The motor theory summarised

Language is the capacity of one individual to alter, through structured sound emission, the mental organisation of another individual. Language is more than speech just as perception is more than the structure and functioning of the eye. In both cases we have also to be concerned with the neural organisation underlying the functions of speech and visual perception. The theory is that language was constructed on the basis of a previously existing complex system, the neural motor system. The programs and procedures which evolved for the construction and execution of simple and sequential motor movements formed the basis of the programs and procedures going to form language.
A principal theme is the mosaic evolution of language, the fitting together of a whole array of elements, anatomical, neural and behavioural. Many elements necessary for mosaic evolution of the language capacity can be found in the anatomical and behavioural repertoires of birds and other animals. If these animals have behavioural elements involved in the evolution of human language capacity, they must also have the neural structures required to produce the behaviours, and in particular the neural motor programs required. A mechanism for the development or acquisition of the elements, in evolutionary terms, must have existed. In humans, the evolution of language would have had a major survival value, particularly for the group which acquired language.
Two important behavioural elements for language are imitation and the categorical perception of speech sound - both abilities found in some animals. Imitation, of speech or other sound or bodily movement, involves a remarkable and complex linking of perception and motor organisation. The capacity to discriminate categorically between human speech sounds, has surprisingly also been found in a variety of animals - and in extremely young human infants. These and other behavioural prerequisites for language depend on the intimate involvement of the motor control system and the existence of cross-modal processes. Development of the language capacity has resulted from the progressive establishment of new cross-modal or trans-functional neural linkages, cerebral reorganisation in the sense that the interconnectedness of different brain regions concerned with what are usually considered distinct functions has substantially increased. This extensive relation between language and the motor system is what one might reasonably expect, given the central role of the motor system in all behaviour and the essentially motor character of speech production. The next step is systematic examination of the relation between each aspect of language and corresponding features of motor activity and the motor system. However, given the close relation between the use and content of language on the one hand and perception on the other, the examination naturally extends also to the relation between the motor system and perception in all its forms. The motor system forms the indispensable mediator between language and perception.
The essential additional hypothesis is that the motor system, prior to the development of language, was built up from a limited number of primitive elements - units of motor action - which could be formed into more extended motor programs. If this is so, then one can look for a direct correspondence between the primitive motor elements and the fundamental elements of spoken language, the phonemic system. The processes of word- formation and syntactic rules for constructing word-sequences would then be derived from the neural rules governing the union of motor elements into simple and more complex actions. If language is in this way derived from the motor system, there is no reason to believe that any aspect of language - sound-elements, words or syntactic structure -is necessarily arbitrary. There is strong experimental evidence that the phonemic system is not arbitrary, and suggestive evidence that word-forms are not arbitrary but are expressive or appropriate to their meaning. There is also considerable evidence for a fundamental relation between the syntax of language and physiological syntax, the syntaxes of action and perception.
Recent research bears on the proposition that motor activity depends on a set of primitive motor elements. It supports the concept of motor programs and motor subprograms as real and not merely formal or theoretical bases for the organisation of action. Common general principles have evolved in neural control of movement in a wide range of animals. The experimental results suggest that the elementary motor programs may well be innate, part of standard human (and even vertebrate) neural structure. The elementary programs may form part of fixed action programs or be formed by a central motor program into novel action-sequences. In humans, research into motor programming bears directly on the relation between arm and head movements and speech.
The relation between motor programming and speech programming can be examined at the phonemic, lexical and syntactic levels. For phonemes, this leads to the idea of an invariant program for each phoneme, or 'auditory targeting', a motor-alphabet underlying speech, related to the elementary motor-patterns underlying other forms of action. Research on categorical speech perception has a direct bearing on this. A range of animals and very young infants have displayed the ability to categorise speech-sounds, natural or synthesised, in ways which match the category boundaries in adult speech; very young infants have been shown to discriminate categorically speech-sounds not found in their mother language.
On the motor theory, the categorisation of speech-sounds is derived from organisation prior to language, and specifically from the categorisation of motor programs used in constructing and executing all forms of bodily action. What the rhesus monkey, or the chinchilla, share with the young human infant is very similar skeletal and muscular organisation. The specificity of the phoneme is the accidental result of the application of the different elementary motor subprograms to the muscles which went to the form the articulatory system.
The link between the motor system and the formation of words follows. The hierarchical structure of the motor system is built on the basis of a limited set of motor elements. These are combined in an unlimited number of ways (motor-words). Words in speech are a read-out of neural structures in the same way as actions or facial expressions. A word, as a neural structure, can be formed from the co-activation of the motor subprograms for phonemes which are then melded or shingled together to form a distinct neural program for the whole word. Experimental approaches with the creation of artificial words have suggested that there can be a lawful relation between speech-sounds and auditory or visual percepts. Research into sound-symbolism suggests that there is an isomorphism at the motor level between speech and the contents of perception. The object seen produces a motor-pattern which is readily transferable as a motor-program to the articulatory system and so becomes the associated word for the thing. The neuromuscular sequences which are the immediate motor programs underlying words are derived from the integration of the neural structures underlying perception in all its forms (visual, auditory, tactile etc.) and motor organisation. It is of interest that in a recent book Hockett has commented on the relation between motor organisation and speech perception. "Although listeners obviously cannot have kinaesthetic feedback from someone else's articulation, they interpret what they hear by implicit motor- matching; actual movements of the organs of speech become unnecessary; the appropriate pattern of impulses within the central nervous system is enough".(Hockett 1987 39) If, as the motor theory proposes, phonemes and word-forms are derived from the motor system, then there must also be a close relation between the structuring of motor activity, motor syntax, and the organisation of language, speech syntax. One looks for evidence of this particularly in word-order.