Longitudinal profiles of semantic impairment for living and nonliving concepts in dementia of Alzheimer's type

Two types of theoretical account have been proposed to explain the phenomen on of category-specific impairment in tests of semantic memory: One stresse s the importance of different cortical regions to the representation of liv ing and nonliving categories, while the other emphasize the importance of s tatistical relationships among features of concepts belonging to these two broad semantic domains. Theories of the latter kind predict that the direct ion of a domain advantage will be determined in large part by the overall d amage to the semantic system, and that the profiles of patients with progre ssive impairments of semantic memory are likely to include a point at which an advantage for one domain changes to an advantage for the other. The pre sent series of three studies employed semantic test data from two separate cohorts of patients with probable dementia of Alzheimer's type (DAT) to loo k for evidence of such a crossover. In the first study, longitudinal test s cores from a cohort of 58 patients were examined to confirm the presence of progressive semantic deterioration in this group. In the second study, Kap lan-Meier survival curves based on serial naming responses and plotted sepa rately for items belonging to living and nonliving domains indicated that t he representations of living concepts (as measured by naming) deteriorated at a consistently and significantly faster rate than those of nonliving con cepts. A third study, carried out to look in detail at the performance of m ildly affected patients, employed an additional cross-sectional cohort of 2 0 patients with mild DAT and utilized a graded naming assessment. This stud y also revealed no evidence for a crossover in the advantage of one domain over the other as a function of disease severity. Taken together with the m odel of anatomical progression in DAT based on the work of Braak and Braak (1991), these findings are interpreted as evidence for the importance of re gional cerebral anatomy to the genesis of semantic domain effects in DAT.