The dependence of ipsilesional aiming deficits on task demands, lesioned hemisphere, and apraxia

Neuroimaging studies as well as neurophysiological and lesion data indicate that the ipsilateral hemisphere plays a role in controlling the active limb. However, the nature and the conditions of this ipsilateral control are not well understood. We measured aiming movements with the ipsilesional limb toward targets with different characteristics which were made by patients with unilateral left brain damage (LBD) or right brain damage (RBD). The movement kinematics were analysed. Performance measures of the pointing movements were impaired in LBD patients, whereas RBD patients performed normally. LBD patients had obvious deficits during all tasks; however, they were exacerbated when high accuracy was required, and when an exocentric target had to be reached without visual feedback. Thus, the motor-dominant hemisphere plays a specific role in the programming and execution of ipsilateral aiming movements, and the importance of ipsilateral control increases with increasing task demands. To assess the relationship between pointing deficits and apraxia in LBD patients, the imitation of meaning gestures was tested. We replicated a recent study, showing that deviations of the final hand position from the demonstration were not correlated with abnormal kinematics of the corresponding arm movement when LBD patients performed this test. However, there were correlations between related kinematic measures during pointing and gesture imitation. These findings suggest a deficit of motor programming and execution after damage to the motor-dominant brain which is unrelated to the spatial errors characteristic of apraxia. This deficit affects different types of goal-directed aiming movements and its severity depends on task demands.