Saturday, April 25, 2009

Part III Biological Mechanisms of Human Language

Vocal Anatomy

In general, human language involves three main components: auditory mechanisms, articulatory mechanisms, and a brain which coordinates the proceedings (Aitchison 1998:19). The basic properties of the ear are common to humans and nonhuman animals (Loritz 1999). In addition, humans and nonhuman primates can reliable distinguish more sounds than they can produce (ibid.; Aitchison 1998).

Human speech is the result of the action of three subsystems: respiratory, phonatory and articulatory (MacNeilage 1998:223). In all mammals, the first two of these subsystems operate as modulated cycles. The expiratory phase of the respiratory system is modulated to produce a power source for phonation (ibid.). At the phonatory level, the vocal fold vibration cycle is modulated to produce variations in fundamental frequency, heard as pitch variations (ibid.). From a physical standpoint, sound production involves the production of syllabic vocal material in a repetitive and rhythmic fashion (Locke 1998:191). This is done by raising and lowering the mandible in particular ways while phonating. The lips and tongue may be actively or passively positioned to produce various points of constriction (ibid.). MacNeilage (1998:501) has proposed that the evolutionary precursor of syllabic structure was the mandibular oscillation associated with chewing and sucking, which provides the ‘frame’ onto which the ‘content’ of specific phonemes is superimposed.

The human vocal tract anatomy differs from that of other primates. Around three months of age, the larynx begins a slow descent to its lower (adult) position, which it reaches between the ages of three to seven years (Lieberman 1998; Arensburg 1994:279). A second decent occurs in human males at puberty (ibid.). This change in larynx position enables the tongue to move both vertically and horizontally within the vocal tract, and is credited with expanding the phonetic repertoire of humans (Studdert-Kennedy 1998:208; Lieberman 1998; Lieberman and McCarthy 1999:489).

Lieberman (1998) has suggested that slight laryngeal lowering could be adaptive for “mouth breathing during extreme physical challenge, probably starting with Homo erectus”. The selection of Homo erectus (approximately 1.8 mya - 300 kya) is based upon associated cranial expansion, tool modifications and migration outside of the African continent (Tattersall and Schwartz 2000). However, there is no evidence to suggest that this physiological change could not have occurred in the Australopithecine lineages. Additionally, many mammals mouth breath when under stress or for cooling by panting, without requiring permanent larynx lowering (MacLarnon and Hewitt 1999; Fitch 2000:263).

Lieberman and McCarthy (1999) have also proposed that the lowered larynx was a by-product of upright bipedal locomotion. However, no other bipedal species has a lowered larynx (i.e. kangaroos). Another difference is that the vocal tracts of humans lack of laryngeal air sacs (Fitch 2000:263). All of the great apes, and many other nonhuman primates, have inflatable, soft walled air sacs that extend outward from the larynx and beneath the skin of the neck and thorax (Schôn Ybarra 1995:186). These sacs can hold up to 6 litres of air and are presumed to serve a vocal function, although their acoustic or adaptive significance is uncertain (ibid.).

Humans also have the ability to suppress vocal reactions, which in other primates are largely automatic (Atichison 1998:21). Finally, due to a lowered larynx and associated vocal tract lengthening, modern humans are capable of vocal mimicry (Fitch 2000:263; Atichison 1998; Donald. 1998:47). The ability to mimic other animals or natural sounds has been suggested as a possible source of spoken language (Beaken 1996:104). For an imitation to be a communicative sign, the attention of both a ‘sender’ and ‘receiver’ of the information is necessary (ibid.). According to Fitch (2000:264), the evolution of vocal imitation in the auditory domain is much easier than other forms of imitation. It is presumed that no nonhuman primate is capable of producing sounds outside of their species specific repertoire (Janik and Slater 2000:2; Snowdon 1990:216). An animal with a lowered larynx has the capacity to duplicate the vocalizations of a larger animal, thus exaggerating the impression of size conveyed by its vocalizations (Fitch 2000:263). However, evidence of vocal mimicry/learning exists in aquatic mammals (cetaceans) and certain avian species (Janik and Slater 2000:2; Hauser 1996:310; Snowdon 1990:216). Additionally, many bird species have an elongated trachea, which in turn elongates the vocal tract (Fitch 2000:264). This serves to lower the vocal frequencies and thus, exaggerate the impression of body size (ibid.)