An update on the Conceptual-Production Systems model of apraxia: evidence from stroke

Limb apraxia is a neurological disorder characterized by an inability to pantomime and/or imitate gestures. It is more commonly observed after left hemisphere damage (LHD), but has also been reported after right hemisphere damage (RHD). The Conceptual-Production Systems model (Roy, 1996) suggests that three systems are involved in the control of purposeful movements: the conceptual, the production and the sensory/perceptual system. Depending on which system is damaged different patterns of apraxia are expressed. To determine the apraxia pattern, pantomime, delayed, and concurrent imitation tasks need to be administered, as well as conceptual tasks assessing one's knowledge of actions. Based on the model, eight patterns of apraxia should emerge. The purpose of this study is to determine whether these patterns are in fact observed in stroke patients and examine their frequency. If the performance of most stroke patients falls into one of the patterns, then we would have strong support for the conceptual-production model. Stroke (34 LHD and 39 RHD) patients and 27 age- and education-matched healthy controls participated in the study. Participants were assessed in four task modalities: pantomime, delayed imitation, concurrent imitation and conceptual knowledge (two tasks were used: tool naming by action and action identification). Patients were categorized as impaired on a task if they scored 2 SD below the mean performance of the controls for gesture production tasks, or below a cut-off score on the conceptual tasks. They were then classified into patterns depending on their performance on the four task modalities. Most patients (86%) fell into one of seven patterns originally predicted from the Conceptual-Production Systems model. The two most common patterns were deficits in pantomime and imitation with preserved gesture recognition and conduction apraxia (selective deficit in imitation). Four new patterns emerged, but mostly single cases of these were found. Overall, the study provides strong support for the Conceptual-Production Systems model.