Memory-driven movements in limb apraxia: is there evidence for impaired communication between the dorsal and the ventral streams?

Memory-driven reaching and grasping movements were analysed in patients with left cerebral hemispheric damage and impaired gesture imitation. The dorsal and ventral streams of the visual pathway model of Milner and Goodale (Milner and Goodale, The Visual Brain in Action, 1995) are thought to operate relatively independently. However, cross-connections between the areas of each pathway are likely to enable interactions essential for higher-level praxis. Apraxic errors such as seen in gesture imitation can possibly be understood as arising from a disconnection of the two visual pathways. If the integrated action of the perceptual and visuomotor systems in patients with apraxia is compromised, then we would expect to find indications of impaired motor programming and misreaching in these patients when making movements driven by stored representations. Such a pattern, however, was not found in our sample of apraxic patients. Patients with limb apraxia produced normal movement kinematics and normal end-point accuracy when making memory-driven reaching movements with or without visual guidance of movement. Furthermore, perceptual information about object size and object distance were incorporated as normal in memory-driven grasping movements of these patients.