Visuomotor sensitivity for shape and orientation in a patient with visual form agnosia.

We have previously demonstrated that a patient with visual form agnosia (DF), who is unable to report the orientation or size of visual targets, can nevertheless use these same visual attributes to control motor acts. In the first of three new experiments, we found that DF is able to grasp everyday tools and utensils proficiently (i.e. with a well-formed hand posture) but has difficulty in visually selecting the correct part of the object to grasp (e.g. the handle) for subsequent use of that object. A second experiment revealed that DF's visuomotor system is able to adjust concurrently to variations in both the size and orientation of target objects; when these visual attributes were both varied, she adjusted both her grip aperture and the orientation of her hand well in advance of target contact. These spared visuomotor abilities do not seem to extend to shape processing per se, however. In the final experiment we found that DF was insensitive to changes in the orientation of a cross-shaped object, where no single principal axis could be extracted to control orientation of the grasp. These observations extend our knowledge of DF's residual visuomotor abilities, and suggest limitations on the visual processing capacities of the human dorsal stream.