Visual-imitative dissociation apraxia.

Liepmann posited that, in right handers, the left parietal lobe contains movement formulas or representations. Therefore, performance failures may be induced by degraded representations, a failure of these representations to influence motor systems or a failure of stimuli to fully access these representations. Imitation may help the performance of subjects with degraded representations. However, patients who have impaired visual access to movement representations may perform more poorly with imitation than to verbal command. Trajectories of repetitive 'slicing' gestures made by a previously reported subject (Raymer et al.) with an infarction in the left visual association cortex (left occipital and inferior temporal lobe) that spared the parietal lobe were contrasted with those of three apraxic subjects with lesions that included the left parietal lobe and four non-brain-damaged control subjects. All subjects were asked to produce the gesture to verbal command and to imitation. Movements of the left hand, wrist, elbow and shoulder were digitized from neighboring views, reconstructed in three dimensions, and analysed graphically and numerically. The apraxic subjects with left parietal damage were unable to maintain the proper linearity and spatiotemporal attributes of their wrist motions and showed interjoint coordination deficits. Their deficits were most pronounced to verbal command, with their movements improving though remaining poorly performed when they imitated. The subject with the left occipital and inferior temporal lesion that spared parietal cortex, however, showed an opposite pattern. This subject exhibited close to normal performance when producing the movement to verbal command, but significant deficits when imitating.