A computer model for the perception of syntactic structure.

Abstract

In his paper ‘The problem of serial order in behavior’ Karl Lashley (1951, p.113) points out that ‘language presents in a most striking form the integrative functions that are characteristic of the cerebral cortex’ adding ‘... the problems raised by the organization of language seem to me to be characteristic of almost all other cerebral activity’. Some idea of the complexity of the integrative processes involved in speech can be gained from the fact that the adult speaker’s ability to produce syllables at an average speed of 210 to 220 a minute (or roughly 14 phonemes per second) means individual muscular events occurring throughout the speech apparatus at a rate of several hundred every second; in the case of some phonemes the total time required to activate the muscles involved in their production being as much as twice as long as the duration of the sound itself. Not very much is known at present about what this involves on the neuronal level, where the rate at which individual events occur must be greater by a large factor, but it is a point of considerable interest that there is at least some evidence to suggest that in some instances the order of neuronal events might be different from that of the muscular events with which they are correlated.* The point Lashley is making in his paper is that any form of behaviour revealing this degree of complexity in its organization cannot be analysed as an associative chain of reflexes. But, as he points out, in the case of speech the evidence against the associative chain hypothesis is particularly compelling. This arises from considerations of two kinds. The first is the fact that the character of certain sounds is determined not only by the sounds that precede them but also by those that follow them. The second is the fact that the character of certain sounds is determined not only by the sounds in their immediate environment but also by the position they occupy with respect to the syntactic structure of the utterance. To take just one example, the speech of Standard English speakers contains at least twelve varieties (allophones) of the phoneme t . But whenever this is the first sound in a word and is immediately followed by a vowel they will always use the aspirated allophone never any of the others. This is clear evidence that in producing utterances speakers follow out principles of organization relating to syntactic structure. To produce a plausible model for speech we have to postulate not only principles of organization more complex than the Markov processes of associative chain theories but hierarchies of organization, elements on one level corresponding to what Lashley calls ‘generalized schemata of action' and Miller, Galanter & Pribram (1960) call ‘plans’ which are carried out on the level below. Evidence in favour of such a model can be obtained from a study of speech disorders, ranging from the transpositions occurring in the speech of a tired or nervous speaker to remarks of aphasics indicating that although for the most part they can only produce strings of unintelligible sounds they still ‘know what they want to say’. All these disorders can be viewed as involving in some degree a breakdown in integrative functions, an inability to carry out successfully plans for utterances.