Anomalous functional language lateralization in semantic variant PPA

Abstract

OBJECTIVE: To describe the lateralization of functional language activity in a case of semantic variant primary progressive aphasia (svPPA). BACKGROUND: Atypical language lateralization, bilateral or right-lateralized, is more common in non-right handed individuals than right-handed individuals. svPPA is an asymmetric language neurodegenerative condition typically affecting the left side. Compared to the general population, we have described an increased rate of non-right handedness in svPPA. Previously, 2 cases of left-handed individuals with predominantly right-sided atrophy have had evidence for atypical language lateralization, one case with fMRI support - raising the possibility that disease vulnerability associates with language dominance. An alternative hypothesis would suggest that anomalous language lateralization is associated with disease vulnerability. However, there are no known reports of anomalous language lateralization in left-sided temporal disease to support this supposition. DESIGN/METHODS: Magnetoencephalography (MEG) data was collected from a non-right-handed, left-sided, svPPA patient (UCSF Memory and Aging Center's research program on frontotemporal dementia) during a syllable reproduction and auditory noun-verb generation task using a 275-channel biomagnetometer (CTF). RESULTS: Time-frequency MEG imaging reconstructions revealed a very strong bilateral network during stimulus encoding in the syllable reproduction task, as well as a strong bilateral change in beta (15-30Hz) oscillatory power during noun reproduction and verb production in the verb generation task. This is in contrast to both the highly left lateralized high-gamma and beta activity found in healthy controls during these tasks. CONCLUSIONS: This is the first description of a left temporal predominant svPPA patient who displays bilateral language lateralization. In the context of our prior findings of increased non-right-handedness in svPPA, this finding may strengthen the theory of anomalous language lateralization as a risk factor associated with development of svPPA while placing in question the alternative theory - disease directly associating with language dominance.