New perspectives on speech motor planning and programming in the context of the four- level model and its implications for understanding the pathophysiology underlying apraxia of speech and other motor speech disorders

Abstract

BACKGROUND : The complexity of speech motor control, and the incomplete conceptualisation of phases in the transformation of the speech code from linguistic symbols to a code amenable to a motor system, tend to obscure the understanding of acquired apraxia of speech (AOS). The four-level framework (FLF) of speech sensorimotor control suggests the differentiation between speech motor planning, programming and execution and locate the locus of disruption in AOS in the motor planning phase. Currently, terminological confusion and uncertainty regarding phases in speech motor control still complicate the characterisation of AOS. This neuromotor disorder is inconsistently described in the literature as a “planning or programming”, “planning and programming”, or as a “planning and/or programming” disorder. PURPOSE : To describe a new version of the FLF, the FL (four-level) model, which further explicates and differentiates between speech motor planning, programming, and execution levels or phases of processing; to integrate concepts from computational modelling into the FL model and propose distinct control architectures for both the planning and programming levels; and to identify the loci and nature of disruption in the motor planning phase which could explain the pathophysiology and core features of AOS. DISCUSSION AND CONCLUSIONS : A four-level model is presented that differentiates two pre-execution phases and an execution phase. The first pre-execution phase is controlled by a motor planner and involves an inverse model, an efference copy, and a forward model for each sound or over-learnt utterance. This phase also involves a forward predictive planner which enables the system to handle the planning of several sounds and to plan coarticulation of sounds. The motor planner is operated according to an auxiliary forward model architecture. AOS is depicted as a breakdown at several possible points in the motor planning phase. The second pre-execution phase is driven by a motor program generator and predictive controller that is governed by an integral forward model architecture. The final execution phase is portrayed as being driven by closed loop control. The conceptualization of the programmer challenges the traditional view of execution and not only that of planning as is generally accepted. The implications for the classification of motor speech disorders are discussed. Future research should address the exact nature of articulatory movements and other features of speech across the range of planning, pure programming, programming-execution and pure execution disorders.