Patients with verbal disassociation apraxia cannot gesture normally to command but perform well with imitation and actual tools and objects. Heilman y described patients who, when asked to pantomime to command, looked at their hand but would not perform any recognizable action. Unlike the patients with ideomotor and conduction apraxia described previously, their ability to imitate motions and use objects was flawless. De Renzi and colleagues y also reported such patients as well as others who had a similar defect in other modalities. Others have reported patients with a disassociation apraxia of only the left hand following a callosal lesion. y , '4 Whereas language in these patients was mediated by the left hemisphere, we posit that movement representations may have been bilaterally represented. Therefore, their callosal abnormalities induced a disassociation apraxia of only the left hand because the verbal command could not get access to the right hemisphere movement representations. Although the patient with callosal disassociation apraxia is not able to correctly carry out skilled learned movements of the left arm on command, these patients can imitate and use actual tools and objects with the left hand because these tasks do not require verbal mediation, and the movement representations stored in the right hemisphere can be activated by visual input.
Right-handed patients in whom both language and movement formulas are represented in the left hemisphere may show a combination of disassociation and ideomotor apraxia in the presence of callosal lesions. [5 When asked to pantomime with their left hands, patients may look at them and perform no recognizable movement (disassociation apraxia), but when imitating or using actual tools and objects, they may make the same spatial and temporal errors seen in patients with ideomotor apraxia.
Left-handed individuals may demonstrate an ideomotor apraxia without aphasia from a right hemisphere lesion. These left-handed individuals are apraxic because their movement representations were stored in the right hemisphere and were destroyed by the lesion. y , y These left-handed persons are not aphasic because language is mediated by the left hemisphere (as is the case in the majority of left-handed people). If these left-handed individuals had a callosal lesion, they might have demonstrated a disassociation apraxia of the left arm and an ideomotor apraxia of the right arm.
The disassociation apraxia described by Heilman y from left hemispheric lesions was incorrectly termed "ideational apraxia." These patients and those of De Renzi and associates^ probably had an intrahemispheric language-movement formula, visual-movement formula, or somesthetic-movement formula disassociation. The locations of the lesions that cause these intrahemispheric disassociation apraxia are not known.
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