Apraxia

Apraxia has been defined as a disorder of voluntary actions that can neither be referred to as elementary motor disorders, such as paresis or tremor, nor general cognitive problems, such as dementia or aphasia. Symptoms can be observed in a variety of settings, from traumatic brain injuries to cerebral infections to congenital disorders. Notions about apraxia have been important in our developing understanding of the workings of the human brain.

The concept of apraxia arose from the debate over cerebral localization of function in the late nineteenth century. In particular, it was linked to another condition, aphasia, a complete or incomplete inability to comprehend or use language (often because of brain injury, infection, or stroke). A central issue of this debate was whether language is an isolated function of the human mind, the neural substrate of which can be localized within the brain. Opponents of this compartmentalization of the human mind into localizable functions emphasized that aphasia is regularly accompanied by disturbances of nonverbal intellectual capacities. The German psychiatrist Carl Maria Finkelnburg observed that communicative gestures of aphasic patients are frequently clumsy and incomprehensible. In a very influential paper published in 1870, he posited that they suffer from a general “asymbolia” preventing the use and comprehension of any communicative signs. Ten years later, the linguist Heymann Steinthal used the term apraxia to denote the faulty use of everyday life objects, such as a fork and knife or a penholder, by patients with aphasia. He considered their errors an “augmentation” of aphasia.

Modern theories of apraxia have been shaped in the early twentieth century by the German psychiatrist Hugo Karl Liepmann. By systematic group studies, he confirmed the frequent occurrence of disturbed communicative gestures and aberrant use of tools and objects in patients with left-hemisphere brain damage and aphasia. To this confirmation, he added the new observation that these patients commit errors also when imitating gestures, an observation central to his conception of apraxia. He reasoned that in imitation the model of the intended movement is unequivocally provided by the demonstration and that errors thus testify to insufficient motor execution. He concluded that apraxia is a disorder of motor control that frequently accompanies aphasia but is itself a distinct and localizable symptom of circumscribed brain damage rather than indicating the augmentation of aphasia to general mental deterioration. For explaining the frequent co-occurrence of aphasia and apraxia after lefthemisphere brain damage, Liepmann speculated that the left hemisphere has a dominance for motor control beyond and perhaps above its dominance for speech.

The dependence of apraxia on left-hemisphere damage has since then been largely, although not completely, confirmed. Whereas the production of communicative gestures on command seems to be very tightly bound to left-hemisphere integrity, imitation of gestures and the use of tools and objects may be sensitive to right-hemisphere brain damage too depending on the exact nature of the task used to assess them. Liepmann's proposal that apraxia is a disorder of motor control fares less well in the light of contemporary research. His idea that faulty imitation implies deficient motor execution of a correctly conceived gesture was challenged by studies showing that patients who cannot imitate gestures have similar difficulties when asked to replicate them on a manikin or to select them from an array of pictures, and by reports of single patients in whom severely defective imitation of gestures contrasted with flawless execution of communicative gestures on command.

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