oscillation threshold pressure. During glottal closure, the air flow is cut off until breath pressure pushes the folds apart and the flow starts up again, causing the cycles to repeat. The textbook entitled Myoelastic Aerodynamic Theory of Phonation by Ingo Titze credits Janwillem van den Berg as the originator of the theory and provides detailed mathematical development of the theory. Neurochronaxic theory This theory states that the frequency of the vocal fold vibration is determined by the chronaxie of the recurrent nerve, and not by breath pressure or muscular tension. Advocates of this theory thought that every single vibration of the vocal folds was due to an impulse from the recurrent laryngeal nerves and that the acoustic center in the brain regulated the speed of vocal fold vibration. Speech and voice scientists have long since abandoned this theory as the muscles have been shown to not be able to contract fast enough to accomplish the vibration. In addition, persons with paralyzed vocal folds can produce phonation, which would not be possible according to this theory. Phonation occurring in excised larynges would also not be possible according to this theory. State of the glottis In linguistic phonetic treatments of phonation, such as those of Peter Ladefoged, phonation was considered to be a matter of points on a continuum of tension and closure of the vocal cords. More intricate mechanisms were occasionally described, but they were difficult to investigate, and until recently the state of the glottis and phonation were considered to be nearly synonymous. If the vocal cords are completely relaxed, with the arytenoid cartilages apart for maximum airflow, the cords do not vibrate. This is voiceless phonation, and is extremely common with obstruents. If the arytenoids are pressed together for glottal closure, the vocal cords block the airstream, producing stop sounds such as the glottal stop. In between there is a sweet spot of maximum vibration. Also, the existence of an optimal glottal shape for ease of phonation has been shown, at which the lung pressure required to initiate the vocal cord vibration is minimum. This is modal voice, and is the normal state for vowels and sonorants in all the world's languages. However, the aperture of the arytenoid cartilages, and therefore the tension in the vocal cords, is one of degree between the end points of open and closed, and there are several intermediate situations utilized by various languages