Journal of Experimental Psychology: General Copyright 1991 by the American Psychological Association, Inc. 1991, Vol. 120, No. 4, 339-357 0096-3445/91/$3.00 A Computational Model of Semantic Memory Impairment: Modality Specificity and Emergent Category Specificity Martha J. Farah and James L. McClelland Carnegie Mellon University It is demonstrated how a modality-specific semantic memory system can account for category- specific impairments after brain damage. In Experiment 1, the hypothesis that visual and functional knowledge play different roles in the representation of living things and nonliving things is tested and confirmed. A parallel distributed processing model of semantic memory in which knowledge is subdivided by modality into visual and functional components is described. In Experiment 2, the model is lesioned, and it is confirmed that damage to visual semantics primarily impairs knowledge of living things, and damage to functional semantics primarily impairs knowledge of non!iving things. In Experiment 3, it is demonstrated that the model accounts naturally for a finding that had appeared problematic for a modality-specific architec- ture, namely, impaired retrieval of functional knowledge about living things. Finally, in Experi- ment 4, it is shown how the model can account for a recent observation of impaired knowledge of living things only when knowledge is probed verbally. How is semantic memory organized? Two general answers to this question have been proposed. One is that semantic memory is organized by taxonomic category, such that differ- ent parts of the system represent knowledge about objects from different categories. Alternatively, semantic memory could be subdivided by modality of knowledge, such that one component is responsible for visual information about ob- jects, another for auditory information, and so on. Patients with selective losses of knowledge after brain dam- age appear to provide a direct source of evidence on the organization of semantic memory. Unfortunately, this evi- dence yields conflicting answers. In most cases, the losses appear to be tied to specific modalities, resulting in impaired recognition of objects in just one modality (e.g., visual or auditory agnosia) or in impaired manipulation of objects with specific uses, despite intact recognition of them (apraxia; e.g., a key might be pulled, rather than turned). These observations are consistent with recent neurophysiological data showing that most cortical neurons are modality-specific, even in regions that were traditionally viewed as supramodal associ- ation areas (e.g., Sereno & Allman, 1991). In some cases, however, brain damage seems to cause category-specific losses of knowledge, which cut across different modalities. Specifi- cally, there are patients who seem to have lost their knowledge of living things, and others who seem to have lost their knowledge of nonliving things. These observations suggest that the architecture of semantic memory incorporates at least This research was supported by Office of Naval Research Grant NS0014-89-J3016, National Institute of Mental Health (NIMH) Grant R01 MH48274, National Institutes of Health Career Devel- opment Award K04-NS01405, NIMH Career Development Award MH00385, and National Science Foundation Grant BNS 88-12048. We thank Robin Rochlin for assistance in data collection. Correspondence concerning this article should be addressed to Martha J. Farah or James L. McClelland, Department of Psychology, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890. two general, taxonomically defined subsystems, for represent- ing knowledge of living and nonliving things. In this article, we attempt to resolve the apparent conflict between these two types of neuropsychological evidence. After reviewing the neuropsychological evidence for category spec- ificity in semantic memory, we present a parallel distributed processing (PDP) model in which the architecture distin- guishes only between modalities of knowledge, but when damaged, displays category specificity similar to that of the patients described in the neuropsychological literature. Impairments in Knowledge of Living and Nonliving Things The most commonly observed semantic memory dissocia- tion is between impaired knowledge of living things with relatively preserved knowledge of nonliving things. In the first report of this phenomenon, Warrington and ShaUice (1984) described 4 patients who were much worse at identifying living things (animals, plants) than nonliving things (inani- mate objects). All 4 of these patients had recovered from herpes encephalitis, and all had sustained bilateral temporal lobe damage. Two of the patients were studied in detail and showed a selective impairment for living things across a range of tasks, both visual and verbal. Table 1 shows examples of their performance in a visual identification task, in which they were to identify by name or description the item shown in a colored picture, and in a verbal definition task, in which they were to provide definitions when the names of these same items were presented auditorily. Examples of their def- initions are also shown in Table 1. Farah, McMullen, and Meyer ( 199 l) studied 2 head-injured patients whose knowledge of living things appeared to be selectively disrupted. We examined their picture recognition performance as a function of the living-nonliving distinction as well as many other possibly confounded factors that might influence performance, including complexity, familiarity, 339340 MARTHA J. FARAH AND JAMES L. McCLELLAND Table 1 Performance of Two Patients With Impaired Knowledge of Living Things on Various Semantic Memory Tasks Case Living thing Nonliving thing Picture identification JBR 6% 90% SBY 0% 75% Spoken word definition JBR 8% 79% SBY 0% 52% JBR Parrot: don't know Daffodil: plant Snail: an insect animal Eel: not well Ostrich: unusual Examples of definitions Tent: temporary outhouse, living home Briefcase: small case used by students to carry papers Compass: tools for telling direction you are going Torch: hand-held light Dustbin: bin for putting rubbish in SBY Duck: an animal Wasp: