Neural basis of category-specific semantic deficits for living things: evidence from semantic dementia, HSVE and a neural network model
1School of Psychological Sciences, University of Manchester, Manchester, UK and 2Department of Psychology, University of Wisconsin-Madison, Madison, WI, USA
Correspondence to: Prof. M. A. Lambon Ralph, School of Psychological Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, UK and Dr T. T. Rogers, Department of Psychology, University of Wisconsin-Madison, Madison, Wisconsin, USA E-mail: matt.lambon-ralph{at}manchester.ac.uk
Studies of patients with semantic impairments following brain damage offer key insights into the cognitive and neural organization of semantic memory. Especially important in this regard are studies of category-specific semantic impairment. We report a direct comparison of semantic deficits in two groups suffering from different diseases: semantic dementia (SD) and herpes simplex virus encephalitis (HSVE). Although pathology in both disorders is centred on the anterior temporal lobes bilaterally, category-specific semantic impairment is rarely observed in SD yet commonly found in HSVE. Using a combination of neuropsychology and computational neuroscience, we tested the possibility that category-specific deficits for living things depend not solely upon the location of damage within the cortical semantic network but also critically upon the type of impairment. When the semantic representations within the model are degraded or ‘dimmed’ then a generalized, global semantic impairment results (as found in SD) but when the representations are distorted then a category-specific pattern emerges (as per HSVE). Three novel predictions from this model were tested and confirmed, thereby adding weight to the hypothesis that both type and distribution of pathology can be critical in producing neuropsychological phenomena.
Key Words: semantic memory disorders; semantic dementia; herpes simplex virus encephalitis; PDP neural network models; category-specific disorders
Abbreviations: CVA, cerebral vascular accident; HSVE, herpes simplex virus encephalitis; PDP, parallel distributed processing; SD, semantic dementia
Received September 26, 2006. Revised January 28, 2007. Accepted January 29, 2007.
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