All humans can acquire at least one natural language. Biolinguistics is the name given to the interdisciplinary enterprise that aims to unveil the biological bases of this unique capacity.
Cedric Boeckx and Pedro Tiago Martins
Sign phonetics is the study of how sign languages are produced and perceived, by native as well as by non-native signers. Most research on sign phonetics has focused on American Sign Language (ASL), but there are many different sign languages around the world, and several of these, including British Sign Language, Taiwan Sign Language, and Sign Language of the Netherlands, have been studied at the level of phonetics. Sign phonetics research can focus on individual lexical signs or on the movements of the nonmanual articulators that accompany those signs. The production and perception of a sign language can be influenced by phrase structure, linguistic register, the signer’s linguistic background, the visual perception mechanism, the anatomy and physiology of the hands and arms, and many other factors. What sets sign phonetics apart from the phonetics of spoken languages is that the two language modalities use different mechanisms of production and perception, which could in turn result in structural differences between modalities. Most studies of sign phonetics have been based on careful analyses of video data. Some studies have collected kinematic limb movement data during signing and carried out quantitative analyses of sign production related to, for example, signing rate, phonetic environment, or phrase position. Similarly, studies of sign perception have recorded participants’ ability to identify and discriminate signs, depending, for example, on slight variations in the signs’ forms or differences in the participants’ language background. Most sign phonetics research is quantitative and lab-based.
The tongue is composed entirely of soft tissue: muscle, fat, and connective tissue. This unusual composition and the tongue’s 3D muscle fiber orientation result in many degrees of freedom. The lack of bones and cartilage means that muscle shortening creates deformations, particularly local deformations, as the tongue moves into and out of speech gestures. The tongue is also surrounded by the hard structures of the oral cavity, which both constrain its motion and support the rapid small deformations that create speech sounds. Anatomical descriptors and categories of tongue muscles do not correlate with tongue function as speech movements use finely controlled co-contractions of antagonist muscles to move the oral structures during speech. Tongue muscle volume indicates that four muscles, the genioglossus, verticalis, transversus, and superior longitudinal, occupy the bulk of the tongue. They also comprise a functional muscle grouping that can shorten the tongue in the x, y, and z directions. Various 3D muscle shortening patterns produce large- or small-scale deformations in all directions of motion. The interdigitation of the tongue’s muscles is advantageous in allowing co-contraction of antagonist muscles and providing nimble deformational changes to move the tongue toward and away from any position.