Monitoring the changing status of semantic memory in Alzheimer's disease: An evaluation of several process measures

The deficits in generating correct words on verbal fluency tasks exhibited by patients with Alzheimer's disease (PLD) are accompanied by fewer switchi ng responses, smaller phonemic and semantic cluster sizes, and greater than normal percentages of errors and category labels. On category fluency task s, patients generate a greater proportion of wards that are prototypical of their semantic class. To determine whether any of these supplementary meas ures of verbal fluency performance might be useful in revealing processes i nvolved in the decline of semantic memory in AD, we studied 219 patients wi th AD and 115 elderly control participants longitudinally. Previously repor ted group differences between patients and controls were replicated, but ch anges in average cluster size, error rates, and prototypicality were not re lated to changes in overall severity of dementia and test-retest stability was only modest. The change in the percentage of labels generated on the Su permarket task was related to changes in dementia severity, but test-retest stability on this measure was quite low. All of these process measures app ear to reflect only the current status of the patient's attention to the ta sk and access to semantic knowledge, but they do not forecast future perfor mance. The numbers of switching responses on the fluency tasks were sensiti ve to differences between clinically deteriorated and clinically stable pat ients and showed fairly high test-retest stability. However, the number of switching responses is so highly correlated with the number of correct word s that it contributes little to the understanding of the processes involved in the progressive decline in performance on fluency tasks by patients wit h AD.