When a person decides to communicate verbally, the message first has to be converted from an abstract idea to meaningful linguistic (language) symbols. Syntactic, morphological, and phonological planning, occur. This linguistic-symbolic planning is a pre-motor (non-motor) phase. The language message must then be transformed into a code that can be handled by a motor (movement) system, such as the speech system. This transformation of the code occurs in three different phases. Speech motor planning, motor programming and execution are the three different phases in the preparation and the actual production of speech movements.
Speech motor planning entails the formulation of the strategy of action by defining motor goals. Each speech sound has a core motor plan that contains a number of motor goals. Motor goals for speech production can be found in the spatial (place and manner of articulation) and temporal (timing) specifications of movements for speech sound production. For example, the motor plan for the /b/ sound includes lip closure, velar lifting and vocal fold closure as motor goals. These movements are adapted to the sound environment in which the sound is to be produced. The motor goals for /b/ in “book” versus /b/ in “big” differ in that the lips are rounded more when a person plans to produce the word “book”. During speech motor planning, the different motor goals for each speech sound are identified and arranged to occur concurrently and sequentially. Next, the strategy (motor plans) must be converted into motor programs (tactics). Motor plans are structure-specific while programs are muscle-specific. During motor programming muscle tone, movement velocity, force and range as well as mechanical stiffness of the joints are specified. The repeated initiation of muscle-specific programs is also controlled during programming. An analogy of the plan-program relationship is found in other movements of the body. When reaching out for an object the motor plan includes the forward movement of the arm, opening of the hand and closing of the hand. The amount of muscle tension in the arm and hand, as well as the velocity, force and range of the movements are specified during programming. Planning and programming of movement occur prior to the execution of movement. During the execution phase the plans and programs are transformed into actual movements.
The different phases of motor planning, motor programming and execution are controlled by the coalition of various areas in the brain. A disorder in each of these phases may lead to different communication problems. The linguistic-symbolic planning phase is controlled by temporal-parietal areas and by Broca’s area. A disorder on this level may lead to aphasia. Motor planning is controlled by areas such as Broca’s area, the supplementary motor area (SMA) and areas 5 and 7 in the parietal lobes. A disorder on this level may lead to apraxia of speech in both its developmental and acquired form. Motor programming is controlled by areas such as the basal ganglia, the lateral cerebellum, the SMA and the fronto-limbic system. A disorder on this level may lead to certain components of dysarthric speech and possibly stuttering. Motor execution is controlled by areas such as the cerebellum, basal ganglia, motor cortex and the motor units in the muscles. A disorder on this level will lead to dysarthria.
Childhood apraxia of speech may be the result of an inability to learn and control motor planning of speech. The child may be unable to learn the core motor plans for all the different speech sounds of the language, to recall the series of motor plans from the sensorimotor memory during continuous speech, to identify the different motor goals of each speech sound and to sequentially and concurrently organize the movements for each speech sound and a series of sounds. These underlying problems may result in the inability to produce some or most speech sounds, to produce utterances consisting of a string of sounds, to produce a certain combination of sounds and to produce sounds accurately in long or unfamiliar utterances. The child may also exhibit struggle behavior during speech production. Within the context of this theoretical framework, treatment of childhood apraxia of speech should address the underlying problems in the different components of speech motor planning.
[Dr. Anita van der Merwe is Professor of Speech Pathology at the University of Pretoria in South Africa. She specializes in motor speech disorders, motor control of speech and voice disorders. She published the Speech Motor Learning Program for Apraxia of Speech in Afrikaans in 1985 and is currently preparing an extended English version. She has presented numerous courses in South Africa on the diagnosis and treatment of apraxia of speech (childhood and acquired). In 2002 she presented a continuing education course for ASHA at the University of Pittsburgh in the USA and a course for the Primary Care Trust in Morecambe Bay in the UK. She was invited to act as Keynote speaker at the 32nd Clinical Aphasiology Conference in the USA in 2002. She contributed a chapter on The Four-level Framework for the characterization of pathological speech sensorimotor control in the book Clinical Management of Sensorimotor Speech Disorders, edited by Dr. M.R. McNeil in 1997. A second edition of this book is currently in preparation.]