Speaking while gesturing: The relationship between speech and limb praxis

Skilled learned purposive movement (praxis) is mediated by a left hemisphere modular (parietal–premotor) network in right-handed people.1 The most sensitive method of screening subjects with neurologic conditions for limb apraxia is to ask them to produce transitive pantomimes to command (in which tools act on an object, e.g., “show me how you would use a screwdriver to put a screw in the wall”). Subjects with neurologic conditions, such as strokes or neurodegenerative disease, may make a number of qualitatively different errors when attempting to pantomime. For example,they may make content errors (pantomiming the use of a hammer instead of a screwdriver) or they may make movement errors such as incorrect joint coordination (e.g., pantomiming the use of a screwdriver by rotating at the wrist rather than the elbow).2

When testing subjects with apraxia, clinicians have casually observed that, unlike normal subjects, these patients often verbally describe a movement while attempting to pantomime (“Well, I would put my hand on the screwdriver and lift it up, then I would turn it”). Their difficulty cannot be attributed to inadequate understanding of the task, and the inappropriate speech can persist despite explicit instruction to respond with gestures alone. In this pilot investigation, we wished to learn …