The expressive pattern of laughter

Laughter as a vocal expressive-communicative signal is one of the least understood and most frequently overlooked human behaviors. The chapter provides an overview of what we know about laughter in terms of respiration, vocalization, facial action, and body movement and attempts to illustrate the mechanisms of laughter and to define its elements. The importance of discriminating between spontaneous and contrived laughter is pointed out and it is argued that unrestrained spontaneous laughter involves inarticulate vocalization. It is argued that we need research integrating the different systems in laughter including experience. Laughter is a conspicuous but frequently overlooked human phenomenon. In ontogenetic development it emerges, later than smiling, around the fourth month; however, cases of gelastic epilepsy (from Greek; gelos = laughter) among neonates demonstrate that all structures are there and functional on date of birth. Further evidence for its innateness comes from twin studies as well as from the fact that laughter is easily observable among deaf-blind children (even among deaf-blind thalidomide children, who could not "learn" laughter by touching people's faces). Man is not the only animal that laughs. Like smiling, laughter has its equivalent in the repertoire of some nonhuman primates. Beginning with Darwin (1872), many writers have been struck by the notable acoustic, orofacial, and contextual similarities between chimpanzee and human laughter. Especially among juvenile chimpanzees a "play-face" with associated vocalization was noted to accompany actions such as play, tickling, or play-biting (Preuschoft, 1995; van Hooff, 1972). 1. Laughter as an Inarticulate Utterance Laughter is estimated to be about 7 million years old (Niemitz, 1990). There is disagreement on how human speech developed phylogenetically. It could have originated from non-verbal vocal utterances, a prelingual gestural system, or sounds initially used to supplement the facial channel. However, it is safe to assume that laughter –like other utterances such as moan, sigh, cry, groan, etc.– was there before man developed speech and served as an expressive-communicative social signal. Considering the stage in the evolution of voice when laughter emerged allows us to make several deductions about the nature of the sound, its generation and the cerebral organization of laughter. If laughter was part of the human vocal repertoire before the speech centers were developed, it is likely that the sound was generated almost exclusively by laryngeal modulations, modified to some degree by supralaryngeal activity but not by articulation. This is because articulation requires voluntary control over the vocal system. Thus the production of speech sounds needs the coordination of respiration, phonation, resonance, and articulation, an analysis of laughter will involve mainly the consideration of the first three. But is it consistent with current knowledge to hypothesize that a laugh-pulse –a vocalization segment initiated by an aspirated "h" type sound followed by the utterance of one of several vowel sounds that is then abruptly terminated– is an inarticulate sound? Indeed, the /h/, an voiceless fricative glottal sound, is the only consonant produced at the level of the larynx; but as laughter is not "speech" we should not expect phonemes of a language to be uttered. Likewise, the assumption of resting articulators would suggest that the laughter "vowel" (but also the sound for other prelingual utterances, such as moaning or utterances of astonishment) is the central vowel schwa, or /e/ (as in about). For the production of this "neutral vowel" one needs to open the mouth and lower the jaw, but keep all other articulators passive. We know from everyday experience that there is a tremendous variety in the quality of the laughing sounds, so obviously deviations from the schwa occur. This does not contradict the hypothesis of an inarticulate vocalization per se but puts forward the task of defining the deviations from the resting articulation position that are due to altered emotional states and separate them from those supralaryngeal conditions modulating the sound that are due to voluntary actions. A second factor that we need to take into account is that there are different ways to generate laughing sounds and it is possible to intentionally modulate the emotion-driven spontaneous laughter. As the cerebral organization of vocal behavior progressed phylogenetically after these early facio-vocal signals became hardwired, higher centers obtained control over the laughter "timer". Thus, in addition to laughing spontaneously (emotional laughter), we can laugh voluntarily or on command (contrived or faked laughter), and we can even speak or sing the laughing sounds, which typically are phonetically represented as "ha ha ha. " These forms of utterances differ in degree of volitional control and –inversely– emotionality, and imply