Abstract
Verbal fluency (VF) is an informative cognitive task. Lesion and functional imaging studies implicate distinct cerebral areas that support letter versus semantic fluency and the understanding of neural and cognitive mechanisms underlying task performance. Most lesion studies include chronic stroke patients. People with primary progressive aphasia (PPA) provide complementary evidence for lesion-deficit associations, as different brain areas are affected in stroke versus PPA. In the present study we sought to determine imaging, clinical and demographic correlates of VF in PPA. Thirty-five patients with PPA underwent an assessment with letter and category VF tasks, evaluation of clinical features and an MRI scan for volumetric analysis. We used stepwise regression models to determine which brain areas are associated with VF performance while acknowledging the independent contribution of clinical and demographic factors. Letter fluency was predominantly associated with language severity (R2 = 38%), and correlated with the volume of the left superior temporal regions (R2 = 12%) and the right dorsolateral prefrontal area (R2 = 5%). Semantic fluency was predominantly associated with dementia severity (R2 = 47%) and correlated with the volume of the left inferior temporal gyrus (R2 = 7%). No other variables were significantly associated with performance in the two VF tasks. We concluded that, independently of disease severity, letter fluency is significantly associated with the volume of frontal and temporal areas whereas semantic fluency is associated mainly with the volume of temporal areas. Furthermore, our findings indicated that clinical severity plays a critical role in explaining VF performance in PPA, compared to the other clinical and demographic factors.
Highlights
Verbal fluency tasks are the most common neuropsychological tests used to assess verbal functioning [1]
Post-hoc analyses revealed that participants with nfvPPA were significantly more impaired in letter compared to semantic fluency (p = 0.001); svPPA were significantly more impaired in semantic compared to letter fluency (p = 0.002); and LvPPA were equivalently impaired in letter and semantic fluency
The goal was to assess whether previous claims from the post-stroke literature—namely that letter fluency involves more frontal areas, but semantic fluency involves more temporal areas—apply to neurodegenerative conditions that involve language, such as progressive aphasia (PPA), while controlling for symptom severity
Summary
Verbal fluency tasks (letter and semantic fluency) are the most common neuropsychological tests used to assess verbal functioning [1]. In the verbal fluency tasks participants are asked to produce as many words as possible starting with a given letter (letter fluency task, called letter fluency, phonemic/phonological and/or word fluency) or words within a specific semantic category (semantic or category fluency task) Both verbal fluency tasks might rely on similar executive cognitive skills, such as initiation (the ability to use attention to generate the word), self-monitoring (suppressing the activation of inappropriate responses—e.g., semantically related words, or repetition), cognitive flexibility (ability to rapidly switch strategies) [2], and other cognitive functions related to memory and language (available semantic and lexical knowledge from which to identify relevant items) and recall (ability to retrieve items from verbal declarative memory) [3]. These two components are shared by both tasks, with the switching component being more related to the frontal lobe functioning [4]
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