Voluntary Movement

Introduction

Voluntary movements are the only means by which the human mind can communicate with the outside world and other minds. Be it speech, facial expression, manipulation of objects, gesturing or simply pressing a button – the mind cannot manifest itself otherwise than by directing movements of the body. Consequently, the range of behaviours that could be subsumed under the heading ‘voluntary movement assessment’ is infinite.

This entry will concentrate on a very limited range of motor behaviours which neuropsychological tradition considers as giving rise to ‘high level’ disorders of motor control. They are summarized in Table 1. The rather vague and ill-defined term is meant to signify that disorders are neither purely motor nor purely mental but arise at the interface between mental processes and motor control or, respectively, between the mind and the body.

One class of such disorders is frequently subsumed under the term ‘apraxia’. They are [Page 1093]characterized by spatially misoriented or awkward movements. Other than in ‘elementary’ motor disorders the same movements which give rise to errors in one condition can successfully be performed in other conditions and success or failure depend on factors not directly related to motor control. Such factors may be the communicative meaning of a gesture or its relationship to tools and objects. Another class of ‘high level’ motor problems is constituted by well executed and apparently purposeful movements which do not conform to the subject's intentions.

Even this restricted range of motor behaviours is very heterogeneous. There is thus no standard assessment for all of them. Selection of appropriate diagnostic measures depends on the examiner's conjectures of likely disturbances in the individual patient. These are guided by knowledge of the brain – behaviour relationships, by the patients' own complaints and by observation of their spontaneous behaviour.

Apraxia

The concept of apraxia was elaborated by Hugo Liepmann (1908) nearly a hundred years ago. Liepmann noted that patients with left sided brain damage committed errors when performing motor actions with either hand. Most of these patients were also aphasic but he found apraxic patients without aphasia, and argued convincingly that faulty motor actions could not be explained as being a sequel of language impairment. He proposed instead that only the left hemisphere is capable of translating a concept or mental image of a desired action into appropriate motor commands. The nature of left hemisphere motor dominance and its relationship to language gave rise to various conflicting interpretations and remains an unsettled question after 100 years of research (Rothi et al., 1997; Heilman & Rothi, 1993; De Renzi, 1990; Geschwind, 1975; Kimura & Archibald, 1974).

Three kinds of actions are traditionally investigated for a clinical diagnosis of apraxia, because they yield clear manifestations of apraxic errors: imitation of gestures, demonstration of meaningful gestures, and use of tools and objects.

...