Neglect and the Dorsal Stream: How Impaired is the Action System?
Hemispatial neglect is generally defined as a failure to attend to stimuli in the contralesional space or to perform actions in that space. We will demonstrate that such patients are indeed not impaired on illusion processing such as the Mueller-Lyer and Judd figures although we failed to find any evidence that the left hand fins of these illusions are implicitly processed. However, although the illusions clearly worked in all but one neglect patient a significant rightward bias remained in their bisection of these illusionary stimuli. This bias may be caused by an inability to process object size (and in particular horizontal line length) veridically. However, this impaired size processing was not reflected in the grasping behaviour of these patients. Their grip scaling while reaching towards objects in varying spatial locations proved normal. Moreover, when neglect patients were subjected to visual feedback training in which, for 2 weeks, they were required to grasp and lift objects in two 20 minute sessions every day, a significant improvement in their neglect symptoms was not only shown via a reduction in their rightward biases in a line bisection task. They also improved on a general test of attentional capacity. It therefore seems possible that the visual dorsal stream that has been linked to visuomotor control and action may be relatively less impaired in hemispatial neglect and that an activation of this system actually improves performance in more general attention tasks.