Mapping category fluency to the ventral and dorsal language streams in PCA and semantic and logopenic PPA

Objective. The main neurocognitive models of language production purport the existence of a ventral, temporal, and a dorsal, parieto-frontal, stream underpinning, respectively, the lexical-semantic and phonological-articulatory components of word retrieval. Our study was aimed at identifying the pathway involved in lexical retrieval deficits in three neurodegenerative diseases: two Primary Progressive Aphasias (PPA), the logopenic (lPPA) and semantic (sPPA) variants, and Posterior Cortical Atrophy (PCA), which may sometimes present with language disturbances in addition to impairment of visual perceptual functions. Material and Methods. We included in the study patients fulfilling diagnostic criteria for PCA (n. 30), lPPA (n. 17) and sPPA (n. 17), matched for socio-demographic characteristics and MMSE score. The category fluency task was administered as part of a general neuropsychological assessment. Patients were asked to produce as many animal or fruit names in 60 secs. We then correlated fluency performance with distribution of hypometabolism on brain FDG-PET using SPM; significance threshold was set at p < 0.001 uncorrected, and only clusters with a minimum size of 100 voxels were considered. Results. Interestingly enough, average score on category fluency was not different in the three groups (PCA=15.6 ±6.0, lPPA= 13.5 ±4.5, sPPA= 13.0 ±6.0, p = 0.249). FDG-PET correlation analysis showed significant correlates of category fluency in posterior middle/inferior temporal gyrus and supramarginal gyrus, in PCA patients, at the temporo-parietal junction and in the planum temporale, in lPPA patients, and in the posterior fusiform, in sPPA patients (all correlates were within the left hemisphere). Discussion. Poor category fluency seems to be associated with conceptual loss due to damage at the far end of the ventral stream in sPPA, and with a deficit of phonological processing within the temporo-parietal region of the dorsal stream in lPPA. As to PCA, we found an involvement of both language streams, suggesting a double source of impairment of category fluency in this form of dementia: a deficit of lexical retrieval at the level of the posterior inferior temporal cortex, and, possibly, a deficit of phonological short-term memory due to dysfunction in the supramarginal gyrus. Conclusion. Patients with PCA, lPPA and sPPA may be similarly impaired in category fluency, and such an impairment appears to be linked to different anatomo-functional loci of damage along the two language pathways in the three diseases.