18 - Imitation, apraxia, and hemisphere dominance

Summary

Introduction

Neuropsychological research on imitation has a history of nearly a hundred years. Liepmann (1908) investigated performance of meaningful gestures on command, like giving a military salute or showing how to turn a key, in patients with damage to the right or left hemisphere and normal controls. He found that only patients with left-brain damage (LBD) committed errors even when they performed the gestures with the nonparetic left hand. As most LBD patients were aphasic they might have had difficulties understanding the verbal instructions. However, they also committed errors when imitating the same gestures. Liepmann ascribed defective gesturing in LBD patients to “apraxia.” He emphasized that, in contrast to other motor sequels of unilateral brain damage, apraxia affects not only the contralesional but also the ipsilesional limbs, and concluded that it interferes with motor actions at a level beyond “elementary” motor control. He conceived of two possibilities for a higher level of disturbances of motor control: apraxia might stem from an inability to conjure up a mental representation of the required action, or from an inability to convert the mental representation into appropriate motor commands. Errors on imitation testified to Liepmann that “there is not only an inexactness of the spatial-temporal image of the movement, but a difficulty or inability to direct the leftsided members according to certain spatial conceptions” (Liepmann, 1908).