A Short Introduction to Laughter ∗

The human speech production system is composed of the lungs, trachea, larynx, nasal cavity and oral cavity as shown in Fig. 1. Speech is simply a sound wave originating from the lungs. Air coming out of the lungs first passes through the trachea and then through the vocal tract. The shape of the vocal tract determines the speech sound produced. The position of the lips, tongue, velum, teeth and jaw (the articulators) define the shape of the vocal tract and therefore can influence the produced sound [65]. The nasal tract is usually closed but it can open when the velum is lowered. A key component in the production of speech is the vocal cords, which are located at the top of the trachea, and define if a sound is voiced or unvoiced. If the vocal cords vibrate when air flows through the trachea then the produced sound is voiced and the rate of the vibration determines the fundamental frequency of the sound. In case they do not vibrate then the produced sound is unvoiced since an aperiodic noise-like sound is produced. Laughter is produced by the same mechanism as speech but there is an important difference between them, speech is articulated but laughter is not. Bachorowski et al. [8] has shown that laughter mainly consists of central sounds and it is not articulated. This agrees with the suggestion by Ruch and Ekman [70] that articulation requires voluntary control over the vocal system which is not present during spontaneous laughter.