Semantic fluency: cognitive basis and diagnostic performance in focal dementias and Alzheimer's disease

Semantic fluency is widely used both as a clinical test and as a basic tool for understanding how humans extract information from the semantic store. Recently, major efforts have been made to devise fine-grained scoring procedures to measure the multiple cognitive processes underlying fluency performance. Nevertheless, it is still unclear how many and which independent components are necessary to thoroughly describe performance on the fluency task. Furthermore, whether a combination of multiple indices can improve the diagnostic performance of the test should be assessed. In this study, we extracted multiple indices of performance on the semantic fluency test from a large sample of healthy controls (n = 307) and patients (n = 145) suffering from three types of focal dementia or Alzheimer's Disease (AD). We found that five independent components underlie semantic fluency performance. We argue that these components functionally map onto the generation and application of a search strategy (component 2), to the monitoring of the overall sequence to avoid repetitions (component 3) and out-of-category items (component 4), and to the full integrity of the semantic store (component 5). The integrated and effective work of all these components would relate to a "general effectiveness" component (component 1). Importantly, while all the focal dementia groups were equally impaired on general effectiveness measures, they showed differential patterns of failure in the other components. This finding suggests that the cognitive deficit that impairs fluency differs among the three focal dementia groups: a semantic store deficit in the semantic variant of primary progressive aphasia (sv-PPA), a strategy deficit in the non-fluent variant of primary progressive aphasia (nfv-PPA), and an initiation deficit in the behavioural variant of fronto-temporal dementia (bv-FTD). Finally, we showed that the concurrent use of multiple fluency indices improves the diagnostic accuracy of semantic fluency both for focal dementias and for AD. More generally, our study suggests that a formal evaluation of fine-grained patterns of performance would improve the diagnostic accuracy of neuropsychological tests.