Anarchic hand syndrome: bimanual coordination and sensitivity to irrelevant information in unimanual reaches

Anarchic hand syndrome is characterised by unintended but purposeful and autonomous movements of the upper limb and intermanual conflict. Based on predictions of internal models of movement generation, we examined the role of visual cues in unimanual and bimanual movements in a patient with anarchic hand syndrome and in a matched control. In Experiment 1, participants made unimanual movements in a sequential button-pressing task. The cue for the next target in a sequence appeared either prior to (exogenous) or after (endogenous) the initiation of movement. For the patient, performance of the anarchic left hand was selectively impaired in the endogenous condition. In Experiment 2, participants made unimanual movements on a digitising tablet to a target, which appeared either alone or with a distractor. While the presence of a distractor was associated with increased Initiation time in general, the patient's anarchic left hand was particularly vulnerable to disruption by the distractor. The findings of Experiments 1 and 2 indicate excessive reliance on salient environmental stimuli for movement production in anarchic hand syndrome. We conclude that in AHS goal-directed actions of the affected limb are particularly vulnerable to disruption by non-relevant information. Finally, in Experiment 3, participants performed unimanual and mirror-image bimanual movements on a digitising tablet to targets in the left or right hemispace. Coupling of the parameters of the two hands was evident such that, compared with a unimanual baseline, Initiation time of the intact right hand deteriorated while it improved for the anarchic left hand.