Apraxia

According to Functions

The investigation of apraxia began over a century ago. The first to study this disorder systematically was Hugo Liepmann, who developed a model of how voluntary gestures are organized in the brain and how the ability to evoke and produce them can be affected by brain damage. Liepmann distinguished three different types of apraxia, according to the function that he hypothesized to be affected: ideational, ideomotor, and melokinetic. This clinical classification is based on the type of function or process that is believed to be damaged. Thus, ideational apraxia (IA) corresponds to a deficit in formulating an action plan (or movement formula); it manifests itself when patients are engaged in complex action sequences (as in preparing breakfast) but also when they use one object at a time (such as drinking from a glass). The ideational deficit has been attributed to different causes. Arnold Pick described patients with dementia who suffered from IA, making gross mistakes in using objects that they were able to identify. Subsequently, Joseph Morlaas suggested that IA was due to a faulty recognition of objects (i.e., agnosia of usage), and Klaus Poeck believed it reflected the disintegration of the sequence organization of object-related actions. According to this latter interpretation, patients should be impaired in all tasks tapping the representation of an action sequence (e.g., everyday activities and sequencing photographs of those activities), sparing the use of single objects.

Poeck's interpretation was challenged by the work of Ennio De Renzi and collaborators who reported patients who made mistakes, such as omissions, object misuse, and mislocations of objects, when they performed complex everyday activities as well as when they used individual objects. De Renzi interpreted IA as a conceptual disorder, caused by “amnesia” for object use that prevents patients from adequately accessing the functional characteristics of objects. However, the impaired ability to use objects is not necessarily caused by damage to semantic memory, as patients with tool use impairment and preserved knowledge about objects have been documented. Tool use deficits are prevalently associated with lesions of the posterior regions of the left hemisphere and, in particular, of the parietal cortex, a region that has been identified in imaging studies with healthy individuals as involved in tool use. IA is often caused by focal lesions of vascular etiology, but it can also be observed in patients with dementia.

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