The drawing of objects by a visual form agnosic: contribution of surface properties and memorial representations

Although edge-based representations of objects are thought to play a central role in object identification, it is clear that real objects convey more information about their form than line drawings. Patients with visual form agnosia, for example, are able to identify real objects more easily than the corresponding line drawings of those objects, even if exactly the same projection planes are used [Goodale et al. Object versus picture identification in a patient with visual form agnosia. Paper presented at the annual meeting of the Association for Research in Vision and Ophthalmology (April, 1991). Sarasota, FL, 1991]. To compare these two modes of representation in another way, we asked a patient (D.F.) with profound visual form agnosia to make line drawings of a series of common objects, either from long-term memory, from the real objects themselves, or from line drawings of those objects. When four independent judges rated the drawings as to how well they represented the target objects, drawings from memory received higher ratings than drawings of real objects which in turn received higher ratings than those based on line drawings. These results complement those of Goodale et al. (1991) and suggest that cues derived from surface properties and depth can assist in the demarcation of the critical features necessary for the accurate portrayal of objects. They also suggest that despite D.F.'s perceptual deficits, her long-term representation of objects is relatively intact.