Conceptual Apraxia

To perform a skilled act, two types of knowledge are needed—conceptual knowledge and production knowledge. Whereas dysfunction of the praxis production system induces ideomotor apraxia, defects in the knowledge needed to successfully select and use tools and objects are termed conceptual apraxia. Therefore, patients with ideomotor apraxia make production errors (e.g., spatial and temporal errors), and patients with conceptual apraxia make content and tool selection errors. Patients with conceptual apraxia may not recall the type of actions associated with specific tools, utensils, or objects (tool-object action associative knowledge) and therefore make content errors. For example, when asked to demonstrate the use of a screwdriver by either pantomiming or using the tool, the patient with a loss of tool-object action knowledge may pantomime a hammering movement or use the screwdriver as if it were a hammer. Content errors (i.e., using a tool as if it were another tool) can also be induced by an object agnosia.

However, researchers have reported a patient who could name tools (and therefore did not have an agnosia) but often used these tools inappropriately. Patients with conceptual apraxia may be unable to recall which specific tool is associated with a specific object (tool-object association knowledge). For example, when shown a partially driven nail, they may select a screwdriver rather than a hammer. This conceptual defect may also be in the verbal domain such that when an actual tool is shown to a patient, the patient may be able to name it (e.g., hammer), but when a patient with conceptual apraxia is asked to name or point to a tool when its function is described, he or she may not be able to correctly point to it. Patients with conceptual apraxia may also be unable to describe the functions of tools.

Patients with conceptual apraxia may also have impaired mechanical knowledge. For example, if they are attempting to drive a nail into a piece of wood and there is no hammer available, they may select a screwdriver rather than a wrench or pliers (which are hard, heavy, and good for pounding). Mechanical knowledge is also important for tool development. Patients with conceptual apraxia may also be unable to correctly develop tools.

In 1920, Liepmann thought that conceptual knowledge was located in the caudal parietal lobe, but in 1988 researchers placed it in the temporal-parietal junction. Later, a patient was reported who was left-handed and rendered conceptually apraxic by a lesion in the right hemisphere, suggesting that both production and conceptual knowledge have lateralized representations and that such representations are contralateral to the preferred hand. Further evidence that these conceptual representations stored in the hemisphere that is contralateral to the preferred hand derives from the observation of a patient who had a callosal disconnection and demonstrated conceptual apraxia of the nonpreferred (left) hand. Researchers studying right-handed patients who had either right or left hemisphere cerebral infarctions found that conceptual apraxia was more commonly associated with left than right hemisphere injury. However, they did not find any anatomic region that appeared to be critical, suggesting that mechanical knowledge may be widely distributed in the left hemisphere of right-handed people. Although conceptual apraxia may be associated with focal brain damage, it is perhaps most commonly seen in degenerative dementia of the Alzheimer's type. It was also noted that the severity of conceptual and IMA did not always correlate. The observation that patients with IMA may not demonstrate conceptual apraxia and patients with conceptual apraxia may not demonstrate IMA provides support for the postulate that the praxis production and praxis conceptual systems are independent. However, for normal function these two systems must interact.

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