Perception and action in neglect and optic ataxia - evidence of a double dissociation
Although a large literature exists relating the direct visual guidance of reaching and grasping movements to the dorsal visual stream in the superior parietal cortex, this work has been based almost entirely on studies where only one stimulus (the target) is present within the workspace. We asked the question whether the processing of non-target objects within the workspace is also carried out by dorsal-stream mechanisms. In our first study, we found that most patients with spatial neglect showed good avoidance of such secondary objects present in the visual field during reaching. In contrast, the same patients took little account of the leftward one of the two objects when asked to make a deliberate bisection response between the two objects. Using open-loop versions of these same tasks, we have now tested patients with superior parietal lesions, who showed impaired visually guided movements with respect to target objects (i.e. optic ataxia). The patients took no account whatever of the varying locations of the two objects when reaching between them, despite taking them fully into account when asked to bisect the space between them. We propose that the reaching task, requiring automatic obstacle avoidance, is a dorsal stream function, abolished in optic ataxia but spared in neglect. Thus the dorsal stream is a necessary part of the brain circuitry that guides actions not only with respect to targets but also with respect to potential obstacles. In contrast, the bisection task recruits a conscious perceptual strategy which optic ataxia patients can implement using their intact ventral stream and its temporo-parietal elaboration. In contrast neglect patients are unable to do this successfully.