development may occur on three levels: (a) rigorous validation of existent measurement approaches; (b) introduction and rigorous validation of novel quantitative and interpretative approaches; and (c) advancement of the measurement technology itself.