Going, going, gone ... ? Implicit and explicit tests of conceptual knowledge in a longitudinal study of semantic dementia

Patients suffering from semantic dementia provide important constraints on theories of the structure and organisation of semantic memory. In this arti cle we report one such patient, AM, whose progressive deterioration of sema ntics enables us to address the much-debated issue of whether conceptual st ructure is hierarchically organised. The hierarchical account predicts that brain damage should impair lower levels of the hierarchy (property informa tion) before affecting higher level (category) information (Warrington and Shallice, Q. J. Exp. Psychol. 1975, 27, 635-657). We evaluate this predicti on by repeated testing of AM in two studies-a semantic priming task and a v erification task-over an 18 month period, contrasting the progressive deter ioration of properties (functional and perceptual) and category relations ( category co-ordinates and category labels). Properties were preserved longe r than category information, arguing against a hierarchical account of sema ntic memory. In addition, functional properties were most robust to brain d amage, supporting our claim that functional information prays a special rol e in semantic representations (Durrant-Peatfield el al., Proc. 19th Ann. Co nf: of the Cognitive Science Society. Erlbaum, Mahwah, NJ, 1997, pp. 193-19 8. Tyler el al.; Cognitive Neuropsychol. 1997, 14, 511-545). (C) 1998 Elsev ier Science Ltd. All rights reserved.