Selective impairments of object knowledge in a case of acquired cortical blindness

A patient (N.B.) is described, who displays distinct deficits of object knowledge related to knowledge type (Functional/Associative vs Visual) and also to knowledge category (Animate vs Inanimate). The patient was first given an orally presented forced-choice test devised to assess orthogonal combinations of knowledge type and knowledge category. In the production of this test it was found that normals took longer to respond to Visual questions than to Functional/Associative questions; therefore, sets of questions were compiled that were matched for both accuracy and latency. There were two main findings concerning N.B.'s semantic memory. First, with careful matching of difficulty level, the patient showed selective preservation of Functional/Associative knowledge of Animate objects compared with Visual knowledge of Animate objects and also compared with Functional/Associative knowledge of Inanimate objects. Second, there was a qualitative difference in patterns of knowledge retrieval for Visual compared with Functional/Associative knowledge. Retrieval of Visual knowledge, both Animate and Inanimate, was inconsistent and, in a word-pair recall test, a high degree of connection of a Visual property to an object did not promote paired-associate learning. In contrast, retrieval of Functional/Associative knowledge (both Animate and Inanimate) was consistent and paired-associate learning was influenced by connection strength. This study provides strong evidence to support the validity of both "knowledge type" and "category" based accounts of the organisation of semantic memory.