To know what it is for, but not how it is: Semantic dissociations in a case of visual agnosia

We report the case of a woman who displayed impaired object recognition following a severe head injury. Her elementary visual functions were substantially preserved, allowing her a coherent percept. On the other hand, she was impaired in accessing stored knowledge from both visual and verbal input. In particular, she showed a dramatic dissociation between fully preserved access to functional knowledge, and severely impaired access to perceptual knowledge so that she could describe what objects are for, but not how they are. Our findings from this case suggest that different categories of object knowledge are represented independently in separate units within the semantic system.