different neural pathways and systems involved (for reviews of neuronal control of vocalization see, for example, Jürgens, 1998; Ploog, 1986; for reviews or models of laughter: Arroyo et al., 1993; Fried et al., 1998). The distinction made between spontaneous and voluntary laughter is consistent with clinical observations; certain patients with degenerative brain disorders may be unable to move their abdomen voluntarily, but demonstrate vigorous expiratory movement of the abdomen when laughing spontaneously (Bright et al., 1986). Finally, it should be noted that there are also voluntary attempts to regulate spontaneous laughter; for sake of brevity the combination of voluntary and spontaneous in regulated laughter is only mentioned here. In spontaneous laughter we are following an impulse, an urge to laugh without restraining ourselves. There is no attempt to suppress the response or exert any control over its expression; the laughing person has been described as abandoning himself or herself to the body response (Plessner, 1941). The involuntary aspect can also be seen in the fact that during laughter our self-awareness and self-attention is diminished. Trying to direct attention during a laughter episode stops or reduces laughter. During laughter the state of consciousness is altered; as Hall and Allin (1897; p. 8-9) put it: "[t]he objective world has vanished and is forgotten, the proprieties and even the presence of others are lost, and the soul is all eye and ear to the one laughable object. Care, trouble, and even physical pain are forgotten, and the mind, as it were, falls back through unnumbered millennia and catches a glimpse of that primeval paradise where joy was intense and supreme. " While descriptions of emotional experience are rather sparse and reports of subjects mainly refer to the awareness of the impulse to laugh, spontaneous laughter is clearly enjoyable. In voluntary laughter we may want to produce a sound pattern like that of natural laughter. A typical everyday situation demanding faked laughter is when we want to signal somebody that we enjoyed a humorous message and join the laughter of others but actually do not feel any enjoyment. While voluntary laughter may pass over into involuntary laughter, we can not voluntarily produce emotional laughter. Interestingly, research has shown that contrived and spontaneous laughter within a person are strikingly similar with respect to the respirational pattern (suggesting a "laughter signature"); however, it is doubtful whether this is also the case for supralaryngeal systems. The many ways in which they differ still need to be described; this will be especially important when studying individual differences as laughter gets socialized during ontogenetic development. Research has made use of the fact that we can voluntarily alter key features, and thus studies of laughter at different vowels, pitch, and voice quality exist (Citardi et al., 1996; Habermann, 1955). Laughing on command is embarrassing to some, and thus the results obtained for contrived laughter are of limited value for describing spontaneous laughter as it may merge voluntary movements with unintended affective states. Speaking or singing "ha ha" allows modulating the sound in many ways (e.g., stretching the length of vowels, emphasizing particular syllables), just as with other spoken words. As with voluntary laughter we intend to produce a particular sound pattern and we have even more control over the outcome; the major difference being that phonation is not based on forced breathing but on well dosed air supply resulting in less tracheal resonance, breathiness, and aspiration. Spoken laugh sounds may be devoid of emotional intonation; thus there will be less variability in melody, and the utterance will be more clearly articulated and often match the phonemes of a language. Emotional laughter and speech are rather independent; in fact, laughter and speech may co-occur, thereby lengthening the duration of laugh-pulses (Nwokah et al., in press). It remains to be verified that damage of the higher speech centers has little impact on the expression of tickle-induced emotional laughter. 2. Description of Laughter Laughter is not a term used consistently, nor is it precisely defined in research articles and even encyclopedias. In everyday life a smiling face is often referred to as "laughter" although the vocal element is entirely missing. Research studies usually focus on one or two systems of laughter rather than the integration of all the components involved. For example, a study of the acoustics of laughter typically restricts the investigation to the phonatory system and may arrive at the conclusion that the duration of laughter is, say, below two seconds. Sound production is contingent on air flow and the deviation from resting breathing exceeds the rhythmic forced exhalations underlying laughter by far; therefore duration estimates of eight seconds are not uncommon in respiration studies (