Visuomotor transformations in apraxia
This thesis investigates whether the clinical neurological condition of apraxia is associated with impaired transformation of visual input into motor actions. Apraxia is generally a result of damage to the left cerebral hemisphere and is characterised by the inability to imitate gesture, to perform movements on verbal command, or to handle objects correctly. The justification for this study is that the mechanisms underlying apraxia are not well understood. Apraxia explained as a defect of visuomotor integration would provide a novel and possibly converging account of the disorder. This work used a hierarchical approach to examine different levels of visuomotor transformation in apraxic patients, looking at processes from input to output and movement classes from simple to complex. The tasks ranged from simple reaching and grasping actions, with or without on-line visual feedback, through delayed reaching and grasping actions, to complex imitative actions. The current work determined that apraxia was independent of general motor programming deficits, perceptual problems, or compromised access of the motor system to stored representations per se. No evidence was found for compromised integrated action of the perceptual and visuomotor systems in apraxia due to the disconnection of the dorsal and ventral streams according to the two visual pathways model (Milner &'38; Goodale 1995). Investigation of imitative actions did suggest that, in the process of imitation, visuomotor transformations based on egocentric body schema, especially when made in the absence of contextual cues from the environment, were essentially impaired in apraxia.