Left unilateral behavioral disorder with diagonistic dyspraxia following callosal and right centrum semiovale lesions: Implications for interhemispheric and fronto-parietal motor connectivity.

We report the case of a 58-year-old right-handed patient who presented with diagonistic dyspraxia of the left hand accompanied by various callosal disconnection symptoms, resulting from infarct lesions involving the right centrum semiovale as well as the corpus callosum. In addition to unintentionally performing actions opposite to those of the right hand, the left hand also exhibited movements identical to the right hand, movements unrelated to right-hand actions, and movements that preceded right-hand actions. Furthermore, the patient occasionally demonstrated symptoms not previously reported in classic diagonistic dyspraxia: the left hand involuntarily grasped objects despite the absence of pathological grasping, became unable to intentionally release grasped objects or, conversely, immediately released held objects. In linguistic tests conducted separately for each hand, errors such as the addition of actions were observed exclusively in the left hand, not only during the verbal command task but also when imitating the demonstrated actions included in the verbal commands, which did not require linguistic processing. The involuntary movements of the left hand observed in this patient suggest a dual disconnection mechanism: interhemispheric disconnection due to callosal lesions and intrahemispheric disconnection between the frontal and parietal lobes. The latter may contribute to impaired coordination between action inhibition and facilitation mechanisms as a result of disrupted association fibers caused by lesions in the right centrum semiovale.